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RE: Yahoo/Hotmail scripting vulnerability, worm propagation
From: "Microsoft Security Response Center" <secure () microsoft com>
Date: Thu, 31 May 2001 16:24:14 -0700

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We are investigating this matter thoroughly and aggressively to
determine whether or not it is valid.  Contrary to the poster's
claim, we have not received any direct communications on this
possible (or alleged) vulnerability. 

Regards,

secure () microsoft com

- -----Original Message-----
From: mparcens () hushmail com [mailto:mparcens () hushmail com] 
Sent: Wednesday, May 30, 2001 5:18 PM
To: bugtraq () securityfocus com
Subject: Yahoo/Hotmail scripting vulnerability, worm propagation


Title: Yahoo/Hotmail scripting vulnerability, worm propagation


Synopsis

Cross-site-scripting holes in Yahoo and Hotmail make it possible to
replicate 
a Melissa-type worm through those webmail services.


Description

An email is sent to the victim, who uses Yahoo Mail or Hotmail.
Inside the 
email is a link to yahoo or hotmail's own server. The link contains
escaped 
javascript that is executed when the page is loaded. That javascript
then 
opens a window that could nagivate through the victim's inbox,
sending messages 
with the malicious link to every email address it finds in the inbox.
Because 
the malicious javascript executes inside a page from the mail
service's 
own server, there is no domain-bounding error when the javascript is
controlling 
the window with the victim's inbox.


Who is vulnerable

Users of the Yahoo Mail and Hotmail service. Although the exploit
requires 
a user to click on a link, two things work for this exploit. (1) The
email 
comes from a familiar user (sent by the worm), and (2) The link is to
a 
familiar, trusted server. Theoretically, more services are
vulnerable, due 
to the proliferation of these holes, but the worm is limited to web
mail 
services.


Proof-of-Concept

Sample links and the worm code can be found at:
http://www.sidesport.com/webworm/


Solution

Escaping all query data that is echoed to the screen eliminates this
problem. 
This must be done on every page on a server that can send or read
mail for 
the service.


Vendor Status

Both Yahoo and Hotmail were notified on May 23 2001.


- -mparcens
mparcens () hushmail com

Free, encrypted, secure Web-based email at www.hushmail.com

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