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This vulnerability affects Cisco IOS XR devices running affected software versions and configured with the BGP routing feature.

The vulnerability manifests itself when a BGP peer announces a prefix with a specific, valid but unrecognized transitive attribute. On receipt of this prefix, the Cisco IOS XR device will corrupt the attribute before sending it to the neighboring devices. Neighboring devices that receive this corrupted update may reset the BGP peering session.

Affected devices running Cisco IOS XR Software corrupt the unrecognized attribute before sending to neighboring devices, but neighboring devices may be running operating systems other than Cisco IOS XR Software and may still reset the BGP peering session after receiving the corrupted update. This is per RFC 4271 that defines the operation of BGP.

After an affected device running Cisco IOS XR Software sends a corrupted update, it will receive a notification from the neighboring router and will create a log message like the following example:

bgp[122]: %ROUTING-BGP-5-ADJCHANGE : neighbor 172.16.1.251 Down - BGP Notification received: update malformed

This vulnerability is documented in Cisco Bug ID CSCti62211 ( registered customers only) and has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2010-3035.

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

http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html .

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

http://tools.cisco.com/security/center/cvssCalculator.x .

Successful exploitation of these vulnerabilities may result in the continuous resetting of BGP peering sessions. This may lead to routing inconsistencies and a denial of service for those affected networks.

When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.

In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco Technical Assistance Center (TAC) or your contracted maintenance provider for assistance.

* Restart of the BGP process in 3.9.1 may trigger a separate issue and result in RIB-failure if BGP is configured for attribute-download. This issue is specific to 3.9.1 and only surfaces if attribute-download is configured. Please refer to Cisco Bug ID CSCti08859 ( registered customers only) for more details and workaround.

Please note that applying the above mentioned SMUs will require a restart of the BGP process. Some of the SMUs will also require a system reboot. This is indicated in the fourth column in the above table.

Installation of the SMU needs to performed in admin mode. An example is shown in the following output:

RP/0/RP1/CPU0:crs#admin
RP/0/RP1/CPU0:crs(admin)#install add ftp://server/hfr-rout-3.9.1.CSCti62211.pie
RP/0/RP1/CPU0:crs(admin)#install activate disk0:hfr-rout-3.9.1.CSCti62211-1.0.0 

There are no workarounds to proactively mitigate this vulnerability. If a route flap is observed, the prefix with the unrecognized attribute can be filtered. For further information on filtering on Cisco IOS XR Software, please consult the document "Implementing Routing Policy on Cisco IOS XR Software" at the following link: http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.0/routing/configuration/guide/rc3rpl.html#wp1118699.

Cisco is releasing software updates that address these vulnerabilities. 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/en/US/docs/general/warranty/English/EU1KEN_.html , or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml .

Do not contact psirt@cisco.com or security-alert@cisco.com for software upgrades.

Customers with Service Contracts

Customers using Third Party Support Organizations

Customers without Service Contracts

An advertisement of an unrecognized but valid BGP attribute resulted in resetting of several BGP neighbors on 27 August 2010. This advertisement was not malicious but inadvertently triggered this vulnerability.

The Cisco PSIRT is not aware of malicious use of the vulnerability described in this advisory.

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

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 :

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100827-bgp

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.

• cust-security-announce@cisco.com
  
• first-bulletins@lists.first.org
  
• bugtraq@securityfocus.com
  
• vulnwatch@vulnwatch.org
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• full-disclosure@lists.grok.org.uk
  
• comp.dcom.sys.cisco@newsgate.cisco.com
  

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.

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 http://www.cisco.com/web/about/security/psirt/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.