# Large Bin Attack

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## Basic Information

For more information about what is a large bin check this page:


{{#ref}}
bins-and-memory-allocations.md
{{#endref}}

It's possible to find a great example in [**how2heap - large bin attack**](https://github.com/shellphish/how2heap/blob/master/glibc_2.35/large_bin_attack.c).

Basically here you can see how, in the latest "current" version of glibc (2.35), it's not checked: **`P->bk_nextsize`** allowing to modify an arbitrary address with the value of a large bin chunk if certain conditions are met.

In that example you can find the following conditions:

- A large chunk is allocated
- A large chunk smaller than the first one but in the same index is allocated
  - Must be smalled so in the bin it must go first
- (A chunk to prevent merging with the top chunk is created)
- Then, the first large chunk is freed and a new chunk bigger than it is allocated -> Chunk1 goes to the large bin
- Then, the second large chunk is freed
- Now, the vulnerability: The attacker can modify `chunk1->bk_nextsize` to `[target-0x20]`
- Then, a larger chunk than chunk 2 is allocated, so chunk2 is inserted in the large bin overwriting the address `chunk1->bk_nextsize->fd_nextsize` with the address of chunk2

> [!TIP]
> There are other potential scenarios, the thing is to add to the large bin a chunk that is **smaller** than a current X chunk in the bin, so it need to be inserted just before it in the bin, and we need to be able to modify X's **`bk_nextsize`** as thats where the address of the smaller chunk will be written to.

This is the relevant code from malloc. Comments have been added to understand better how the address was overwritten:

```c
/* if smaller than smallest, bypass loop below */
assert (chunk_main_arena (bck->bk));
if ((unsigned long) (size) < (unsigned long) chunksize_nomask (bck->bk))
  {
    fwd = bck; // fwd = p1
    bck = bck->bk; // bck = p1->bk

    victim->fd_nextsize = fwd->fd; // p2->fd_nextsize = p1->fd (Note that p1->fd is p1 as it's the only chunk)
    victim->bk_nextsize = fwd->fd->bk_nextsize; // p2->bk_nextsize = p1->fd->bk_nextsize
    fwd->fd->bk_nextsize = victim->bk_nextsize->fd_nextsize = victim; // p1->fd->bk_nextsize->fd_nextsize = p2
  }
```

This could be used to **overwrite the `global_max_fast` global variable** of libc to then exploit a fast bin attack with larger chunks.

You can find another great explanation of this attack in [**guyinatuxedo**](https://guyinatuxedo.github.io/32-largebin_attack/largebin_explanation0/index.html).

### Other examples

- [**La casa de papel. HackOn CTF 2024**](https://7rocky.github.io/en/ctf/other/hackon-ctf/la-casa-de-papel/)
  - Large bin attack in the same situation as it appears in [**how2heap**](https://github.com/shellphish/how2heap/blob/master/glibc_2.35/large_bin_attack.c).
  - The write primitive is more complex, because `global_max_fast` is useless here.
  - FSOP is needed to finish the exploit.

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