Products & Services

Cisco Secure Intrusion Detection System Signature Obfuscation Vulnerability

Advisory ID: cisco-sa-20010906-intrusion-detection

Revision 1.5

For Public Release 2001 September 6 00:00  UTC (GMT)


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 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

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.


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.

Vulnerability Scoring Details

Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.

CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.

Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

Cisco has provided an FAQ to answer additional questions regarding CVSS at .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .


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 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.


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

Obtaining Fixed Software

Cisco has made free software available to address this vulnerability for affected customers. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.

Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, customers agree to be bound by the terms of Cisco's software license terms found at, or as otherwise set forth at Downloads at

Do not contact either "" or "" for software upgrades.

Customers with Service Contracts

Customers with contracts should obtain upgraded software through their regular update channels. For most customers, this means that upgrades should be obtained through the Software Center on Cisco's worldwide website at

Customers using Third Party Support Organizations

Customers whose Cisco products are provided or maintained through prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.

Customers without Service Contracts

Customers who purchase direct from Cisco but who do not hold a Cisco service contract and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting 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:

Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

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: Final


A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.


This advisory is posted on Cisco's worldwide website at

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 advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.

Revision History

Revision 1.5

2003-JAN 24

Removed beta code information and location from Software Versions and Fixes section.

Revision 1.4


Updated Summary, Details, Software Versions and Fixes, and Status of This Notice.

Revision 1.3


Updated details of Software Versions & Fixes and Status of this Notice.

Revision 1.2


Updated details of instructions for obtaining fixed software.

Revision 1.1


Updated details of workarounds and instructions for obtaining fixed software.

Revision 1.0


Initial public release.

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 website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at