Double Free Exploitation

Double-Free Heap Exploitation

A skill for understanding and exploiting double-free vulnerabilities in C programs.

What is a Double-Free?

A double-free occurs when the same memory block is freed more than once. This corrupts the heap allocator's internal data structures and can lead to:

• Memory corruption: Allocator metadata gets corrupted
• Overlapping pointers: Two pointers reference the same memory location
• Arbitrary writes: Attacker can control memory through one pointer affecting another
• Code execution: Can lead to ROP chains or shellcode execution

How It Works

The Mechanism

1. First free: Memory block goes into a free list (fast bin or tcache)
2. Second free: If another chunk is freed in between, the double-free check is bypassed
3. Result: The same block appears twice in the free list
4. Exploitation: When reallocating, you get overlapping pointers

Fast Bin Dup (Classic Double-Free)

// Fill tcache with 7 freed chunks
free(a); free(b); free(c); free(d); free(e); free(f); free(g);

// Double-free with intervening free
free(h);  // First free of h
free(i);  // Intervening free (bypasses double-free check)
free(h);  // Second free of h - now h is in fast bin twice!

// Reallocate - i1 and i2 get the SAME address
char *i1 = malloc(10);  // Gets h's address
char *i2 = malloc(10);  // Also gets h's address (duplicate!)

Tcache Poisoning Variant

When tcache is involved, you can use null-byte overflows to corrupt size fields:

1. Allocate chunks A, B, C of size 0x110
2. Free B, then free A
3. Reallocate A and overflow with null byte
4. B's size becomes 0x100 instead of 0x111
5. Free B again - now you have two tcache entries pointing to same address

Identifying Double-Free Vulnerabilities

Code Patterns to Look For

// Pattern 1: Direct double-free
void *ptr = malloc(100);
free(ptr);
// ... some code ...
free(ptr);  // VULNERABLE

// Pattern 2: Conditional double-free
if (condition) {
    free(ptr);
}
// ... code that might free ptr again ...
free(ptr);  // VULNERABLE if condition was true

// Pattern 3: Function call double-free
void process(void *data) {
    // ... uses data ...
    free(data);  // Frees it
}
// Caller also frees data - VULNERABLE

Debugging with GDB/Pwndbg

# Set breakpoints on free
break free

# Watch for suspicious patterns
watch *(void **)ptr

# Use pwndbg heap analysis
heap
heap chunks
vmmap

Exploitation Techniques

Technique 1: Fast Bin Dup for Address Disclosure

When you can't directly overwrite __malloc_hook:

1. Fill tcache with 7 freed chunks of the same size
2. Double-free a chunk (with intervening free)
3. Reallocate to get overlapping pointers
4. Write PIE address through one pointer
5. Read through other pointer to leak address
6. Calculate base address and proceed with ROP

Technique 2: Fast Bin Dup for Code Execution

1. Target main_arena (contains PIE addresses, near __malloc_hook)
2. Allocate chunk at specific offset of main_arena
3. Continue allocating until reaching __malloc_hook
4. Overwrite __malloc_hook with shellcode address
5. Trigger malloc to execute shellcode

Technique 3: Tcache Poisoning

1. Create double-free in tcache (not fast bin)
2. Use overlapping pointers to corrupt tcache fd pointer
3. Redirect malloc to arbitrary address
4. Overwrite GOT entry or function pointer

Practical Example

Vulnerable Code Analysis

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *a = malloc(10);
    char *b = malloc(10);
    char *c = malloc(10);
    
    free(a);
    free(b);
    free(a);  // DOUBLE FREE - a is now in fast bin twice!
    
    char *a1 = malloc(10);  // Gets a's address
    char *a2 = malloc(10);  // Also gets a's address!
    
    // Now a1 and a2 point to same memory
    strcpy(a1, "controlled");  // Also modifies a2!
    
    return 0;
}

Exploitation Steps

1. Identify the double-free: Look for free(ptr) called twice on same pointer
2. Verify with debugger: Check if addresses overlap after reallocation
3. Plan the attack: Decide what you want to overwrite (GOT, function pointer, etc.)
4. Craft the payload: Write through one pointer to affect the other
5. Trigger the exploit: Call the function or trigger the code path

Common CTF Patterns

Dragon Army (HTB)

• Can only allocate fast-bin-sized chunks except 0x70
• Can't directly overwrite __malloc_hook
• Use PIE addresses (start with 0x56) as target
• Target main_arena offset to reach __malloc_hook

Zero to Hero (PicoCTF)

• Uses tcache bins with null-byte overflow
• Corrupts size field from 0x111 to 0x100
• Creates double-free in tcache
• Leverages tcache poisoning for exploitation

Prevention and Mitigation

For Developers

// Always NULL out pointers after freeing
void *ptr = malloc(100);
free(ptr);
ptr = NULL;  // Prevents double-free

// Use smart pointers in C++
std::unique_ptr<char[]> ptr(new char[100]);
// Automatically freed, can't double-free

For Security Testing

1. Static analysis: Use tools like AddressSanitizer
2. Dynamic analysis: Run with ASan to detect double-frees
3. Code review: Look for all free() calls and trace pointer ownership

References

• Heap Exploitation - Double Free
• Dragon Army HTB Writeup
• Zero to Hero PicoCTF Writeup

Quick Reference