Advisory ID: cisco-sa-19990608-ios-grs-acl
For Public Release 1999 June 8 15:00 UTC (GMT)
Vulnerability Scoring Details
Software Versions and Fixes
Obtaining Fixed Software
Exploitation and Public Announcements
Status of This Notice: Final
Cisco Security Procedures
Cisco 12000 series Gigabit Switch Routers running certain versions of Cisco IOS software forward unauthorized traffic due to an error encountered while processing the established keyword in an access-list statement. The resulting vulnerability could be exploited to circumvent a site's security policy.
Only Cisco Gigabit Switch Routers (currently the 12008 and 12012 GSRs) running Cisco IOS software release 11.2(14)GS2 through 11.2(15)GS3 are vulnerable.
This error is corrected in release 11.2(15)GS5 and later versions.
This error is not present in any version of Cisco IOS software release 12.0S and later. Non-GSR releases are not affected.
The bug ID associated with this error is CSCdm36197.
This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19990608-ios-grs-acl.
This section provides details on affected products.
A GSR running release 11.2(14)GS2 through 11.2(15)GS3 is vulnerable if the keyword established is used in an access-list statement.
The Cisco 12000 series Gigabit Switch Router is a large rack-mount device, approximately twenty to sixty inches (0.5 to 1.5meters) tall and twenty inches (0.5 meters) deep, that requires specialized power connections to supply forty to sixty amps of electricity. GSRs are typically used by major Internet Service Providers at their most important interconnection points. If you do not have a Cisco 12000 series GSR, then you are not affected by the vulnerability described in this notice.
A GSR running release 11.2(15)GS5 and later or any version of release 12.0S is not affected.
The Cisco 12000 series Gigabit Switch Router (GSR) is the only Cisco product that is affected by this vulnerability. Currently the 12008 GSR and the 12012 GSR are the only two models in the series.
No other Cisco products are currently known to be affected by these vulnerabilities.
When an affected Cisco Gigabit Switch Router (GSR) executes the following command on an interface:
access-list 101 permit tcp any any established
the established keyword is ignored. This will cause the GSR to forward all TCP traffic for the relevant interface, contrary to the restriction intended in the access-list statement.
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 vulnerability can be exploited to circumvent your security policy, resulting in unauthorized access to systems and unauthorized release of information. This may be inadvertent or intentional. Exploiting the flaw requires no special tools or knowledge. It can be determined if your system is vulnerable by attempting to exploit the vulnerability. It is not necessary to make an attempt if it can be determined that you are running one of the affected releases of software on a GSR and a copy of the configuration can be obtained or reverse-engineered.
This bug, documented as CSCdm36197, initially appears in 11.2(14)GS2, the first release of Cisco IOS software to support access lists on the GSR.
The bug is present in versions of Cisco IOS software from 11.2(14)GS2 to 11.2(15)GS3, inclusive. The earliest repaired version is 11.2(15)GS5.
If you are running any vulnerable version of 11.2GS and wish to resolve this problem with the least possible change to your existing version of software, you should upgrade to 11.2(15)GS5 or later.
This bug is not present in any release of 12.0S, so upgrading to 12.0S or later will also remove the vulnerability.
If you need the functionality provided by the established keyword for an access-list command, there is no reasonable workaround.
Customers may wish to consider modifying the policies on other network components, if possible, to limit exploitation of this vulnerability until such time as they have downloaded a fixed version of software to the affected GSR.
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 http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
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 http://www.cisco.com.
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.
Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.
Cisco knows of no public announcements or discussion of this vulnerability before the date of the public release of this notice. No incidents of malicious exploitation of this vulnerability have been reported to Cisco.
This vulnerability was reported to Cisco by a customer.
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This notice is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19990608-ios-grs-acl on Cisco's Worldwide Web site. In addition to Worldwide Web posting, the initial public version of this notice is being distributed via the following mailing lists and Usenet newsgroups:
email@example.com (includes CERT/CC)
Various internal Cisco mailing lists
Future updates of this advisory, 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 above URL for any updates.
Fix boilerplate problems.
Initial public release.
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