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From: Cisco Systems Product Security Incident Response Team (psirtcisco.com)
Date: Wed Sep 05 2001 - 23:30:00 CDT
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Cisco Security Advisory: Cisco Secure Intrusion Detection System Signature
For release 2001 September 5 05:00 PM US/Pacific (UTC -0700)
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
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.
The complete notice will be available at
The following products are affected:
* Cisco Secure Intrusion Detection System, formerly known as NetRanger,
* 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
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
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
October 2001 release. Cisco will update this advisory when more detailed
delivery information for the service pack is available.
Getting Fixed Software
Cisco is offering free software upgrades to remedy this vulnerability for
all affected customers.
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
Customers without contracts 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)
* e-mail: taccisco.com
See http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for
additional TAC contact information, including instructions and e-mail
addresses for use in various languages.
Give the URL of this notice as evidence of your entitlement to a free
upgrade. Free upgrades for non-contract customers must be requested
the TAC. Please do not contact either "psirtcisco.com" or
"security-alertcisco.com" for software upgrades.
Workarounds for this issue would include implementing a large number of
custom string match entries, each one an iteration of the proprietary
encoding obfuscation method for the expected attack. This workaround
possibly address a short term problem, but is not scalable for the
of customers, and the product upgrade or service pack is recommended.
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
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
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:
* first-teamsfirst.org (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 1.0 For public release 2001-SEP-05 17:00 US/Pacific (UTC-0700)
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
includes instructions for press inquiries regarding Cisco security
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.
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