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Remotely Exploitable Cross-Site Scripting in Hotmail and Yahoo (GM#005-MC)
From: GreyMagic Software <security () greymagic com>
Date: Tue, 23 Mar 2004 19:41:13 +0200

GreyMagic Security Advisory GM#005-MC

By GreyMagic Software, Israel.
23 Mar 2004.

Available in HTML format at

Topic: Remotely Exploitable Cross-Site Scripting in Hotmail and Yahoo.

Discovery date: 06 Mar 2004.

Affected applications:

* Hotmail web-based email service (when used with IE).
* Yahoo web-based email service (when used with IE).

Note that many other web-based services may be vulnerable to this method of
exploitation, as it is a completely new way to embed script. 


Both Hotmail and Yahoo make tremendous efforts to sanitize incoming emails
from potentially unsafe HTML content. Flawed filtering of such unsafe
content may result in severe consequences that would occur as soon as a user
opens an email for reading, including: 

* Theft of login and password. 
* Content disclosure of any email in the mailbox. 
* Automatically send emails from the mailbox. 
* Exploitation of known vulnerabilities in the browser to access the user's
file system and eventually take over the machine. 
* Distribution of a web-based email worm. 
* Disclosure of all contacts within the address book. 


GreyMagic devised a method to inject such arbitrary (potentially malicious)
content to a Yahoo or Hotmail email message. The method is not limited to
Hotmail and Yahoo alone though, it may apply to other web-based services
that attempt to filter HTML input. 

The vulnerability makes use of an Internet Explorer technology called
HTML+TIME (based on SMIL), which is meant to add timing and media
synchronization support to HTML pages. 

One of the features included in HTML+TIME is the ability to manipulate any
attribute on an element via special control elements. For example, the
<t:set> element exposes the attributes "attributeName" and "to", which make
it possible to inject ANY HTML content to the document when "attributeName"
is set to "innerHTML" and "to" is set to any HTML the attacker would like to
execute, including script. 


For the HTML+TIME module to be activated, the document must fulfill two
requirements. It must declare the designated namespace and it must bind the
namespace to the HTML+TIME behavior implementation. 

In order to fulfill the first requirement it is usually necessary to be able
to access the <html> element, with the syntax <html
xmlns:t="urn:schemas-microsoft-com:time">. However, Hotmail completely
filters out that element, so another method of namespace declaration is
needed. It so happens that Internet Explorer provides one other mechanism to
declare a namespace, via the non-standard <?xml:namespace> processing
instruction, which may be used anywhere in the document and does not get

The second requirement usually involves the use of the CSS "behavior"
property, with the syntax "behavior:url(#default#time)". However, Hotmail
blocks all instances of "url(...)" in the incoming mail, so another way to
bind the behavior must be used. It comes in the form of the <?import>
element, which was added in Internet Explorer 5.5 and enables namespace to
implementation binding. 

So after evading all filters, the final code looks like this: 

<?xml:namespace prefix="t" ns="urn:schemas-microsoft-com:time" />
<?import namespace="t" implementation="#default#time2">
Optional text here...
<t:set attributeName="innerHTML" to="&lt;script
defer&gt;alert()&lt;/script&gt;A" />


We put together a proof of concept demonstration, which can be found at


GreyMagic started work on this issue with Microsoft on 11-Mar-2004. They
have quickly confirmed our findings and were able to produce a fix less than
two days later. As a result, Hotmail is no longer vulnerable to this method
of exploitation. 

All attempts to contact Yahoo unfortunately failed. Mail was sent to
security and secure at yahoo.com and at yahoo-inc.com, no replies were
received to date. 

Tested on: 



The information in this advisory and any of its demonstrations is provided
"as is" without warranty of any kind. 

GreyMagic Software is not liable for any direct or indirect damages caused
as a result of using the information or demonstrations provided in any part
of this advisory. 

- Copyright © 2004 GreyMagic Software.

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