Summary

• A buffer overflow vulnerability exists in an ActiveX control used by the WebEx Meeting Manager. Exploitation of this vulnerability could allow a remote attacker to execute arbitrary code on the user client machine. The WebEx Meeting Manager is a client-side program that is provided by the Cisco WebEx meeting service. The Cisco WebEx meeting service automatically downloads, installs, and configures Meeting Manager the first time a user begins or joins a meeting.

  When users connect to the WebEx meeting service, the WebEx Meeting Manager is automatically upgraded to the latest version. There is a manual workaround available for users who are not able to connect to the WebEx meeting service.

  Cisco WebEx is in the process of upgrading the meeting service infrastructure with fixed versions of the affected file.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20080814-webex.

Affected Products

• Vulnerable Products

  The WebEx Meeting Manager downloads several components to meeting participants before they join a WebEx meeting. The vulnerability in this Security Advisory affects the atucfobj.dll library.

  Products Confirmed Not Vulnerable

  No other Cisco products are currently known to be affected by this vulnerability.

Details

• The WebEx meeting service is a hosted multimedia conferencing solution that is managed by and maintained by Cisco WebEx. When a meeting participant connects to the WebEx meeting service through a web browser, the WebEx meeting service installs several components of the WebEx Meeting Manager browser plugin on the meeting participant's system.

  WebEx Meeting Manager includes atucfobj.dll, a DLL that allows meeting participants to view Universal Communication Format (UCF) contents. This library contains a buffer overflow vulnerability that could allow an attacker to execute arbitrary code.

  The WebEx meeting service currently maintains three different versions of software. WebEx meeting service servers run one of the following versions: WBS 23, WBS 25, or WBS 26.

  Note: In addition to the three currently maintained versions of WebEx software, pre WBS 23 software versions may also be impacted by this vulnerability.

  This vulnerability is documented in WebEx Bug IDs 292551 for WBS 26 and 306639 for WBS 25. This vulnerability has been assigned Common Vulnerabilities and Exposures (CVE) identifier CVE-2008-3558.

  Identifying WebEx Meeting Service Version

  The following procedure will allow a meeting participant to identify the version of client software provided by a WebEx site. The procedure varies slightly depending on the software version. The URL in all the following examples is provided to meeting participants in the WebEx meeting invitation.

  Client build numbers adhere to the format of XX.YY.ZZ.WWWW. The first number indicates the major version number of the software release. For example, a client build number of 26.49.9.2838 indicates a WBS 26-based software client.

  For the WBS 26 version:

  1. Browse to the WebEx meeting server at https://.webex.com/.
     
  2. Select Support from the left side of the web page.
     
  3. Select Downloads from the left side of the web page.
     
  4. The version of the client software that is provided by the server is listed next to Client build.
     

  For WebEx servers that are running WBS 26, the first fixed version is 26.49.9.2838. Client build versions prior to 26.49.9.2838 are vulnerable.

  For the WBS 25 version:

  1. Browse to the WebEx meeting server at https://.webex.com/.
     
  2. Select Assistant on the left side of the page.
     
  3. Select the Support link.
     
  4. Select the Version link, which is displayed on the right side of the top of the page.
     
  5. The Client Build version is displayed in a pop-up window.
     

  There is currently no fixed version for the WBS 25-based WebEx meeting service. This section of the Security Advisory will be updated when fixed version information is available.

  For the WBS 23 version:

  Servers that run WBS 23-based WebEx meeting service display version information using the following URL format:

  https://.webex.com/version/wbxversionlist.do?siteurl=

  On the redisplayed page the Client versions in files field will indicate the Client Build.

  For example: The 'T23' in WBXclient-T23L10NSP33EP13-1092.txt indicates a WBS 23-based system.

  Cisco WebEx is not planning to repair WBS 23-based software. Affected WBS 23-based servers will be upgraded to fixed WBS 26-based software.

  Attack Vector Details

  This Security Advisory addresses a vulnerable ActiveX control (atucfobj.dll). If atucfobj.dll is present on a client's computer, it may be possible for an attacker to embed malicious code into HTML content that calls an affected function in atucfobj.dll via ActiveX.

  Users could encounter the malicious HTML in several ways. The most common manners are:

  • Browsing to a web-site that contains the malicious content
    

  • HTML that is embedded in e-mail messages
    

  • HTML that is delivered via instant messaging applications
    

  WebEx Upgrade Timeline

  Upgrades from WBS 23 versions to WBS 26 are expected to be complete by the end of September 2008.

  Fixed versions of WBS 25 are expected to be deployed by the end of September 2008.

  Deployed versions of WBS 26 are fixed.

Workarounds

• WebEx meeting participants who join a WebEx meeting that is hosted by a server with fixed software will download a fixed version of atucfobj.dll prior to joining the meeting.

  Several other workarounds are described below in the following subsections.

  Additional mitigation techniques that can be deployed on Cisco devices within the network are available in the Cisco Applied Mitigation Bulletin companion document for this advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20080814-webex.

  Manually Upgrading WebEx Meeting Manager

  Users can verify that the WebEx meeting service server they are connecting to is running fixed code via the method that is described in the subsection entitled Identifying WebEx Version subsection of the Details section of this Security Advisory.

  If the WebEx server is running a version of software that is fixed, users can manually download and install the Meeting Manager client to ensure their versions of atucfobj.dll are not vulnerable.

  Removing WebEx Meeting Manager

  It is possible to remove the WebEx Meeting Manager component from Microsoft Windows by using the Add or Remove Programs utility in the Windows Control Panel:

  1. In Windows, choose Start > Control Panel.
     
  2. Double-click Add or Remove Programs.
     
  3. Double-click WebEx.
     
  4. In the pop-up menu, check the Meeting Manager box and click Uninstall.
     
  5. Follow the prompts to complete the uninstall process and restart the system.
     

  NOTE: After uninstalling the WebEx Meeting Manager, users that join a WebEx meeting that is hosted by a vulnerable version will again download and install a vulnerable atucfobj.dll.

  Disabling atucfobj.dll by Setting the Kill Bit

  It is possible to disable the execution of atucfobj.dll by using a configuration setting in Microsoft Windows. This method is called setting the kill bit for the DLL. Once set, this method prevents atucfobj.dll from loading, which prevents exploitation of the vulnerability.

  Instructions for setting the kill bit in Microsoft Windows are available at the following location:

  http://support.microsoft.com/kb/240797

  Setting the kill bit for atucfobj.dll will persist even after a fixed version of the DLL is installed. To re-enable the use of atucfobj.dll, the kill bit will need to be unset.

  To disable atucfobj.dll users must know the CLSID for the DLL. The CLSID for atucfobj.dll is {32E26FD9-F435-4A20-A561-35D4B987CFDC}

Fixed Software

• The following table contains information on the WebEx meeting service software releases:

  Software release	First Fixed release
  WBS19	Vulnerable. Migrate to WBS25
  WBS20	Vulnerable. Migrate to WBS25
  WBS23	Vulnerable. Migration to WBS25 expected to be completed by the end of September 2008.
  WBS25	Fixed release expected to be deployed by the end of September 2008.
  WBS26	26.49.9.2838

  Clients will receive an upgrade automatically in accordance with the process that is outlined in the Obtaining Fixed Software section of this advisory within the time frame that is outlined in the WebEx Upgrade Timeline subsection of this advisory.

  Cisco WebEx will not offer the modified atucfobj.dll as a separate download.

Exploitation and Public Announcements

• This issue has been publicly announced on multiple external forums and mailing lists.

  Exploit code has been made available.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20080814-webex

Revision History

• Revision 1.3	2008-August-29	Update to Software Fixes section and update to the software naming scheme.
  Revision 1.2	2008-August-18	Updated CVE identifier.
  Revision 1.1	2008-August-15	Addition of AMB link, adjustment of site terminology, and update of legacy software impact in Details section.
  Revision 1.0	2008-August-14	Initial public release

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