Fast Bin Attack

A fast bin attack exploits the singly-linked list structure of fast bins in glibc's malloc implementation. By modifying the

next

pointer of a freed chunk, you can cause

malloc

to return a pointer to an arbitrary memory address.

Core Attack Pattern

Prerequisites

• Use-after-free (UAF) vulnerability to write to freed chunks
• Ability to control chunk contents (overflow, write primitive, etc.)
• Fast bin sizes: typically 0x10-0x80 (varies by configuration)
• Tcache must be full or disabled for fast bins to be used

Basic Exploitation Flow

1. Allocate two chunks of the same size (e.g., 0x20)
   ptr0 = malloc(0x20)
   ptr1 = malloc(0x20)

2. Free both chunks (they go to fast bin)
   free(ptr0)
   free(ptr1)

3. Use UAF to modify ptr1's next pointer
   *ptr1 = (unsigned long)&<target_address>

4. Allocate to consume ptr0
   ptr2 = malloc(0x20)  // Gets ptr0

5. Allocate to get chunk at target address
   ptr3 = malloc(0x20)  // Gets chunk at &<target_address>

Why This Works

Fast bins are singly-linked lists with minimal validation:

• No size checks on the

  next

  pointer
• No forward/backward consistency checks
• The

  next

  pointer is simply the first 8 bytes of the freed chunk
• When you allocate, malloc returns the address stored in

  next

Common Attack Scenarios

1. Overwrite GOT Entry

Use fast bin attack to allocate a chunk at

&free@got

or

&printf@got

, then write a one-gadget address:

1. Leak libc address (via unsorted bin or other technique)
2. Calculate &free@got = libc_base + free_offset
3. Fast bin attack to allocate chunk at &free@got
4. Write system() address to that chunk
5. Call free() on a chunk containing "/bin/sh"

2. Overwrite __malloc_hook

Allocate a chunk at

&__malloc_hook - 0x10

(to account for chunk header), then write a one-gadget:

1. Leak libc address
2. Calculate &__malloc_hook = libc_base + __malloc_hook_offset
3. Fast bin attack to allocate at &__malloc_hook - 0x10
4. Write one-gadget address to the chunk
5. Trigger malloc to execute the gadget

3. Overwrite Function Pointers

If the target has a global array of function pointers or chunk pointers:

1. Leak address of the pointer array
2. Fast bin attack to allocate chunk at the array
3. Overwrite pointers to controlled addresses
4. Trigger the function call

4. global_max_fast Manipulation

If you can't use fast bins due to size restrictions:

1. Use unsorted bin attack to leak libc
2. Overwrite global_max_fast with a large value (e.g., 0x1000)
3. Now larger chunks go to fast bins instead of unsorted bins
4. Perform fast bin attack with larger chunks

Note: This works only 1/16 times due to ASLR (need to match 12 bits out of 16).

Implementation Template

Use the

scripts/generate_fastbin_exploit.py

script to create a basic exploit template:

python scripts/generate_fastbin_exploit.py \
  --chunk-size 0x30 \
  --target-address 0x404040 \
  --output exploit.py

Key Considerations

Fast Bin vs Tcache

• Tcache is checked before fast bins
• If tcache is not full, freed chunks go there instead
• To force fast bin usage:
  • Free 7+ chunks of the same size (fills tcache)
  • Or disable tcache in the binary

Size Constraints

• Fast bins only accept chunks up to

  global_max_fast

  (default 0x80)
• Chunk size includes the 8-byte header
• Actual payload size = requested_size - 8

Alignment

• Allocations are 8-byte aligned
• Target address must be properly aligned
• If targeting

  &__malloc_hook

  , allocate at

  &__malloc_hook - 0x10

Double Free Protection

• Fast bins don't check for double frees
• This can be exploited to have the same chunk appear twice in the list
• Useful for creating overlapping chunks

Common CTF Patterns

Pattern 1: Infoleak + Fast Bin

1. Use unsorted bin attack to leak libc address
2. Calculate target addresses
3. Perform fast bin attack to overwrite GOT/hook
4. Execute shellcode or one-gadget

Pattern 2: UAF + Fast Bin

1. Trigger UAF to get write primitive
2. Modify freed chunk's next pointer
3. Allocate to get arbitrary write
4. Overwrite function pointer or GOT

Pattern 3: Overflow + Fast Bin

1. Use overflow to create fake prev_size
2. Consolidate chunks to leak addresses
3. Use leaked addresses for fast bin attack
4. Overwrite malloc hook or GOT

Debugging Tips

Check Fast Bin State

# In GDB with pwndbg/gef
heap_analysis
fastbins

Verify Chunk Contents

# After freeing, check what's in the chunk
x/10gx ptr0
x/10gx ptr1

Confirm Attack Success

# After allocation, verify you got the target address
p ptr3
# Should equal your target address

Safety Notes

• Only use this skill for authorized security research and CTF challenges
• Fast bin attacks are specific to glibc's malloc implementation
• Modern systems may have additional protections (ASLR, PIE, etc.)
• Always verify your target environment before attempting exploitation

References

• Guy In A Tuxedo - Fastbin Attack
• 0CTF Babyheap Walkthrough
• CSAW 17 Auir Walkthrough
• BlackHat MEA Robot Factory