AUSCERT External Security Bulletin Redistribution

                 ESB-2002.262 -- CERT Advisory CA-2002-16
               Multiple Vulnerabilities in Yahoo! Messenger
                                6 June 2002


        AusCERT Security Bulletin Summary

Product:                Yahoo! Messenger version  5,0,0,1064 and  prior
Vendor:                 Yahoo!
Operating System:       Windows
Impact:                 Execute Arbitrary Code/Commands
                        Modify Arbitrary Files
                        Denial of Service
Access Required:        Remote

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CERT Advisory CA-2002-16 Multiple Vulnerabilities in Yahoo! Messenger

   Original release date: June 05, 2002
   Last revised: --
   Source: CERT/CC

   A complete revision history can be found at the end of this file.

Systems Affected

     * Yahoo!  Messenger  version  5,0,0,1064  and  prior  for  Microsoft


   There are multiple vulnerabilities in Yahoo! Messenger. Attackers that
   are  able  to  exploit  these  vulnerabilities  may be able to execute
   arbitrary  code  with  the  privileges of the victim user. We have not
   seen  active  scanning for these vulnerabilities, nor have we received
   any reports of these vulnerabilities being exploited, but users should
   upgrade to version 5,0,0,1065 or later.

I. Description

   Yahoo! Messenger is a widely used program for communicating with other
   users  over the Internet. On May 27, 2002, a buffer overflow and a URL
   validation  vulnerability  were  discovered  in  the  Yahoo! Messenger
   client for Microsoft Windows. Details of each vulnerability follow:

     VU#137115  - Yahoo! Messenger contains a buffer overflow in the URI

     The  buffer  overflow  occurs  during  the processing of the Yahoo!
     Messenger  URI  handler  (ymsgr:). This URI handler is installed at
     the system level for applications that use the underlying operating
     system when processesing URIs (such as Microsoft Internet Explorer,
     Netscape  Navigator  6, Microsoft Outlook, or the command shell). A
     URI  can  be  sent  by  another Yahoo! Messenger user in a message,
     embedded  in  a  web  site,  or  sent  in  an HTML-renderable email

     This vulnerability has been assigned as CAN-2002-0031 by the Common
     Vulnerabilities and Exposures (CVE) group:


     VU#172315  -  Yahoo!  Messenger  "addview"  function allows for the
     automatic execution of malicious script contained in web pages

     A  vulnerability  exists in the Yahoo! Messenger "addview" function
     that permits a remote attacker to execute arbitrary script and HTML
     in  the  Internet security zone of the local machine. The "addview"
     function  is  only  supposed to accept view information from Yahoo!
     servers. However, an attacker can send malicious script and HTML to
     the client using the Yahoo! URL redirection service. This script or
     HTML is interpreted by the Yahoo! Messenger client and is displayed
     in the client's web browser.

     This vulnerability has been assigned as CAN-2002-0032 by the Common
     Vulnerabilities and Exposures (CVE) group:


   These  vulnerabilities  were  resolved  in  Yahoo!  Messenger  version
   5,0,0,1065,  released May 22, 2002; however, a bug in the distribution
   server  may  have  inadvertantly  installed  Yahoo!  Messenger version
   5,0,0,1036  on  systems that downloaded Yahoo! Messenger after May 22,
   2002. The bug in the distribution server has since been resolved.

   In  February  2002,  the  following  vulnerabilities  were reported to
   affect Yahoo! Messenger:

     * http://www.kb.cert.org/vuls/id/393195
     * http://www.kb.cert.org/vuls/id/419419
     * http://www.kb.cert.org/vuls/id/755755
     * http://www.kb.cert.org/vuls/id/887319
     * http://www.kb.cert.org/vuls/id/952875

   All of these vulnerabilities were resolved in Yahoo! Messenger version
   5,0,0,1058,  released February 25, 2002, or by server-side resolutions
   around the same time.

II. Impact

   A  remote  attacker  can execute arbitrary code with the privileges of
   the  victim  user,  cause  a  denial of service, or modify data in the
   victim's buddy list.

III. Solution

   Upgrade to the latest version of Yahoo! Messenger

     On  May  22,  2002,  Yahoo!  released  a  fixed  version  of Yahoo!
     Messenger  (5,0,0,1065)  and began issuing a patch (5,0,0,1066) via
     the  AutoUpdater to address this issue. All users should upgrade to
     version   5,0,0,1065   or  later.  Users  with  versions  prior  to
     5,0,0,1066  that  have "Auto Update" enabled will receive a message
     informing  them  that  an  upgrade  is  available. All users should
     accept this upgrade.

     Users who downloaded Yahoo! Messenger after May 22, 2002, should be
     aware  that a bug in the distribution server may have inadvertantly
     installed  Yahoo! Messenger version 5,0,0,1036, which is vulnerable
     to  all issues in this advisory. The bug in the distribution server
     has since been resolved.

     Users  should upgrade and verify the version of Yahoo! Messenger by
     selecting  the  "About  Yahoo!  Messenger..."  option from the Help

   Implement a firewall and filtering

     Yahoo! Messenger listens for peer-to-peer requests on port 5101/TCP
     but  users  can  implement a firewall to block inbound and outbound
     access  to port 5101/TCP. However, since Yahoo! Messenger URI's can
     be  embedded  in  a web site or email message, blocking requests to
     and from port 5101/TCP is not a completely effective solution. Mail
     and  Internet filters should also be applied to filter the "ymsgr:"
     URI handler from email messages and web sites.

Appendix A. - Vendor Information

   This  appendix  contains  information  provided  by  vendors  for this
   advisory.  When  vendors  report  new  information  to the CERT/CC, we
   update this section and note the changes in our revision history. If a
   particular  vendor  is  not  listed  below, we have not received their

Yahoo!, Inc.

     Yahoo!  encourages  users to upgrade to the latest version whenever
     prompted by the AutoUpdater or regularly check for updated versions
     of the client at http://messenger.yahoo.com.

   The     CERT     Coordination    Center    thanks    Scott    Woodward
   <scott@phoenixtechie.com>, Phuong Nguyen <dphuong@yahoo.com>, and Adam
   Lang <themeetup@hotmail.com> for their discovery and analysis of these
   vulnerabilities.   We  also  thank  Yahoo!  for  their  assistance  in
   analyzing and responding to these issues.

   Feedback can be directed to the author: Jason A. Rafail

Appendix B. - References

    1. http://www.kb.cert.org/vuls/id/137115
    2. http://www.kb.cert.org/vuls/id/172315
    3. http://www.kb.cert.org/vuls/id/393195
    4. http://www.kb.cert.org/vuls/id/419419
    5. http://www.kb.cert.org/vuls/id/755755
    6. http://www.kb.cert.org/vuls/id/887319
    7. http://www.kb.cert.org/vuls/id/952875

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   Revision History
      June 05, 2002:  Initial release

Version: PGP 6.5.8


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