This is my first and last Bug Bounty Writeup this year 😀
I am sharing with you my latest XSS finding, which I’ve found 2 weeks ago.
This was the fastest and a bit unusual flow that I normally do when I search for XSS.
So let’s dive in…
Company asked me to retest an old XSS report.
I’ve checked that XSS and confirmed that it was fixed properly.
The specific endpoint had
namea param that was vulnerable to Reflected XSS injection.
- I’ve started to search for a bypass and used the Search function in Chrome Developer tools to search this endpoint
/profilein all JS files to check for another vulnerable param, but found another endpoint:
The first idea that came to my mind was to put this URL in the google search engine and see if this endpoint was cached somewhere on the google web space with params.
After the first try, I found a cached endpoint with params on the first page of the results, the endpoint had ID param and some other params.
- I’ve added my payload
qwe'"<X</to the ID param and started to check if anything is reflected somewhere on the webpage’s source code.
Besides that, I’ve opened the Network tab in Chrome Developer tools to check all requests that this endpoint might send somewhere.
After the second refresh of the page, I found an interesting AJAX request that used the JSONP callback param together with the ID param from the endpoint itself. The AJAX request URL was similar to this:
The first thing that I tested was the JSONP param itself, to see if I can change it to an
alertfunction with a custom parameter
To my surprise, there was no check for JSONP value, so I easily changed it to
Now it was time to check the ID param once again and see if it accepts other symbols, for example,
%sign to craft an encoded payload in order to add custom parameters to AJAX URL.
I’ve changed the endpoint URL to
- When JS processed it, it transformed
%26to & and
%23to #. Everything that is behind the # (hashtag) symbol is ignored by the browser. The final AJAX call looked like this:
- Using this AJAX URL manipulation (parameter pollution attack) I have successfully triggered an alert box with text
1337. This confirmed the DOM XSS vulnerability existence and I have received a $350 bounty, with an additional $50 for a retest of an old report.
Thanks for reading!
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Happy Holidays & Happy New Year! 🎄
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