ctf-pwn — Claude Code skill: what it does, how to install

---
name: ctf-pwn
description: Provides binary exploitation techniques for CTF challenges. Use when you already have a vulnerable native target or service and need to turn memory corruption or low-level primitives into code execution or privilege escalation, such as buffer overflows, format strings, heap bugs, ROP, ret2libc, shellcode, kernel exploitation, seccomp bypass, sandbox escape, or Windows/Linux exploit chains. Do not use it when the main blocker is understanding what the binary does; use reverse engineering first. Do not use it for pure web bugs, disk or packet forensics, or standalone crypto/math challenges.
license: MIT
compatibility: Requires filesystem-based agent (Claude Code or similar) with bash, Python 3, and internet access for tool installation.
allowed-tools: Bash Read Write Edit Glob Grep Task WebFetch WebSearch
metadata:
  user-invocable: "false"
---

# CTF Binary Exploitation (Pwn)

Quick reference for binary exploitation (pwn) CTF challenges. Each technique has a one-liner here; see supporting files for full details.

## Prerequisites

**Python packages (all platforms):**
```bash
pip install "pwntools==4.15.0" "ROPgadget==7.7" "ropper==1.13.13"
# or via script: bash scripts/install_ctf_tools.sh python
```

**uv alternative:**
```bash
uv venv && uv pip install "pwntools==4.15.0" "ROPgadget==7.7" "ropper==1.13.13"
```

**Linux (apt):**
```bash
apt install gdb binutils strace ltrace qemu-system-x86
```

**Optional cross-arch (ARM/MIPS — Debian/Ubuntu):**
```bash
apt install qemu-user qemu-user-static gdb-multiarch binutils-multiarch libc6-armhf-cross libc6-arm64-cross libc6-mips-cross
```

**macOS (Homebrew):**
```bash
brew install gdb binutils qemu
```

**Ruby gems (all platforms):**
```bash
gem install one_gadget seccomp-tools
```

**Manual install:**
- pwndbg — Linux: [GitHub](https://github.com/pwndbg/pwndbg), macOS: `brew install pwndbg/tap/pwndbg-gdb`
- checksec — included with pwntools (`checksec --file=binary`)

**MCP servers (optional):** WinDbg MCP ([svnscha/mcp-windbg](https://github.com/svnscha/mcp-windbg)) for Windows crash dumps, live breakpoint-driven exploitation, and kernel/driver debugging; IDA Pro MCP / Ghidra MCP for decompiling the vulnerable function — see [tools-mcp.md](tools-mcp.md).

## Additional Resources

- [overflow-basics.md](overflow-basics.md) - Stack/global buffer overflow, ret2win, canary bypass, canary byte-by-byte brute force on forking servers, struct pointer overwrite, signed integer bypass, hidden gadgets, stride-based OOB read leak, parser stack overflow via unchecked memcpy length with callee-saved register restoration
- [rop-and-shellcode.md](rop-and-shellcode.md) - Core ROP chains (ret2libc, syscall ROP, rdx control, shell interaction), ret2csu, bad character XOR bypass, exotic x86 gadgets (BEXTR/XLAT/STOSB/PEXT), stack pivot via xchg rax,esp, sprintf() gadget chaining for bad character bypass, canary XOR epilogue as RDX zeroing gadget, stub_execveat syscall as execve alternative via read() return value
- [rop-advanced.md](rop-advanced.md) - Advanced ROP techniques: double stack pivot to BSS via leave;ret, SROP (Sigreturn-Oriented Programming) with UTF-8 constraints, seccomp bypass, RETF architecture switch (x64→x32) for seccomp bypass, shellcode with input reversal, .fini_array hijack, ret2vdso, pwntools template, x32 ABI syscall aliasing for seccomp bypass, time-based blind shellcode exfiltration
- [format-string.md](format-string.md) - Format string exploitation (leaks, GOT overwrite, blind pwn, filter bypass, canary leak, __free_hook, .rela.plt patching, saved EBP overwrite for .bss pivot, argv[0] overwrite for stack smash info leak, .fini_array loop for multi-stage exploitation, __printf_chk bypass with sequential %p, single-call leak + GOT overwrite, ROT13-encoded format string exploit through input transformation)
- [advanced.md](advanced.md) - Seccomp advanced techniques, UAF, JIT, esoteric GOT, heap overlap via base conversion, tree data structure stack underallocation, ret2dlresolve, kernel exploitation (basic)
- [heap-techniques.md](heap-techniques.md) - House of Apple 2 (+ setcontext SUID variant), House of Einherjar, House of Orange/Spirit/Lore/Force, heap grooming, custom allocators (nginx, talloc), classic unlink, musl libc heap (meta pointer + atexit hijack), tcache stashing unlink attack, unsafe unlink + top chunk consolidation
- [heap-techniques-2.md](heap-techniques-2.md) - CTF-writeup heap variants: UAF vtable pointer encoding shell argument, uninitialized chunk residue pointer leak, tcache strcpy null-byte overflow + backward consolidation, adjacent-struct fn-pointer overflow for libc leak + GOT overwrite, hidden-menu tcache poisoning, tcache double-free + fake _IO_FILE vtable stdout hijack, tcache-to-fastbin promotion cross-bin attack, 6-bit index OOB + written_bytes accumulator, IS_MMAPED bit-flip for unsorted bin leak on calloc'd chunk, filename-regex-constrained fastbin via LSB-only heap pointer overwrite, custom allocator unsafe unlink to GOT
- [heap-fsop.md](heap-fsop.md) - FILE-structure (_IO_FILE) exploitation: fastbin stdout vtable two-stage hijack for PIE + Full RELRO, _IO_buf_base null-byte stdin hijack, glibc 2.24+ _IO_FILE vtable validation bypass, unsorted-bin attack on stdin _IO_buf_end, unsorted-bin corruption via mp_ structure, realloc(ptr, 0) as free() UAF, single-byte reference counter wraparound UAF
- [advanced-exploits.md](advanced-exploits.md) - Advanced exploit techniques (part 1): VM signed comparison, BF JIT shellcode, type confusion, off-by-one index corruption, DNS overflow, ASAN shadow memory, format string with encoding constraints, custom canary preservation, signed integer bypass, canary-aware partial overflow, CSV injection, MD5 preimage gadgets, VM GC UAF slab reuse, path traversal sanitizer bypass, FSOP + seccomp bypass via openat/mmap/write
- [advanced-exploits-2.md](advanced-exploits-2.md) - Advanced exploit techniques (part 2): bytecode validator bypass via self-modification, io_uring UAF with SQE injection, integer truncation int32->int16, GC null-reference cascading corruption, leakless libc via multi-fgets stdout FILE overwrite, signed/unsigned char underflow heap overflow, XOR keystream brute-force write primitive, tcache pointer decryption heap leak, unsorted bin promotion via forged chunk size, FSOP stdout TLS leak, TLS destructor hijack via `__call_tls_dtors`, custom shadow stack pointer overflow bypass, signed int overflow negative OOB heap write, XSS-to-binary pwn bridge
- [advanced-exploits-4.md](advanced-exploits-4.md) - Advanced exploit techniques (part 4): Windows SEH overwrite + pushad VirtualAlloc ROP, IAT-relative resolution, detached process shell stability, SeDebugPrivilege SYSTEM escalation, ARM buffer overflow with Thumb shellcode, Forth interpreter system word exploitation, GF(2) Gaussian elimination for multi-pass tcache poisoning, single-bit-flip exploitation primitive (mprotect + iterative code patching), Game of Life shellcode evolution via still-lifes, UAF via menu-driven strdup/free ordering, Windows CFG bypass via system() as valid call target, neural network output as function pointer index OOB, shellcode unique-byte limit bypass via counter overflow
- [advanced-exploits-3.md](advanced-exploits-3.md) - Advanced exploit techniques (part 3): stack variable overlap / carry corruption OOB, 1-byte overflow via 8-bit loop counter, game AI arithmetic mean OOB read, arbitrary read/write GOT overwrite to shell, stack leak via __environ + memcpy overflow, JIT sandbox escape via uint16 jump truncation, DNS compression pointer stack overflow with multi-question ROP, ELF code signing bypass via program header manipulation, game level format signed/unsigned coordinate mismatch, file descriptor inheritance via missing O_CLOEXEC, sign extension integer underflow in metadata parsing, ROP chain construction with read-only primitive, 4-byte shellcode with timing side-channel via persistent registers, CRC oracle as arbitrary read, UTF-8 case conversion buffer overflow
- [advanced-exploits-5.md](advanced-exploits-5.md) - Advanced exploit techniques (part 5): data-interpretation exploitation — Chip-8 emulator OOB memory for ret2libc, double-precision float quicksort canary repositioning, bloom filter abs(INT_MIN) negative index OOB write
- [sandbox-escape.md](sandbox-escape.md) - Custom VM exploitation, FUSE/CUSE devices, busybox/restricted shell, shell tricks, process_vm_readv sandbox bypass, named pipe file size bypass, CPU emulator print opcode Python eval injection (cross-references ctf-misc/pyjails.md for Python jail techniques)
- [kernel.md](kernel.md) - Linux kernel exploitation fundamentals: environment setup, QEMU debug, heap spray structures (tty_struct, poll_list, user_key_payload, seq_operations), kernel stack overflow, canary leak, privilege escalation (ret2usr, kernel ROP), modprobe_path overwrite, core_pattern overwrite, kmalloc size mismatch heap overflow + struct file f_op corruption
- [kernel-techniques.md](kernel-techniques.md) - Kernel exploitation techniques: tty_struct kROP (fake vtable + stack pivot), AAW via ioctl register control, userfaultfd race stabilization, SLUB allocator internals (freelist hardening/obfuscation), leak via kernel panic, MADV_DONTNEED race window extension (DiceCTF 2026), cross-cache CPU-split attack (DiceCTF 2026), PTE overlap file write (DiceCTF 2026), addr_limit bypass via failed file open for kernel memory read/write
- [kernel-bypass.md](kernel-bypass.md) - Kernel protection bypass: KASLR/FGKASLR bypass (__ksymtab), KPTI bypass (swapgs trampoline, signal handler, modprobe_path/core_pattern via ROP), SMEP/SMAP bypass, GDB kernel module debugging, initramfs/virtio-9p workflow, exploit templates, exploit delivery
- [field-notes.md](field-notes.md) - Detailed pwn notes: heap exploitation quick reference, additional exploit notes, useful commands
- [attack-playbook.md](attack-playbook.md) - Linux userland attack playbook: recon of input/privilege boundaries, leak selection by mitigation, and primitive ranking workflow
- [static-signals.md](static-signals.md) - Exploit-relevant static signals (libc version, seccomp policy, RELRO/PIE/canary/NX evidence) for exploitability assessment
- [pwnable-tw/README.md](pwnable-tw/README.md) - 43 pwnable.tw writeups (orw, start, calc, hacknote, babystack, tcache_tear, death_note, dubblesort, applestore, bookwriter, and more) with per-challenge environment, protections, root-cause defects, and concrete exploitation chains
- [tools-mcp.md](tools-mcp.md) - MCP server integration for pwn: WinDbg MCP session model (`open_cdb_dump`/`open_cdb_remote`/`open_kd_session` + `session_id`), crash-dump triage, in-dump `flag{` memory scanning, live breakpoint/hardware-watchpoint/memory-patch exploitation, `!dh -f` Windows mitigation check (the `checksec` equivalent), kernel-mode token stealing with kd; IDA/Ghidra MCP for exploit development (decompile the vulnerable function first, drop to assembly only for exact gadget bytes)

---

## When to Pivot

- If you do not yet understand what the binary does, switch to `/ctf-reverse` before trying to exploit it.
- If the service is really a restricted shell, encoding puzzle, or sandbox language challenge, switch to `/ctf-misc`.
- If the exploit path depends on a web endpoint, session bug, or upload primitive more than memory corruption, switch to `/ctf-web`.
- If the vulnerability requires breaking a cryptographic primitive before exploitation, switch to `/ctf-crypto`.

## Quick Start Commands

```bash
# Binary analysis
checksec --file=binary
file binary
readelf -h binary

# Find gadgets
ROPgadget --binary binary | grep "pop rdi"
ropper -f binary --search "pop rdi"
one_gadget /lib/x86_64-linux-gnu/libc.so.6

# Debug
gdb -q binary -ex 'start' -ex 'checksec'

# Pattern for offset finding
python3 -c "from pwn import *; print(cyclic(200))"
python3 -c "from pwn import *; print(cyclic_find(0x61616168))"

# libc identification
./libc-database/find puts <leaked_addr_last_3_nibbles>
```

## Source Code Red Flags

- Threading/`pthread` -> race conditions
- `usleep()`/`sleep()` -> timing windows
- Global variables in multiple threads -> TOCTOU

## Race Condition Exploitation

```bash
bash -c '{ echo "cmd1"; echo "cmd2"; sleep 1; } | nc host port'
```

## Common Vulnerabilities

- Buffer overflow: `gets()`, `scanf("%s")`, `strcpy()`
- Format string: `printf(user_input)`
- Integer overflow, UAF, race conditions

## Protection Implications for Exploit Strategy

| Protection | Status | Implication |
|-----------|--------|-------------|
| PIE | Disabled | All addresses (GOT, PLT, functions) are fixed - direct overwrites work |
| RELRO | Partial | GOT is writable - GOT overwrite attacks possible |
| RELRO | Full | GOT is read-only - need alternative targets (hooks, vtables, return addr) |
| NX | Enabled | Can't execute shellcode on stack/heap - use ROP or ret2win |
| Canary | Present | Stack smash detected - need leak or avoid stack overflow (use heap) |

**Quick decision tree:**
- Partial RELRO + No PIE -> GOT overwrite (easiest, use fixed addresses)
- Full RELRO -> target `__free_hook`, `__malloc_hook` (glibc < 2.34), or return addresses
- Stack canary present -> prefer heap-based attacks or leak canary first

## Stack Buffer Overflow

1. Find offset: `cyclic 200` then `cyclic -l <value>`
2. Check protections: `checksec --file=binary`
3. No PIE + No canary = direct ROP
4. Canary leak via format string or partial overwrite
5. Canary brute-force byte-by-byte on forking servers (7*256 attempts max)

**ret2win with magic value:** Overflow -> `ret` (alignment) -> `pop rdi; ret` -> magic -> win(). **Stack alignment:** SIGSEGV in `movaps` = add extra `ret` gadget. **Offset:** buffer at `rbp - N`, return at `rbp + 8`, total = N + 8. **Input filtering:** assert payload avoids `memmem()` banned strings. **Gadgets:** `ROPgadget --binary binary | grep "pop rdi"`, or pwntools `ROP()` for hidden gadgets in CMP immediates. See [overflow-basics.md](overflow-basics.md) for full exploit code.

## Parser Stack Overflow (Unchecked memcpy)

**Pattern:** Custom file parser (PCAP, image, archive) allocates fixed stack buffer but input records can exceed it. `memcpy` copies before length validation, overflowing saved registers and return address. Must restore callee-saved registers: `rbx` to readable memory (BSS), loop counters to exit values, then `ret` gadget + win function. See [overflow-basics.md](overflow-basics.md#parser-stack-overflow-via-unchecked-memcpy-length-metactf-flash-2026).

## Struct Pointer Overwrite (Heap Menu Challenges)

**Pattern:** Menu create/modify/delete on structs with data buffer + pointer. Overflow name into pointer field with GOT address, then write win address via modify. See [overflow-basics.md](overflow-basics.md) for full exploit and GOT target selection table.

## Signed Integer Bypass

**Pattern:** `scanf("%d")` without sign check; negative quantity * price = negative total, bypasses balance check. See [overflow-basics.md](overflow-basics.md).

## Canary-Aware Partial Overflow

**Pattern:** Overflow `valid` flag between buffer and canary. Use `./` as no-op path padding for precise length. See [overflow-basics.md](overflow-basics.md) and [advanced.md](advanced.md) for full exploit chain.

## Global Buffer Overflow (CSV Injection)

**Pattern:** Adjacent global variables; overflow via extra CSV delimiters changes filename pointer. See [overflow-basics.md](overflow-basics.md) and [advanced.md](advanced.md) for full exploit.

## ROP Chain Building

Leak libc via `puts@PLT(puts@GOT)`, return to vuln, stage 2 with `system("/bin/sh")`. See [rop-and-shellcode.md](rop-and-shellcode.md) for full two-stage ret2libc pattern, leak parsing, and return target selection.

**DynELF libc discovery:** `pwntools.DynELF(leak_func, pointer_in_libc)` resolves libc symbols remotely without knowing the libc version. See [rop-and-shellcode.md](rop-and-shellcode.md#dynelf-automated-libc-discovery-rc3-ctf-2016).

**Constrained shellcode in small buffers:** When buffer is too small, use `read()` shellcode stub (< 20 bytes) to pull full stage-2 shellcode. See [rop-and-shellcode.md](rop-and-shellcode.md#constrained-shellcode-in-small-buffers-tum-ctf-2016).

**Raw syscall ROP:** When `system()`/`execve()` crash (CET/IBT), use `pop rax; ret` + `syscall; ret` from libc. See [rop-and-shellcode.md](rop-and-shellcode.md).

**ret2csu:** `__libc_csu_init` gadgets control `rdx`, `rsi`, `edi` and call any GOT function — universal 3-argument call without libc gadgets. See [rop-and-shellcode.md](rop-and-shellcode.md#ret2csu--__libc_csu_init-gadgets-crypto-cat).

**Bad char XOR bypass:** XOR payload data with key before writing to `.data`, then XOR back in place with ROP gadgets. Avoids null bytes, newlines, and other filtered characters. See [rop-and-shellcode.md](rop-and-shellcode.md#bad-character-bypass-via-xor-encoding-in-rop-crypto-cat).

**Exotic gadgets (BEXTR/XLAT/STOSB/PEXT):** When standard `mov` write gadgets are unavailable, chain obscure x86 instructions for byte-by-byte memory writes. See [rop-and-shellcode.md](rop-and-shellcode.md#exotic-x86-gadgets--bextrxlatstosbpext-crypto-cat).

**Stack pivot (xchg rax,esp):** Swap stack pointer to attacker-controlled heap/buffer when overflow is too small for full ROP chain. Requires `pop rax; ret` to load pivot address first. See [rop-and-shellcode.md](rop-and-shellcode.md#stack-pivot-via-xchg-raxesp-crypto-cat).

**rdx control:** After `puts()`, rdx is clobbered to 1. Use `pop rdx; pop rbx; ret` from libc, or re-enter binary's read setup + stack pivot. See [rop-and-shellcode.md](rop-and-shellcode.md).

**Canary XOR epilogue as rdx zeroing gadget:** When no `pop rdx; ret` exists, jump into the canary check epilogue `xor rdx, fs:28h` -- it zeros RDX when the canary is intact. See [rop-and-shellcode.md](rop-and-shellcode.md#stack-canary-xor-epilogue-as-rdx-zeroing-gadget-volgactf-2017).

**stub_execveat as execve alternative:** When no `pop rax; ret` exists, use `stub_execveat` (syscall 322/0x142) instead of `execve` -- send exactly 0x142 bytes so `read()` return value sets rax. See [rop-and-shellcode.md](rop-and-shellcode.md#stub_execveat-syscall-as-execve-alternative-asis-ctf-2018).

**Shell interaction:** After `execve`, `sleep(1)` then `sendline(b'cat /flag*')`. See [rop-and-shellcode.md](rop-and-shellcode.md).

## Format String Through Input Transformation

**ROT13-encoded format string:** When input is ROT13/Caesar-transformed before reaching `printf`, pre-encode the format string payload with the inverse transform so it arrives intact. See [format-string.md](format-string.md#format-string-exploit-through-rot13-encoding-sunshinectf-2018).

## Kernel Exploitation

**addr_limit bypass via failed file open:** When a kernel module sets `addr_limit = KERNEL_DS` but fails to restore it on error paths, force the error (e.g., make target file a directory) to retain kernel memory access from userspace `read()`/`write()`. See [kernel-techniques.md](kernel-techniques.md#kernel-addr_limit-bypass-via-failed-file-open-midnight-sun-ctf-2018).

## Sandbox and Emulator Escape

**CPU emulator eval injection:** When an emulator's print opcode uses `eval('"' + buf + '"')` for escape sequences, build `"+__import__("os").system("cmd")#` in emulator memory via ADD opcodes to escape the string and execute Python. See [sandbox-escape.md](sandbox-escape.md#cpu-emulator-print-opcode-python-eval-injection-midnight-sun-ctf-2018).

## Advanced Exploit Primitives

**Neural network function pointer OOB:** When a binary uses NN output as an index into a function pointer array without bounds checking, retrain weights/biases to produce an out-of-bounds index that reads a target address from the biases array. See [advanced-exploits-4.md](advanced-exploits-4.md#neural-network-output-as-function-pointer-index-oob-swampctf-2018).

**Shellcode unique-byte limit bypass via counter overflow:** When shellcode is limited to N unique bytes, spray the stack to corrupt the `seen[256]` counter, then re-execute main (skipping `memset`) so the overflowed counter allows arbitrary bytes on the second run. See [advanced-exploits-4.md](advanced-exploits-4.md#shellcode-unique-byte-limit-bypass-via-counter-overflow-blaze-ctf-2018).

## Deep-Dive Notes

Use [field-notes.md](field-notes.md) once you have confirmed the challenge is truly exploitation-heavy.

- Heap and allocator notes: House of Apple, tcache, unsafe unlink, talloc, UAF, FSOP
- Advanced exploit notes: seccomp bypass, ret2vdso, io_uring, integer truncation, ASAN, timing oracles
- Sandbox and hybrid notes: pyjail crossover, busybox escapes, custom VMs, shell tricks, path sanitizers
- Kernel and Windows notes: kernel playbooks, SEH, CFG bypass, privilege escalation
- Historical case notes: older but still reusable CTF exploit patterns