[Full-disclosure] YouTube Persistent Messaging XSS Vulnerability

YouTube Persistent Messaging XSS Vulnerability
September 30, 2006

Overview:
As I'm sure everyone knows; YouTube is the internet's leading video
destination site and one of the fastest-growing websites on the web.
It's ranked as the 10th most popular website on Alexa, far outpacing
even MySpace's growth.

Vulnerability:
YouTube has a robust message/email functionality to allow users
communicate and share content within the YouTube user environment. The
YouTube messaging system is vulnerable to a persistent XSS
vulnerability via the subject field allowing the theft of user session
information and other bad stuff.

This vulnerability stems from YouTube failing to sanitize the value of
the HIDDEN HTML tag "field_sendmessage_subject" which is rendered as
part of the message read process.

<input type="hidden" name="field_sendmessage_subject" value="MY SUBJECT">

A malicious user can send a message with a subject which terminates
the 'value' attribute and inserts malicious code.

Example Subject:

Check out my video!         "><script>alert(document.cookie);</script>


Thank you,
Darren Bounds

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