05 - Infecting the plt/got

The objective of this tutorial is to hook an imported function in an ELF binary.

Scripts and materials are available here: materials

By Romain Thomas - @rh0main


Hooking imported functions by infecting the .got section is a well-known technique [1] [2] and this tutorial will be focused on its implementation using LIEF.

These figures illustrate the plt/got mechanism:

../_images/pltgot.png

With lazy binding, the first time that the function is called the got entry redirects to the plt instruction.

../_images/pltgot3.png

The Second time, got entry holds the address in the shared library.

Basically the infection is done in two steps:

  • Firstly, we inject our hook
  • Secondly, we redirect the targeted function to our hook by patching the got

It can be summed up by the following figure:

../_images/pltgot2.png

As example, we will use a basic crackme which performs a memcmp(3) on the flag and user’s input.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Damn_YoU_Got_The_Flag
char password[] = "\x18\x3d\x31\x32\x03\x05\x33\x09\x03\x1b\x33\x28\x03\x08\x34\x39\x03\x1a\x30\x3d\x3b";

inline int check(char* input);

int check(char* input) {
  for (int i = 0; i < sizeof(password) - 1; ++i) {
    password[i] ^= 0x5c;
  }
  return memcmp(password, input, sizeof(password) - 1);
}

int main(int argc, char **argv) {
  if (argc != 2) {
    printf("Usage: %s <password>\n", argv[0]);
    return EXIT_FAILURE;
  }

  if (strlen(argv[1]) == (sizeof(password) - 1) && check(argv[1]) == 0) {
    puts("You got it !!");
    return EXIT_SUCCESS;
  }

  puts("Wrong");
  return EXIT_FAILURE;

}

The flag is xored with 0x5C. To validate the crackme, the user has to enter Damn_YoU_Got_The_Flag:

$ crackme.bin foo
Wrong
$ crackme.bin Damn_YoU_Got_The_Flag
You got it !!

The hook will consist in printing arguments of memcmp and returning 0:

#include "arch/x86_64/syscall.c"
#define stdout 1

int my_memcmp(const void* lhs, const void* rhs, int n) {
  const char msg[] = "Hook memcmp\n";
  _write(stdout, msg, sizeof(msg));
  _write(stdout, (const char*)lhs, n);
  _write(stdout, "\n", 2);
  _write(stdout, (const char*)rhs, n);
  _write(stdout, "\n", 2);
  return 0;
}

As the hook is going to be injected into the crackme, it must have the following requirements:

  • Assembly code must be position independant (compiled with -fPIC or -pie/-fPIE flags)
  • Don’t use external libraries such as libc.so (-nostdlib -nodefaultlibs flags)

Due to the requirements, the hook is compiled with: gcc -nostdlib -nodefaultlibs -fPIC -Wl,-shared hook.c -o hook.

Injecting the hook

The first step is to inject the hook into the binary. To do so we will add a Segment:

import lief

crackme = lief.parse("crackme.bin")
hook    = lief.parse("hook")

segment_added  = crackme.add(hook.segments[0])

All assembly code of the hook stands in the first LOAD segment of hook.

Once the hook added, its virtual address is virtual_address of segment_added and we can processed to the got patching.

Patching the got

LIEF provides a function to easily patch the got entry associated with a Symbol:

Binary.patch_pltgot(*args, **kwargs)

Overloaded function.

  1. patch_pltgot(self: _pylief.ELF.Binary, symbol_name: str, address: int) -> None

Patch the imported symbol’s name with the address

  1. patch_pltgot(self: _pylief.ELF.Binary, symbol: LIEF::ELF::Symbol, address: int) -> None

Patch the imported Symbol with the address

The offset of the memcmp function is stored in the value attribute of the associated dynamic symbol. Thus its virtual address will be:

  • my_memcpy: value + segment_added.virtual_address
my_memcmp      = hook.get_symbol("my_memcmp")
my_memcmp_addr = segment_added.virtual_address + my_memcmp.value

Finally we can patch the memcmp from the crakme with this value:

crackme.patch_pltgot('memcmp', my_memcmp_addr)

And rebuild it:

crackme.write("crackme.hooked")

Run

As a check on the input size is performed before checking the flag value, we have to provide an input with the correct length (no matter its content):

$ crackme.hooked XXXXXXXXXXXXXXXXXXXXX
Hook add
Damn_YoU_Got_The_Flag
XXXXXXXXXXXXXXXXXXXXX
You got it !!

References

[1]http://vxheaven.org/lib/vrn00.html
[2]http://phrack.org/issues/56/7.html