Summary

• Intrusion Detection Systems inspect network traffic for suspect or malicious packet formats, data payloads and traffic patterns. Intrusion detection systems typically implement obfuscation defense - ensuring that suspect packets cannot easily be disguised with UTF and/or hex encoding and bypass the Intrusion Detection systems. Recently, the CodeRed worm has targeted an unpatched vulnerability with many MicroSoft IIS systems and also highlighted a different encoding technique supported by MicroSoft IIS systems. This encoding technique known as %u can be used to circumvent intrusion detection systems, and has been made public by eEye security in their announcement located at http://www.eeye.com/html/Research/Advisories/AD20010705.html .

  Cisco has corrected this vulnerability in the Cisco Secure Intrusion Detection System, formerly known as Netranger, with a service pack that is now available to customers. This vulnerability also affects the Cisco Catalyst 6000 Intrusion Detection System Module, and is repaired in release 3.0(4)S20, released in May 2002. Cisco has provided a workaround for this issue, which is listed in the Workaround section of this advisory.

  The complete notice will be available at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010906-intrusion-detection.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  The following products are affected:

  • Cisco Secure Intrusion Detection System, formerly known as NetRanger, Sensor component
    
  • Cisco Catalyst 6000 Intrusion Detection System Module
    

  Additionally, selected workarounds such as the use of NBAR, or the Cisco Cache Engine, for filtering the CodeRed worm exploit will not detect %u encoding attack obfuscation, unless specifically configured for all possibilities.

  Products Confirmed Not Vulnerable

  The Cisco Secure Intrusion Detection System Director for both Unix and NT platforms are management components of the IDS, and do not participate in packet obfuscation detection, and are not affected by this vulnerability.

  The following products implement a limited subset of Intrusion Detection attack signatures, and the signatures included do NOT detect MicroSoft IIS targeted attacks, and are therefore NOT vulnerable to the %u encoding method of attack obfuscation.

  • Cisco Secure PIX Firewall
    
  • Cisco IOS Firewall Feature Set with Intrusion Detection
    

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• The "CodeRed" worm utilized an obscure unicode encoding technique to deliver the payload of the worm. The %u encoding method is a different encoding method that is understood and parsed by the IIS web server. This encoding can be applied to other portions of the url to effectively obfuscate the attack, preventing detection by many intrusion detection systems available. Cisco Secure Intrusion Detection System Sensor decoding algorithms have been modified to detect and parse this unicode form. Cisco Catalyst 6000 Intrusion Detection Systems Modules did NOT implement obfuscation detection at the original posting of this notice, but include this feature as of May 2002.

  This vulnerability is documented as Cisco Bug ID CSCdv20287. This vulnerability is also listed in the Mitre CVE as CAN-2001-0669.

Workarounds

• Workarounds for this issue exist for both the Cisco Secure Intrusion Detection System Sensor, and the Catalyst 6000 Intrusion Detection System Module.

  A custom string match signature can be defined to address the %u unicode obfuscation vulnerability.

  This custom string match will detect uses of the unicode obfuscation. There may be legitimate uses of the unicode strings that are non-threatening that we are unaware of, so this signature may alarm on legitimate traffic patterns. Careful monitoring of associated alarms must accompany this signature.

  Signature 1
  ****************************************** 
  Unicode Obfuscation String:    
  "[%][uU][0-9a-FA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]" 
  Occurrences: 
  1 
  Port: 
  80    
  If you have Web servers listening on other TCP ports (for example, 8080), you will 
  need to create a separate custom string match for each port number. 
  Recommended Alarm Severity Level: 
  High (CSPM) 
  5 (Unix Director) 
  Direction: 
  TO 
  ***************************************************************  

  For more information on Custom String Match features, please refer to the documentation available at http://www.cisco.com/univercd/cc/td/doc/product/iaabu/csids/csids5/csidscog/.

Fixed Software

• This vulnerability is repaired in service pack 3.0(2)S6 for the Cisco Secure Intrusion Detection System Sensor, and will be included in all versions forward.

  This vulnerability will be repaired in service pack 3.0 for the Cisco Catalyst 6000 Intrusion Detection Module. Basic obfuscation detection was originally slated for the 3.0 release, which is due to be available in early October 2001. A service pack to the 3.0 release will include this additional method of obfuscation, but will not be available until after the October 2001 release. This service pack release is 3.0(4)S20, and is available as of May 2002.

Exploitation and Public Announcements

• This vulnerability has been announced by the eEye security team, and is published on the eEye security site at the following location http://www.eeye.com/html/Research/Advisories/AD20010705.html .

  Cisco has no knowledge of exploitation of this method of obfuscation.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010906-intrusion-detection

Revision History

• Revision 1.5	2003-JAN 24	Removed beta code information and location from Software Versions and Fixes section.
  Revision 1.4	2002-SEP-27	Updated Summary, Details, Software Versions and Fixes, and Status of This Notice.
  Revision 1.3	2001-OCT-17	Updated details of Software Versions & Fixes and Status of this Notice.
  Revision 1.2	2001-SEP-27	Updated details of instructions for obtaining fixed software.
  Revision 1.1	2001-SEP-14	Updated details of workarounds and instructions for obtaining fixed software.
  Revision 1.0	2001-SEP-05	Initial public release.

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