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89wc th/thh kalm  11mo ago (text post) 302 thread views #227504

Any C hackers in here 👀👀

Just curious, and I haven't googled because I already know it's not going to answer the question ....

So say hypothetically I launch an API to run in-line C code from the terminal. It'd just be a basic shell script that has a skeleton of a .c file and injects STDIN straight into the main function. The only header would be <stdio.h> Is it possible to do this securely? Or is the simply allowing arbitrary C even with these restrictions a recipe for pwnage.

So it'd be just

In the endpoint directory:

index.php
run.sh

index.php would receive the string and call shell to remove comments

:a;s@//.*@@;ta; :a;s@/\*@@;ta; :a;s@\*/@@;ta;

then it would just echo out

#include <stdio.h>
int main(void) {

**

<<user input>>

**

return 0; }

into a temporary.c , compile, and then php can capture the output into a variable and with that reply to the API call.

I'm gonna do this anyway on a throwaway server but I was wondering mostly out of curiosity whether even this can be exploited. (i.e. somehow adding more headers and executing a fork bomb or something)

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Yes that's just arbitrary code execution. That's the worst thing you can do security wise.

When you compile C code, #include is a preprocessor directive to copy the library's code directly into the file.

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so one could even load a library from inside of main? I thought I was being slick by hardcoding that :marseycry:

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C std has system() that passes arbitrary commands to the shell

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An #include is never necessary, you can always supply your own defines and type and function definitions, and can inject arbitrary code easily. See this stupid example:

int main(void) {
   unsigned char buf[] = { 0x48, 0x89, 0xfe, 0xbf, 0x00, 0x00, 0x00, 0x00, 0xba, 0x10, 0x00, 0x00, 0x00, 0xb8, 0x01, 0x00, 0x00, 0x00, 0x0f, 0x05, 0xc3 };

   int mprotect(void* addr, unsigned long len, int prot);
   mprotect(((void*)(((unsigned long)buf) / 4096 * 4096)), 4096, 7);

   typedef void (*fptr)(const char*);

   fptr my_clandestine_print = (fptr)buf;
   my_clandestine_print(":marseywave:");
}

(Only works on Intel and AMD systems, because the payload is AMD64)

:marseywave2:

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:marseyitneverbegan:

C is so awesome :marseyadmire: 's#(void*|void *)#foid#g'

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