Heap Freed Chunk Exploitation

This skill covers heap exploitation techniques that involve overwriting pointers inside freed chunks. These are fundamental to many heap-based attacks in binary exploitation.

Core Concept

Several heap exploitation techniques require the ability to overwrite pointers inside freed chunks. The goal is to understand what vulnerabilities grant this access and how to leverage them.

Vulnerability Types

1. Simple Use After Free (UAF)

What it is: The attacker can write data into a chunk that has already been freed.

How it works:

• A chunk is freed but the program still holds a reference to it
• Attacker writes malicious data to the freed chunk
• When the chunk is reallocated, the attacker controls its contents
• This allows overwriting critical pointers

When to use: Look for cases where freed memory is still accessible through dangling pointers.

2. Double Free

What it is: The same chunk is freed twice, potentially with other frees in between.

How it works:

1. Free the same chunk twice (possibly with other frees between them)
2. The chunk ends up in the same bin twice
3. Allocate the chunk later to get a reference
4. Write the needed pointers to it
5. Allocate it again to trigger the attack (fastbin attack, tcache attack, etc.)

When to use: When you can trigger multiple frees on the same allocation.

3. Heap Overflow

What it is: Overflowing an allocated chunk that has a freed chunk as its neighbor.

How it works:

• Allocate a chunk with a freed chunk immediately after it in memory
• Overflow the allocated chunk
• Modify headers/pointers of the freed chunk
• When the freed chunk is reallocated, your modified pointers take effect

When to use: When you have a buffer overflow adjacent to freed heap memory.

4. Off-by-One Overflow

What it is: Writing one byte past the end of a buffer, modifying the size field of the next chunk.

How it works:

1. Overflow by one byte to modify the size of the following chunk
2. Make an allocated chunk appear larger than it really is
3. Free the chunk - it gets added to a bin of the (fake) larger size
4. Allocate a chunk of the fake size
5. You now have access to a chunk larger than its real size
6. This creates an overlapping chunks situation
7. Exploit the overlap the same way as a heap overflow

When to use: When you can write exactly one byte past a buffer boundary.

Attack Patterns

Fastbin Attack

• Exploits the fastbin allocator's lack of validation
• Requires double free or UAF to control the fd pointer
• When a chunk is allocated from fastbin, the fd pointer becomes the next chunk's address

Tcache Attack

• Similar to fastbin but for tcache (glibc 2.26+)
• Tcache has a limit of 7 chunks per bin
• Can be exploited with UAF or double free

Practical Considerations

Memory Layout

• Understand the heap layout: chunks are allocated sequentially
• Each chunk has headers (size, flags) before the data
• The

  prev_size

  and

  size

  fields are critical for exploitation

Chunk Structure

[prev_size][size][data...][prev_size][size][data...]

Key Pointers to Overwrite

• fd

  (forward) pointer in bins

• bk

  (backward) pointer in bins

• prev_size

  in chunk headers

• size

  field to control chunk boundaries

Debugging Tips

1. Use GDB with heap debugging:

   • heap

     command to view heap state

   • vmmap

     to see memory regions

   • x/20wx $heap

     to examine chunk headers

2. Track allocations:

   • Note which chunks are freed
   • Monitor which bins chunks go to
   • Watch for double frees or UAF conditions

3. Check for:

   • Dangling pointers after free
   • Multiple frees of same address
   • Buffer boundaries and adjacent chunks

When This Skill Applies

Use this skill when:

• Analyzing heap vulnerabilities in CTF challenges
• Debugging memory corruption bugs
• Learning heap exploitation techniques
• Working with glibc malloc implementations
• Preparing for binary exploitation competitions

Related Techniques

• Heap Feng Shui (controlling heap layout)
• Unlink attacks (older glibc versions)
• House of X attacks (various heap exploitation patterns)
• Tcache poisoning
• Fastbin corruption

Safety Note

These techniques are for educational purposes and authorized security research only. Always ensure you have permission before testing exploitation techniques on systems you don't own.