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 multiple persistent XSS
vulnerabilities via the subject and message body fields allowing the
theft of user session information and other bad stuff.

XSS #1:
YouTube fails 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>


XSS #2:
YouTube fails to sanitize the content of the "field_sendmessage_body"
textarea which is rendered as part of the read process. allowing a
malicious user terminate the textarea scope and inject malicious code.

<textarea name="field_sendmessage_body">MY MESSAGE BODY</textarea>

A malicious user can send a message body which terminates
the textarea scope and inserts malicious code.

Message Body Example:

This is my message!</textarea><script>alert(document.cookie);</script>


Thank you,

Darren Bounds