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TUCoPS :: Cisco :: ciacm001.txt

Cisco Secure IDS Signature Obfuscation Vulnerability


                       The U.S. Department of Energy
                     Computer Incident Advisory Center
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

              Cisco Secure IDS Signature Obfuscation Vulnerability

October 1, 2001 17:00 GMT                                         Number M-001
PROBLEM:       Two Cisco Intrusion Detection Systems have a vulnerability that 
               allows the %u encoding technique to circumvent them. 
PLATFORM:      * Cisco Secure Intrusion Detection System, formerly known as 
                 NetRanger, Sensor component 
               * Cisco Catalyst 6000 Intrusion Detection System Module
DAMAGE:        The vulnerability may allow attacks to go undetected and thus 
SOLUTION:      Apply the given patch and/or workaround. 
VULNERABILITY  The risk is MEDIUM. Circumvented Intrusion Detection Systems 
ASSESSMENT:    can impair virus infection investigations. 

[******  Start Cisco Security Advisory ******]


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

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 will be repaired in a service pack for version 3.0, 
which is not yet released. 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

Affected 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.

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 

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


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 
do NOT yet implement obfuscation detection.

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


This method of obfuscation can allow malicious exploitation to bypass current 
intrusion detection technology.

Software Versions and Fixes

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 service pack is still officially BETA code until the testing cycle is 
complete; however, due to the nature of the repairs and the public notification 
of this vulnerability,  the code is posted for customer download at the 
following location:

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. 
Cisco will update this advisory when more detailed delivery information for the 
service pack is available.

Obtaining Fixed Software

Cisco is offering software upgrades to remedy this vulnerability for all 
affected customers who have current SmartNet contracts on their sensors. 
Customers with contracts should obtain upgraded software through their regular 
update channels. For most customers, this means that upgrades should be obtained 
via the Software Center on Cisco's Worldwide Web site at

Customers without contracts can purchase either SmartNet contracts or one time 
software upgrades (IDS-SW-U) for each affected sensor by contacting the Customer 
Service Center. Customer Service contacts are as follows:

  +1 800 553 6378 (toll-free from within North America) 
  +1 408 526 1868 (toll call from anywhere in the world)

See for more information on 
ordering Cisco products and upgrades.

Customers can also contact the Cisco Technical Assistance Center (TAC). TAC 
contacts are as follows:

     * +1 800 553 2447 (toll-free from within North America). 
     * +1 408 526 7209 (toll call from anywhere in the world). 
     * E-mail:

See for additional 
TAC contact information, including instructions and e-mail addresses for use in 
various languages.

Please do not contact either "" or "" for 
software upgrades.


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:    
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) 
For more information on Custom String Match features, please refer to the 
documentation available at

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

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

Status of This Notice: INTERIM

This is an interim notice. Although Cisco cannot guarantee the accuracy of all 
statements in this notice, all of the facts have been checked to the best of our 
ability.   Cisco will issue updated versions of this notice when there is 
updated information on product release dates. Should there be a significant 
change in the facts, Cisco may update this notice. This notice will be updated 
by 2001-OCT-15.


This notice will be posted on Cisco's Worldwide Web site at
/cisco-intrusion-detection-obfuscation-vuln-pub.shtml. In addition to 
Worldwide Web posting, a text version of this notice is clear-signed with
the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet
news recipients:

     * (includes CERT/CC) 
     * Various internal Cisco mailing lists

Future updates of this notice, if any, will be placed on Cisco's Worldwide Web 
server, but may or may not be actively announced on mailing lists or newsgroups. 
Users concerned about this problem are encouraged to check the URL given above 
for any updates.

Revision History

      Revision 1.0 	2001-SEP-05 17:00 US/Pacific 	Initial public release.
      Revision 1.1	2001-SEP-14			Updated details of 
							workarounds and 
							instructions obtaining 
							fixed software.
      Revision 1.2	2001-SEP-27			Updated details of 
							instructions for 
							obtaining fixed software.

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, 
obtaining assistance with security incidents, and registering to receive 
security information from Cisco, is available on Cisco's Worldwide Web site at This includes 
instructions for press inquiries regarding Cisco security notices.

This notice is copyright 2001 by Cisco Systems, Inc. This notice may be 
redistributed freely after the release date given at the top of the text, 
provided that redistributed copies are complete and unmodified, including all 
date and version information.

[******  End Cisco Security Advisory ******]


CIAC wishes to acknowledge the contributions of Cisco Systems, Inc. for the 
information contained in this bulletin.

CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
    Voice:    +1 925-422-8193 (7x24)
    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:
   Anonymous FTP:

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation or favoring by the United States Government or the
University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.

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