Advisory ID: cisco-sa-20010906-intrusion-detection
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
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
This section provides details on affected products.
The following products are affected:
Cisco Secure Intrusion Detection System, formerly known as NetRanger,
Cisco Catalyst 6000 Intrusion Detection System
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
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
No other Cisco products are currently known to be affected by these
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.
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.
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
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
Workarounds for this issue exist for both the Cisco Secure Intrusion
Detection System Sensor, and the Catalyst 6000 Intrusion Detection System
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
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:
5 (Unix Director)
For more information on Custom String Match features, please refer to
the documentation available at
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 Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
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 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 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 at obtaining fixed software through their point of sale should get
their upgrades 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)
Have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
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This advisory is posted on Cisco's worldwide website at
In addition to worldwide web posting, a text version of this notice is
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Future updates of this advisory, if any, will be placed on Cisco's
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newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.
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Updated Summary, Details, Software Versions and Fixes, and
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Initial public release.