While doing research on one of the open source web applications I came across a Cross-Site Scripting trick that seems dumb but is actually useful in some situations. In this post I will describe such situation that is interesting enough and not so obvious to exploit.
Vulnerable API
As I mentioned the culprit of this post is a web application (for the greater good I cannot share any details about it) which I found vulnerable in some of it’s API endpoints. Long story short, the endpoint was designed to retrieve local files from the host but when file was not found it returned error message. Unfortunatelly the Content-Type header of the response was set to text/html not text/plain. Add to this that the name of the missing file was returned in the error message and that the user can control the filename. All of this resulted in a pretty straightforward reflected XSS.
Normally I would stop any pentesting at this point but following the POC||GTFO rule I decided to make a more “exploitable” payload than the standard <img> one (couldn’t use standard <script> tags as the CSP was blocking inline scripts 
).
The / issue
Due to the properly configured CSP all the efforts to steal session cookies were unsuccessful so I decided to stick with redirecting user on other page and executing JavaScript code there. And here came the problem. As this was API it treated every slash / as a new path segment. So when I tried to redirect victim using stardard URL I ended up with this (the slashes in the protocol section of the URL messed the API path).
Now I needed to find another way to pass URL to the JS document.location variable. Passed string need to be without slashes and any encodings as the API was either decoding all my payloads or returning more errors upon submitting them.
Weird but useful trick
Than I thought of jsfuck. As we can read on it’s website:
JSFuck is an esoteric and educational programming style based on the atomic parts of JavaScript. It uses only six different characters to write and execute code.
Using JSFuck we can convert our desired JavaScript string to be represented by only six different characters and still be properly interpreted by the JS engine. Luckily slash is not one of these characters. We can use the JSFuck converter to represent our Burp Collaborator link using only brackets, exclamation marks and plus signs.
The final payload looked like this. Sure it’s not compact but at least its working 
.
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I ended up with payload that resulted in user redirection to my desired webpage where I could execute any JavaScript code I wanted or spoof the webpage to get credentials or other secrets via social engineering. Sometimes it is good to experiment with not so obvious techniques that once upon a time make themselves useful.
References:
- http://www.jsfuck.com/