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.

Additional Resources

• overflow-basics.md - Stack/global buffer overflow, ret2win, canary bypass, struct pointer overwrite, signed integer bypass, hidden gadgets
• rop-and-shellcode.md - ROP chains (ret2libc, syscall ROP), shellcode with input reversal, seccomp bypass, .fini_array hijack, pwntools template
• format-string.md - Format string exploitation (leaks, GOT overwrite, blind pwn, filter bypass, canary leak, __free_hook, .rela.plt patching)
• advanced.md - Heap, UAF, JIT, esoteric GOT, custom allocators, DNS overflow, MD5 preimage, ASAN, rdx control, canary-aware overflow, CSV injection, path traversal, kernel
• sandbox-escape.md - Python sandbox escape, custom VM exploitation, FUSE/CUSE devices, busybox/restricted shell, shell tricks

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 __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

ret2win with magic value: Overflow -> ret (alignment) -> pop rdi; ret -> magic -> win(). See overflow-basics.md for full exploit code.

Stack alignment: Modern glibc needs 16-byte alignment; SIGSEGV in movaps = add extra ret gadget. See overflow-basics.md.

Offset calculation: Buffer at rbp - N, return at rbp + 8, total = N + 8. See overflow-basics.md.

Input filtering: memmem() checks block certain byte sequences; assert payload doesn't contain banned strings. See overflow-basics.md.

Finding gadgets: ROPgadget --binary binary | grep "pop rdi", or use pwntools ROP() which also finds hidden gadgets in CMP immediates. 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.

Raw syscall ROP: When system()/execve() crash (CET/IBT), use pop rax; ret + syscall; ret from libc. 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.

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

Use-After-Free (UAF) Exploitation

Pattern: Menu create/delete/view where free() doesn't NULL pointer. Create -> leak -> free -> allocate same-size object to overwrite function pointer -> trigger callback. Key: both structs must be same size for tcache reuse. See advanced.md for full exploit code.

Seccomp Bypass

Alternative syscalls when seccomp blocks open()/read(): openat() (257), openat2() (437, often missed!), sendfile() (40), readv()/writev().

Check rules: seccomp-tools dump ./binary

See rop-and-shellcode.md for quick reference and advanced.md for conditional buffer address restrictions, shellcode without relocations, scmp_arg_cmp struct layout.

Stack Shellcode with Input Reversal

Pattern: Binary reverses input buffer. Pre-reverse shellcode, use partial 6-byte RIP overwrite, trampoline jmp short to NOP sled. See rop-and-shellcode.md.

.fini_array Hijack

Writable .fini_array + arbitrary write -> overwrite with win/shellcode address. Works even with Full RELRO. See rop-and-shellcode.md for implementation.

Path Traversal Sanitizer Bypass

Pattern: Sanitizer skips char after banned char match; double chars to bypass (e.g., ....//....//etc//passwd). Also try /proc/self/fd/3 if binary has flag fd open. See advanced.md.

Kernel Exploitation

OOB via vulnerable lseek, heap grooming with fork(), SUID exploits. Check CONFIG_SLAB_FREELIST_RANDOM and CONFIG_SLAB_MERGE_DEFAULT. See advanced.md.

Format String Quick Reference

• Leak stack: %p.%p.%p.%p.%p.%p | Leak specific: %7$p
• Write: %n (4-byte), %hn (2-byte), %hhn (1-byte), %lln (8-byte full 64-bit)
• GOT overwrite for code execution (Partial RELRO required)

See format-string.md for GOT overwrite patterns, blind pwn, filter bypass, canary+PIE leak, __free_hook overwrite, and argument retargeting.

.rela.plt / .dynsym Patching (Format String)

When to use: GOT addresses contain bad bytes (e.g., 0x0a). Patch .rela.plt symbol index + .dynsym st_value to redirect function resolution to win(). Bypasses all GOT byte restrictions. See format-string.md for full technique and code.

Heap Exploitation

• tcache poisoning (glibc 2.26+), fastbin dup / double free
• House of Force (old glibc), unsorted bin attack
• House of Apple 2 (glibc 2.34+): FSOP via _IO_wfile_jumps when __free_hook/__malloc_hook removed. Fake FILE with _flags = " sh", vtable chain → system(fp).
• House of Einherjar: Off-by-one null clears PREV_INUSE, backward consolidation with self-pointing unlink.
• Safe-linking (glibc 2.32+): tcache fd mangled as ptr ^ (chunk_addr >> 12).
• Check glibc version: strings libc.so.6 | grep GLIBC
• Freed chunks contain libc pointers (fd/bk) -> leak via error messages or missing null-termination
• Heap feng shui: control alloc order/sizes, create holes, place targets adjacent to overflow source

See advanced.md for House of Apple 2 FSOP chain, custom allocator exploitation (nginx pools), heap overlap via base conversion, tree data structure stack underallocation.

JIT Compilation Exploits

Pattern: Off-by-one in instruction encoding -> misaligned machine code. Embed shellcode as operand bytes of subtraction operations, chain with 2-byte jmp instructions. See advanced.md.

BF JIT unbalanced bracket: Unbalanced ] pops tape address (RWX) from stack → write shellcode to tape with +/-, trigger ] to jump to it. See advanced.md.

Type Confusion in Interpreters

Pattern: Interpreter sets wrong type tag → struct fields reinterpreted. Unused padding bytes in one variant become active pointers/data in another. Flag bytes as type value trigger UNKNOWN_DATA dump. See advanced.md.

Off-by-One Index / Size Corruption

Pattern: Array index 0 maps to entries[-1], overlapping struct metadata (size field). Corrupted size → OOB read leaks canary/libc, then OOB write places ROP chain. See advanced.md.

Double win() Call

Pattern: win() checks if (attempts++ > 0) — needs two calls. Stack two return addresses: p64(win) + p64(win). See advanced.md.

Esoteric Language GOT Overwrite

Pattern: Brainfuck/Pikalang interpreter with unbounded tape = arbitrary read/write relative to buffer base. Move pointer to GOT, overwrite byte-by-byte with system(). See advanced.md.

DNS Record Buffer Overflow

Pattern: Many AAAA records overflow stack buffer in DNS response parser. Set up DNS server with excessive records, overwrite return address. See advanced.md.

ASAN Shadow Memory Exploitation

Pattern: Binary with AddressSanitizer has format string + OOB write. ASAN may use "fake stack" (50% chance). Leak PIE, detect real vs fake stack, calculate OOB write offset to overwrite return address. See advanced.md.

Format String with RWX .fini_array Hijack

Pattern (Encodinator): Base85-encoded input in RWX memory passed to printf(). Write shellcode to RWX region, overwrite .fini_array[0] via format string %hn writes. Use convergence loop for base85 argument numbering. See advanced.md.

Custom Canary Preservation

Pattern: Buffer overflow must preserve known canary value. Write exact canary bytes at correct offset: b'A' * 64 + b'BIRD' + b'X'. See advanced.md.

MD5 Preimage Gadget Construction

Pattern (Hashchain): Brute-force MD5 preimages with eb 0c prefix (jmp +12) to skip middle bytes; bytes 14-15 become 2-byte i386 instructions. Build syscall chains from gadgets like 31c0 (xor eax), cd80 (int 0x80). See advanced.md for C code and v2 technique.

Python Sandbox Escape

AST bypass via f-strings, audit hook bypass with b'flag.txt' (bytes vs str), MRO-based __builtins__ recovery. See sandbox-escape.md.

VM Exploitation (Custom Bytecode)

Pattern: Custom VM with OOB read/write in syscalls. Leak PIE via XOR-encoded function pointer, overflow to rewrite pointer with win() ^ KEY. See sandbox-escape.md.

FUSE/CUSE Character Device Exploitation

Look for cuse_lowlevel_main() / fuse_main(), backdoor write handlers with command parsing. Exploit to chmod /etc/passwd then modify for root access. See sandbox-escape.md.

Busybox/Restricted Shell Escalation

Find writable paths via character devices, target /etc/passwd or /etc/sudoers, modify permissions then content. See sandbox-escape.md.

Shell Tricks

exec<&3;sh>&3 for fd redirection, $0 instead of sh, ls -la /proc/self/fd to find correct fd. See sandbox-escape.md.

Useful Commands

checksec, one_gadget, ropper, ROPgadget, seccomp-tools dump, strings libc | grep GLIBC. See rop-and-shellcode.md for full command list and pwntools template.