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To aid in the wide distribution of essential security information, the CERT Coordination Center is forwarding the following information from Cisco Systems. NOTE THAT THIS IS NOT NEW INFORMATION; CISCO RELEASED THIS ADVISORY TO THE PUBLIC IN JUNE 1995. If you have already seen this and are certain that you have correctly configured your routers, you do not need to take any further action. However, if you have not seen this Cisco Security Advisory or are not certain that your routers are configured correctly, we urge you to review this bulletin and check the configuration of your router. Cisco contact information is included in the forwarded text below; please contact them if you have any questions or need further information.
The following describes a vulnerability in Cisco's IOS software when the 'established' keyword is used in extended IP access control lists. This bug can, under very specific circumstances and only with certain IP host implementations, allow unauthorized packets to circumvent a filtering router. This vulnerability is present in the following IOS software versions: 10.3(1) through 10.3(2) 10.2(1) through 10.2(5) 10.0(1) through 10.0(9) ...and all previous versions of Cisco software. If you are running any of these IOS versions on a product that uses IP extended access lists, and you are using the 'established' keyword in these lists, then Cisco strongly recommends that you take immediate action to remove the vulnerability. You can determine what version of IOS you are running by issuing the following command: show version. The recommended action is to upgrade to a more recent version of IOS, or take one of the immediate workaround actions described below. The vulnerability is fixed by in the following official software releases: 10.0(10) or later, 10.2(6) or later 10.3(3) or later (For reference, the Cisco update identifier for this fix is "CSCdi34061".) Customers may obtain software upgrades without going through the Cisco's Technical Assistance Center (TAC) via the Cisco Information Online (CIO) service. Instructions for downloading are available at the end of this message. You may also contact your Cisco distributor or contact Cisco's TAC for more information. The TAC can be reached by phone at 800-553-2447, by e-Mail at tac@cisco.com or via the World Wide Web at http://www.cisco.com (for registered CIO users only). In Europe, you can contact the TAC by phone at 32-2-778-42-42 or via e-Mail to euro-tac@cisco.com.
A bug in Cisco's extended IP access list implementation can, under very specific circumstances, allow a user to bypass IP packet filtering. This may permit unintended IP traffic to pass through your firewall setup. To determine if you are vulnerable, look through your configuration. The configuration can be displayed by enabling and then entering the command "write term". If you see an access list line using a list number in the range of 100 through 199 that permits or denies TCP traffic and contains the word 'established' near the end of the line, you may be vulnerable. An example line might look like: In IOS 10.3: access-list 100 permit tcp any any established In IOS 10.2 or earlier: access-list 100 permit tcp 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 established. If you do not meet this test, then you are not vulnerable. You do not need to do anything.
The following actions will remove the vulnerability:
Example: router(config)#interface ethernet 0 router(config-if)#shutdown
Obtain and install the appropriate release of IOS software as described above. For assistance, contact Cisco's TAC.
This problem is caused by an obscure but common design flaw that we believe exists in many router/firewall vendor's packet filtering implementations. Owners of non-Cisco hardware who use IP packet filtering features similar to Cisco's "extended access lists" as part of a firewall system may wish to contact their vendor to confirm that this vulnerability does not exist in their system. (Technical discussions about the problem have already occurred in the appropriate forum.) This vulnerability can only be exploited with certain IP host implementations (we do not have information on which implementations are susceptible). Cisco suggests that all routers configured to filter IP packets based upon the 'established' mechanism be upgraded.
Software upgrades may be obtained via any of the following mechanisms:
If you believe that your system has been compromised, contact the CERT Coordination Center or your representative in the Forum of Incident Response and Security Teams (FIRST). If you wish to send sensitive incident or vulnerability information to CERT staff by electronic mail, we strongly advise that the e-mail be encrypted. The CERT Coordination Center can support a shared DES key, PGP (public key available via anonymous FTP on info.cert.org), or PEM (contact CERT staff for details). Internet e-mail: cert@cert.org Telephone: +1 412-268-7090 (24-hour hotline) CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4), and are on call for emergencies during other hours. Fax: +1 412-268-6989 Postal address: CERT Coordination Center , Software Engineering Institute, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA. CERT advisories and bulletins are posted on the USENET news group comp.security.announce. If you would like to have future advisories and bulletins mailed to you or to a mail exploder at your site, please send mail to cert-advisory-request@cert.org. Past CERT publications, information about FIRST representatives, and other information related to computer security are available by anonymous FTP from info.cert.org.
Thanks for visiting DataSure's CERT Advisory Pages. As the pages are developed further we hope to provide more, so please bear with us during the ongoing development process.
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