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Basic Information ​

This was a very interesting technique that allowed for RCE without leaks via fake fastbins, the unsorted_bin attack and relative overwrites. However it has ben patched.

Code ​

Goal ​

  • RCE by abusing relative pointers

Requirements ​

  • Edit fastbin and unsorted bin pointers
  • 12 bits of randomness must be brute forced (0.02% chance) of working

Attack Steps ​

Part 1: Fastbin Chunk points to __malloc_hook ​

Create several chunks:

  • fastbin_victim (0x60, offset 0): UAF chunk later to edit the heap pointer later to point to the LibC value.
  • chunk2 (0x80, offset 0x70): For good alignment
  • main_arena_use (0x80, offset 0x100)
  • relative_offset_heap (0x60, offset 0x190): relative offset on the 'main_arena_use' chunk

Then free(main_arena_use) which will place this chunk in the unsorted list and will get a pointer to main_arena + 0x68 in both the fd and bk pointers.

Now it's allocated a new chunk fake_libc_chunk(0x60) because it'll contain the pointers to main_arena + 0x68 in fd and bk.

Then relative_offset_heap and fastbin_victim are freed.

c
/*
Current heap layout: 
	0x0:   fastbin_victim       - size 0x70 
	0x70:  alignment_filler     - size 0x90
	0x100: fake_libc_chunk      - size 0x70 (contains a fd ptr to main_arena + 0x68)
	0x170: leftover_main        - size 0x20
	0x190: relative_offset_heap - size 0x70 

	bin layout: 
			fastbin:  fastbin_victim -> relative_offset_heap
			unsorted: leftover_main
*/
  • fastbin_victim has a fd pointing to relative_offset_heap
  • relative_offset_heap is an offset of distance from fake_libc_chunk, which contains a pointer to main_arena + 0x68
  • Just changing the last byte of fastbin_victim.fd it's possible to make fastbin_victim points to main_arena + 0x68

For the previous actions, the attacker needs to be capable of modifying the fd pointer of fastbin_victim.

Then, main_arena + 0x68 is not that interesting, so lets modify it so the pointer points to __malloc_hook.

Note that __memalign_hook usually starts with 0x7f and zeros before it, then it's possible to fake it as a value in the 0x70 fast bin. Because the last 4 bits of the address are random there are 2^4=16 possibilities for the value to end pointing where are interested. So a BF attack is performed here so the chunk ends like: 0x70: fastbin_victim -> fake_libc_chunk -> (__malloc_hook - 0x23).

(For more info about the rest of the bytes check the explanation in the how2heap example). If the BF don't work the program just crashes (so start gain until it works).

Then, 2 mallocs are performed to remove the 2 initial fast bin chunks and the a third one is alloced to get a chunk in the __malloc_hook:

c
malloc(0x60);
malloc(0x60);
uint8_t* malloc_hook_chunk = malloc(0x60);

Part 2: Unsorted_bin attack ​

For more info you can check:

But basically it allows to write main_arena + 0x68 to any location by specified in chunk->bk. And for the attack we choose __malloc_hook. Then, after overwriting it we will use a relative overwrite) to point to a one_gadget.

For this we start getting a chunk and putting it into the unsorted bin:

c
uint8_t* unsorted_bin_ptr = malloc(0x80);	
malloc(0x30); // Don't want to consolidate

puts("Put chunk into unsorted_bin\n");
// Free the chunk to create the UAF
free(unsorted_bin_ptr);

Use an UAF in this chunk to point unsorted_bin_ptr->bk to the address of __malloc_hook (we brute forced this previously).

❌

Note that this attack corrupts the unsorted bin (hence small and large too). So we can only use allocations from the fast bin now (a more complex program might do other allocations and crash), and to trigger this we must alloc the same size or the program will crash.

So, to trigger the write of main_arena + 0x68 in __malloc_hook we perform after setting __malloc_hook in unsorted_bin_ptr->bk we just need to do: malloc(0x80)

Step 3: Set __malloc_hook to system ​

In the step one we ended controlling a chunk containing __malloc_hook (in the variable malloc_hook_chunk) and in the second step we managed to write main_arena + 0x68 in here.

Now, we abuse a partial overwrite in malloc_hook_chunk to use the libc address we wrote there(main_arena + 0x68) to point a one_gadget address.

Here is where it's needed to bruteforce 12 bits of randomness (more info in the how2heap example).

Finally, one the correct address is overwritten, call malloc and trigger the one_gadget.

References ​

Learn AWS hacking from zero to hero with htARTE (HackTricks AWS Red Team Expert)!

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