Python - Race Condition File-Write

Running the app on Docker

$ sudo docker pull blabla1337/owasp-skf-lab:racecondition-file-write

$ sudo docker run -ti -p 127.0.0.1:5000:5000 blabla1337/owasp-skf-lab:racecondition-file-write

Now that the app is running let's go hacking!

Reconnaissance

Step1

We can download a file from the server by doing a GET request to the server.

Let's try:

Once we download the file we can see whatever we add to the URL is being written in a file called shared-file.

Step 2

As the application suggests, there is a Race condition vulnerability in this app, let's try to find it.

If we look at the code we see that the application gets the query parameter, writes to a file called shared-file.txt, then opens the file and send it back as a response.

@app.route("/<string:value>", methods=['GET'])
def home(value):
    # Create a Python file object using open() and the with statement
    with open("shared-file.txt", 'w') as f:
        f.write(value)
        f.closed
        f.closed
    file = open("shared-file.txt", "r")
    response = make_response(send_file("shared-file.txt", attachment_filename="shared-file.txt"))
    response.headers.set("Content-Type", "text/html; charset=utf-8")
    response.headers.set("Content-Disposition", "attachment; filename=shared-file.txt")
    return response

Step 3

How can we exploit this?

We have a very small window between the writing of the file:

with open("shared-file.txt", 'w') as f:
        f.write(value)
        f.closed
        f.closed

and the response:

 file = open("shared-file.txt", "r")
    response = make_response(send_file("shared-file.txt", attachment_filename="shared-file.txt"))
    response.headers.set("Content-Type", "text/html; charset=utf-8")
    response.headers.set("Content-Disposition", "attachment; filename=shared-file.txt")
    return response

Maybe if we have multiple users on this application at the same time we might be able to intercept someone else's query.

Exploitation

In order to do that we must send requests with high frequency.

Doing it manually is practically impossible, so we create a script that does that for us:

#!/bin/bash

while true; do

	curl -i -s -k  -X $'GET' -H $'Host: localhost:5000' $'http://localhost:5000/111' | grep "111"

done

and in the meantime we will send a couple requests from Burp:

If we look in the logs we will see:

Additional sources

https://wiki.owasp.org/index.php/Testing_for_Race_Conditions_(OWASP-AT-010)