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From: dshalev at (Dror Shalev)
Subject: Yahoo! E-mail Service Vulnerability

Yahoo! E-mail Service Vulnerability


Release Date:

August 23, 2004



Critical (Potential web-based e-mail worm)


Systems Affected:

Other web-based e-mail systems may be vulnerable.

Internet Explorer and any software application used for reading Yahoo 

e-mail messages. 

(The ActiveX payload is relevant only for Internet Explorer)


Finjan Software notification sent to Yahoo! on May 24, 2004.



Yahoo! has already patched their Web-based e-mail services 

on July 16, 2004.

Other web-based e-mail systems may be vulnerable. 




Finjan Software identified a new critical cross site scripting
vulnerability in Yahoo's Web-based e-mail service.  

This vulnerability allowed hackers to develop an attack that could have
caused significant computer damage during regular Internet use.



This vulnerability resulted from the failure of Yahoo's active content
filter to adequately block ActiveX controls and other active content
components, and affected all Windows based system platforms that read
e-mail messages using Yahoo Web-mail service. Active X controls are
downloadable programs that run with the same rights and privileges as
the user, allowing access to files and personal information stored on a
local hard drive or shared folder.  A no-click attack could have
launched automatically once a user opened an e-mail message.  

For example, the vulnerability could have also potentially allowed a
worm to read Windows address book, replicate and send itself to everyone
in the address book, and have this process repeat at an exponential
rate.  It could have also harvested email addresses from local files,
just like any other worm, and use the Yahoo web-mail vulnerability to
send the email messages. Other web-based e-mail systems may be
vulnerable to this vulnerability. 




Technical details:

The potential worm could do anything that the user could do. 

It is a potentially automatic attack. 

Users had to simply read the infected email message.

This was a cross-site scripting vulnerability of the Yahoo! Web-based
e-mail service. 

There are two variants of this vulnerability.

The purpose of Yahoo's active content filter is to block the injection
of any active content into Yahoo! messages. 

However, the basic failure that allowed this vulnerability is that there
was no blocking of a backslash that is used instead of the import rule. 


An example:

<style><!--@\ "";--></style>


The injected JavaScript code inside the CSS file is responsible for:


-Getting cookies.

-Automatic launching of malicious code.

-A possible identity theft using a spoofed re-login window.

-Sending an e-mail message.

The injected ActiveX control can be used for a destructive payload of
the propagating worm. 

The basic attack does not require an ActiveX control. 

The ActiveX control is the payload that can be used to extend the attack
to non-web mail users, or to perform any malicious activity, including
formatting of the hard disk.

Upon using the ActiveX control, end user may get a security warning. 

It depends on the security setting of the browser. 


An example:

(Click on the 'test me' button after reading the disclaimer)



Bitlance Winter provided the initial tip.

Finjan Software's  Malicious Code Research Center (MCRC) has expanded



This specific vulnerability has been eliminated by Yahoo based on Finjan
Software notification. 

Finjan's content security products provided proactive defense against
this Yahoo! vulnerability prior to its detection and correction.

Finjan's patented behavior blocking engine will protect computer users
from similar future vulnerabilities and comparable potential exploits.




Credit: Bitlance Winter , Dror Shalev and Menashe Eliezer.



Finjan Software  

Malicious Code Research Center (MCRC) department

Prevention is the best cure!


Finjan Software

This e-mail and any attached files are confidential and may be legally
privileged. The unauthorized use, disclosure or copying of this email or
any information contained within it is strictly prohibited. This also
confirms that Finjan Software's Vital Security for E-Mail has scanned this
message for the presence of known viruses and potentially malicious

Finjan Software - Prevention is the Best Cure!
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