ctf-pwn skill

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):

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:

uv venv && uv pip install "pwntools==4.15.0" "ROPgadget==7.7" "ropper==1.13.13"

Linux (apt):

apt install gdb binutils strace ltrace qemu-system-x86

Optional cross-arch (ARM/MIPS — Debian/Ubuntu):

apt install qemu-user qemu-user-static gdb-multiarch binutils-multiarch libc6-armhf-cross libc6-arm64-cross libc6-mips-cross

macOS (Homebrew):

brew install gdb binutils qemu

Ruby gems (all platforms):

gem install one_gadget seccomp-tools

Manual install:

• pwndbg — Linux: GitHub, macOS: brew install pwndbg/tap/pwndbg-gdb
• checksec — included with pwntools (checksec --file=binary)

Additional Resources

• 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 - 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 - 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 exploitation (leaks, GOT overwrite, blind pwn, filter bypass, canary leak, freehook, .rela.plt patching, saved EBP overwrite for .bss pivot, argv[0] overwrite for stack smash info leak, .finiarray 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 - Seccomp advanced techniques, UAF, JIT, esoteric GOT, heap overlap via base conversion, tree data structure stack underallocation, ret2dlresolve, kernel exploitation (basic)
• 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 - 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 IOFILE vtable stdout hijack, tcache-to-fastbin promotion cross-bin attack, 6-bit index OOB + writtenbytes accumulator, ISMMAPED 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 - FILE-structure (IOFILE) exploitation: fastbin stdout vtable two-stage hijack for PIE + Full RELRO, IObufbase null-byte stdin hijack, glibc 2.24+ IOFILE vtable validation bypass, unsorted-bin attack on stdin IObufend, unsorted-bin corruption via mp_ structure, realloc(ptr, 0) as free() UAF, single-byte reference counter wraparound UAF
• 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 exploit techniques (part 2): bytecode validator bypass via self-modification, iouring 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 calltlsdtors, custom shadow stack pointer overflow bypass, signed int overflow negative OOB heap write, XSS-to-binary pwn bridge
• 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 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 OCLOEXEC, 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

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

# 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 -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

Quick decision tree:

• Partial RELRO + No PIE -> GOT overwrite (easiest, use fixed addresses)
• Full RELRO -> target freehook, mallochook (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
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 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.

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 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.

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 and 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 and 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 for full two-stage ret2libc pattern, leak parsing, and return target selection.

DynELF libc discovery: pwntools.DynELF(leakfunc, pointerin_libc) resolves libc symbols remotely without knowing the libc version. See rop-and-shellcode.md.

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.

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

ret2csu: libccsu_init gadgets control rdx, rsi, edi and call any GOT function — universal 3-argument call without libc gadgets. See rop-and-shellcode.md.

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.

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.

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.

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.

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.

stubexecveat as execve alternative: When no pop rax; ret exists, use stubexecveat (syscall 322/0x142) instead of execve -- send exactly 0x142 bytes so read() return value sets rax. See rop-and-shellcode.md.

Shell interaction: After execve, sleep(1) then sendline(b'cat /flag*'). See 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.