Format String Exploitation

Format String Exploitation

A skill for exploiting format string vulnerabilities in C binaries using pwntools.

When to Use This Skill

Use this skill when:

• You're given a binary with a format string vulnerability (e.g., printf(user_input) without format specifiers)
• You need to leak stack memory or overwrite GOT entries
• You want to redirect function calls (printf → system) for shellcode execution
• You're working on CTF challenges involving format string bugs

Quick Start

1. Configure the connection in scripts/format_string_exploit.py:

   • Set LOCAL = True for local testing
   • Set REMOTETTCP = True for remote TCP
   • Set REMOTESSH = True for SSH (e.g., OverTheWire)

2. Run the exploit:

   python scripts/format_string_exploit.py
   

3. Review the output - the script will:

   • Automatically find the format string offset
   • Overwrite GOT entries
   • Spawn an interactive shell

How Format String Exploitation Works

The Vulnerability

Format string vulnerabilities occur when user input is passed directly to format functions like printf(), fprintf(), or sprintf() without a format string argument:

// VULNERABLE
char *input = get_user_input();
printf(input);  // User can use %x, %s, %n to read/write memory

// SAFE
printf("%s", input);  // Format string is controlled

Attack Phases

1. Find the offset: Determine which position in the format string corresponds to your input
2. Leak memory: Use %x or %p to read stack values
3. Overwrite GOT: Use %n to write addresses to memory
4. Redirect execution: Point printf GOT entry to system PLT
5. Trigger shell: Find where printf is called with controlled input

Configuration Options

Example Usage

Local Binary

LOCAL = True
LOCAL_BIN = "./vulnerable_binary"

Remote TCP

REMOTETTCP = True
# Update in connect_binary():
P = remote('10.10.10.10', 1338)

SSH (OverTheWire)

REMOTESSH = True
REMOTE_BIN = "./tyler"
# SSH credentials configured in connect_binary()

Advanced: Loop Back for Extra Execution

Some binaries require looping back to the vulnerability for a second exploitation. Uncomment in the script:

P_FINI_ARRAY = ELF_LOADED.symbols["__init_array_end"]
INIT_LOOP_ADDR = 0x8048614  # Address to return to
format_string.write(P_FINI_ARRAY, INIT_LOOP_ADDR)

Troubleshooting

Offset not found

• Increase the search range in get_formatstring_config()
• Check if the binary has ASLR enabled (setarch -R)
• Verify the binary actually has a format string vulnerability

GOT overwrite fails

• Check if the binary is PIE (Position Independent Executable)
• Verify GOT entry is writable
• Ensure you're writing the correct address

Shell doesn't spawn

• Find where printf is called with user-controlled input
• The input should contain /bin/sh or similar
• Check if the binary has a second call to printf after GOT overwrite

References

• HackTricks Format Strings
• pwntools FmtStr
• Format String Attacks - Phrack 59

Script Files

• scripts/format_string_exploit.py - Main exploitation script

Run the script after configuring your connection settings. The script will automatically:

1. Find the format string offset
2. Overwrite printf GOT with system PLT
3. Spawn an interactive shell