[CKEditor library problem] Drupal + CKEditor 3.6.2 - Persistent EventHandler XSS

The following is a security advisory for the Drupal CKEditor. Please note this has also been submitted to the CKEditor project too.

# Exploit Title: Drupal CKEditor 3.6.2 - Persistent EventHandler XSS
# Google Dork: "inurl:"sites/all/modules/ckeditor" -drupalcode.org"
# Google Results: Approximately 379.000 results
# Date: 9th December 2011
# Author: MaXe @InterN0T
# Software Link: http://ckeditor.com/ & http://drupal.org/node/1332022
# Version: 3.6.2 (Drupal module: 6.x-1.8)
# Screenshot: If attached, see image file(s).
# Tested on: Windows + FireFox 8.0 & Internet Explorer 8.0

Drupal CKEditor - Persistent / Stored Cross-Site Scripting

Versions Affected: 3.6.2 (Possibly all versions that supports eventhandler injection.)

Info:
CKEditor is a text editor to be used inside web pages. It's a WYSIWYG editor, which
means that the text being edited on it looks as similar as possible to the results users
have when publishing it. It brings to the web common editing features found on desktop
editing applications like Microsoft Word and OpenOffice.

External Links:
http://ckeditor.com/
http://drupal.org/node/1332022

Credits: MaXe (@InterN0T)

-:: The Advisory ::-
CKEditor is prone to Persistent Cross-Site Scripting within the actual editor, as
it is possible for an attacker could maliciously inject eventhandlers serving java-
script code in preview / editing in html mode.

If an attacker injects an eventhandler into an image, such as "onload='alert(0);'",
then the javascript will execute, even if the data is saved and previewed in editing
mode later on. (The XSS will only executing during preview / editing in html mode.)

If an administrator tries to edit the comment afterward, or is logged in and browses
to the edit page of the malicious comment, then he or she will execute the javascript,
allowing attacker controlled code to run in the context of the browser.

Proof of Concept:
Switching to "raw mode" in CKEditor and then writing:

Will become this when it is saved:

If one searches for alert(0); in Firebug after the code has been injected and executed, the location of the script will be: $full_url_to_script/event/seq/4/onload
Where $full_url_to_script is e.g. the following: http://localhost/drupal/drupal-6.22/?q=comment/edit/3/event/seq/4/onload

The content of this script is:
function onload(event) {
alert(0);
}

As there is a HTML filter in Drupal, it does not matter whether the tag is allowed in this case, as it was possible to execute the eventhandler either way. (And even store the data.)

-:: Solution ::-
* Awaits developer response *
Note: It shouldn't be possible to use eventhandlers unless explicitly specified by the administrator of the target site.
All eventhandler input should also be sanitized / encoded to their equivalent htmlentities and encapsulated in quotes.

Disclosure Information:
6th December 2011 - Vulnerability found during a Penetration Test
7th December 2011 - Researched and confirmed the vulnerability
4th January 2012 - Reported to Drupal and CKEditor via http://drupal.org/project/ckeditor and http://dev.ckeditor.com/ and http://cksource.com/contact

There has been no public disclosure of this advisory yet. Please respond back to us whenever a solution is available, or if this is deemed a "non-issue".