Cisco IOS XR Software Border Gateway Protocol Vulnerabilities

The "Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update" vulnerability affects all Cisco IOS XR Software devices after and including software release 3.4.0 configured with BGP routing.

The other two vulnerabilities affect all Cisco IOS XR Software devices configured with BGP routing.

Vulnerable Products

Products Confirmed Not Vulnerable

These vulnerabilities affect Cisco IOS XR devices running affected software versions and configured with the BGP routing feature. Details per vulnerability are outlined below:

• Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update.
  On August 17th, 2009, a widely-distributed Border Gateway Protocol (BGP) route update contained a BGP Update message with a specific invalid attribute. When the invalid BGP Update message was processed by Cisco IOS XR Software, it began resetting BGP peering sessions over which the update was received.
  
  When a affected device receives the invalid/corrupt update, Cisco IOS XR Software will create a log message like the following example:
  
  

  RP/0/RP0/CPU0:Aug 17 13:47:05.896 GMT: bgp[122]: %ROUTING-BGP-5-ADJCHANGE : neighbor 192.168.0.1 Down - BGP Notification sent: invalid or corrupt AS path 

  The peering session will flap until the sender stops sending the invalid/corrupt BGP update message.
  
  This vulnerability is documented in Cisco Bug ID CSCtb42995 ( registered customers only) and has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2009-2055.
  
  
• Cisco IOS XR BGP process will crash when sending a long length BGP update message
  The BGP process on an affected Cisco IOS XR device may reload when sending a long length BGP update. The number of AS numbers required to exceed the total/maximum length of update message and cause the crash are well above normal limits seen within production environments.
  
  When an affected device BGP process crashes because of this long length BGP update message, Cisco IOS XR Software may create a log message like the following example:
  

  bgp[122]: %ROUTING-BGP-3-INTERNAL_ERROR : [10] : Internal error (Write buffer too small to generate update)

  The above error message is not always displayed and the BGP process may crash before IOS XR has the chance to generate the error message.
  This vulnerability is documented in Cisco Bug ID CSCtb05382 ( registered customers only) and has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2009-1154.
  
  
• Cisco IOS XR BGP process will crash when constructing a BGP update with a large number of AS prepends
  If the Cisco IOS XR BGP process is configured to prepend a very large number of AS Numbers to the AS path, the BGP process will crash. The number of AS numbers required to be prepended to cause the crash are well above normal limits seen within production environments.
  
  An example of AS path prepending in Cisco IOS XR Software is shown below:
  

  route-policy prepend-example
  	prepend as-path 65534 3
  	prepend as-path 65531 2
  end-policy
  
  router bgp 65534
    neighbor 192.168.0.1 
    remote-as 65531
    address-family ipv4 unicast 
      route-policy prepend-example out

  When an affected device BGP process crashes because of this large AS path prepend, no log message will be generated, prior to the crash.
  
  This vulnerability is documented in Cisco Bug ID CSCtb12726 ( registered customers only) and has been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2009-2056.
  

The above three vulnerabilities have been fixed in a single SMU and released under an umbrella Cisco Bug ID CSCtb18562 ( registered customers only)

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://intellishield.cisco.com/security/alertmanager/cvss .

Successful exploitation of these vulnerabilities may result in the continuous resetting of BGP peering sessions, or the continuous resetting of the BGP process itself. 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.

Note: Currently the SMUs are being posted to Cisco.com. This section will be updated accordingly once the SMUs are available for download.

Each individual vulnerability has a different workaround. Following are the mitigations and workarounds recommended for these vulnerabilities, prior to applying a SMU or software upgrade. The workarounds should be applied to both eBGP and iBGP peers.

• Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update.
  There are no workarounds on the affected device itself. Co-ordination is required with the peering neighbor support staff to filter the invalid update on their outbound path. The following procedure explains how to help mitigate this vulnerability:
  
  Using the peer IP address in the log message that was generated when the Cisco IOS XR Software device received the invalid update; capture the notification message hex dump from the CLI command show bgp neighbor and contact the Cisco TAC, who can assist with a decode. Details on how to contact Cisco TAC are contained within the "Obtaining Fixed Software" section of this advisory.
  
  For illustrative purposes, the following example shows a log message generated by an affected device when it receives an invalid/corrupt update message:
  
  

  RP/0/RP0/CPU0:Aug 17 13:47:05.896 GMT: bgp[122]: %ROUTING-BGP-5-ADJCHANGE : neighbor 192.168.0.1 Down - BGP Notification sent: invalid or corrupt AS path 

  These details can be captured and provided to Cisco TAC to decode the update message. show bgp neighbors [ip address of neighbor from above log message]:
  

  RP/0/RP0/CPU0:CRS#show bgp neighbors 192.168.0.1        
  
  <capture output and provide to Cisco TAC>

  Working with Cisco TAC, the decode of the above will display the AS path in a manner illustrated below.
  

  ATTRIBUTE NAME:  AS_PATH
  
   AS_PATH: Type 2 is AS_SEQUENCE
   AS_PATH: Segment Length is 4 (0x04) segments long
   AS_PATH: 65533  65532 65531 65531

  Working cooperatively with your peering partner, request that they filter outbound prefix advertisements from the identified source AS (in this example 65531) for your peering session. The filters configuration methods will vary depending on the routing device operating system used. For Cisco IOS XR Software the filters will be applied using Routing Policy Language (RPL) policies or with Cisco IOS Software via applying route-maps that deny advertisements matching that AS in their AS-PATH. Once these policies are applied, the peering session will be re-established.
  
  For further information on Cisco IOS XR RPL 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.
  
  For further information on Cisco IOS route maps with BGP, consult the document "Cisco IOS BGP Configuration Guide, Release 12.4T" at the following link: http://www.cisco.com/en/US/docs/ios/12_2sr/12_2srb/feature/guide/tbgp_c.html.
  
  
• Cisco IOS XR BGP process will crash when sending a long length BGP update message
  While the long length BGP update message can be caused by any number of attributes, then most common would be the AS Path. Limiting the number of AS numbers in the AS Path Attribute should mitigate this vulnerability. The following shows an example of filtering on AS paths within Cisco IOS XR Software:
  

  route-policy maxas-limit
  # Check number of AS Numbers in AS Path attribute.
  # If greater than 100 drop the update.
  # If less than 100 pass the update.
    if as-path length ge 100 then 
          drop
    else
          pass
    endif
  end-policy
  
  router bgp 65533
   neighbor 192.168.0.1
    remote-as 65534
     address-family ipv4 unicast
       policy maxas-limit in
       policy maxas-limit out

  For further information on Cisco IOS XR RPL 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 IOS XR BGP process will crash when constructing a BGP update with a large number of AS prepends
  There is no workaround for this vulnerability, other than reducing the number of AS path prepends configured within Cisco IOS XR.
  

Cisco will be releasing free 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

On August 17, 2009 around 16:30-17:00 UTC several ISP's began experiencing connectivity issues as BGP sessions were being repeatedly reset, which corresponds to the vulnerability "Cisco IOS XR will reset a BGP peering session when receiving a specific invalid BGP update" disclosed in this advisory. Cisco TAC was engaged with a number of customers all seeing similar issues. Stability came a few hours afterward as workarounds were applied. At this time, it is not believed that the connectivity issues were the result of malicious activity.

The other two BGP process crash vulnerabilities were discovered by Cisco during internal negative testing.

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This advisory is posted on Cisco's worldwide website at :

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090818-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.