Summary

• Cisco IOS XR Software contains multiple vulnerabilities in the Border Gateway Protocol (BGP) feature. These vulnerabilities include:

  • Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update.
    The vulnerability manifests when a BGP peer announces a prefix with a specific invalid attribute. On receipt of this prefix, the Cisco IOS XR device will restart the peering session by sending a notification. The peering session will flap until the sender stops sending the invalid/corrupt update. This vulnerability was disclosed in revision 1.0 of this advisory.
    
    
  • Cisco IOS XR BGP process will crash when sending a long length BGP update message
    When Cisco IOS XR sends a long length BGP update message, the BGP process may crash. 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.
    
    
  • 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 Autonomous System (AS) Numbers to the AS path, the BGP process will crash. The number of AS numbers required to be prepended and cause the crash are well above normal limits seen within production environments.
    
    

  All three vulnerabilities are different vulnerabilities from what was disclosed in the Cisco Security Advisory "Cisco IOS Software Border Gateway Protocol 4-Byte Autonomous System Number Vulnerabilities" on the 2009 July 29 1600 UTC at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090729-bgp.

  Cisco has released a free software maintenance upgrade (SMU) that addresses these vulnerabilities.

  Workarounds that mitigates these vulnerabilities are available.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090818-bgp

Affected Products

• 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

  To determine the Cisco IOS XR Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The system banner confirms that the device is running Cisco IOS XR Software by displaying text similar to "Cisco IOS XR Software". The software version is displayed after the text "Cisco IOS XR Software".

  The following example identifies a Cisco CRS-1 that is running Cisco IOS XR Software Release 3.6.2:

  RP/0/RP0/CPU0:CRS#show version
  Tue Aug 18 14:25:17.407 AEST
  
  Cisco IOS XR Software, Version 3.6.2[00]
  Copyright (c) 2008 by Cisco Systems, Inc.
  
  ROM: System Bootstrap, Version 1.49(20080319:195807) [CRS-1 ROMMON],
  
  CRS uptime is 4 weeks, 4 days, 1 minute
  System image file is "disk0:hfr-os-mbi-3.6.2/mbihfr-rp.vm"
  
  cisco CRS-8/S (7457) processor with 4194304K bytes of memory.
  7457 processor at 1197Mhz, Revision 1.2
  
  17 Packet over SONET/SDH network interface(s)
  1 DWDM controller(s)
  17 SONET/SDH Port controller(s)
  8 TenGigabitEthernet/IEEE 802.3 interface(s)
  2 Ethernet/IEEE 802.3 interface(s)
  1019k bytes of non-volatile configuration memory.
  38079M bytes of hard disk.
  981440k bytes of ATA PCMCIA card at disk 0 (Sector size 512 bytes).
  
  Configuration register on node 0/0/CPU0 is 0x102
  Boot device on node 0/0/CPU0 is mem:
  
  
  !--- output truncated
  
  

  The following example identifies a Cisco 12404 router that is running Cisco IOS XR Software Release 3.7.1:

  RP/0/0/CPU0:GSR#show version
  
  Cisco IOS XR Software, Version 3.7.1[00]
  Copyright (c) 2008 by Cisco Systems, Inc.
  
  ROM: System Bootstrap, Version 12.0(20051020:160303) SOFTWARE
  Copyright (c) 1994-2005 by cisco Systems,  Inc.
  
  GSR uptime is 3 weeks, 6 days, 3 hours, 20 minutes
  System image file is "disk0:c12k-os-mbi-3.7.1/mbiprp-rp.vm"
  
  cisco 12404/PRP (7457) processor with 2097152K bytes of memory.
  7457 processor at 1266Mhz, Revision 1.2
  
  1 Cisco 12000 Series Performance Route Processor
  1 Cisco 12000 Series - Multi-Service Blade Controller
  1 1 Port ISE Packet Over SONET OC-48c/STM-16 Controller (1 POS)
  1 Cisco 12000 Series SPA Interface Processor-601/501/401
  3 Ethernet/IEEE 802.3 interface(s)
  1 SONET/SDH Port controller(s)
  1 Packet over SONET/SDH network interface(s)
  4 PLIM QoS controller(s)
  8 FastEthernet/IEEE 802.3 interface(s)
  1016k bytes of non-volatile configuration memory.
  1000496k bytes of disk0: (Sector size 512 bytes).
  65536k bytes of Flash internal SIMM (Sector size 256k).
  
  Configuration register on node 0/0/CPU0 is 0x2102
  Boot device on node 0/0/CPU0 is disk0:
  
  
  !--- output truncated
  
  

  Additional information about Cisco IOS XR Software release naming conventions is available in the "White Paper: Cisco IOS Reference Guide" at the following link: http://www.cisco.com/warp/public/620/1.html#t6.

  Additional information about Cisco IOS XR Software time-based release model is available in the "White Paper: Guidelines for Cisco IOS XR Software" at the following link: http://www.cisco.com/en/US/prod/collateral/iosswrel/ps8803/ps5845/product_bulletin_c25-478699.html.

  BGP is configured in Cisco IOS XR Software with the configuration command router bgp [AS Number] or router bgp [X.Y]. The device is vulnerable if it is running an affected Cisco IOS XR Software version and has BGP configured.

  The following example shows a Cisco IOS XR Software device configured with BGP:

  RP/0/0/CPU0:GSR#show running-config | begin router bgp
  Building configuration...
  router bgp 65535
   bgp router-id 192.168.0.1
   address-family ipv4 unicast
    network 192.168.1.1/32
   !
   address-family vpnv4 unicast
   !
   neighbor 192.168.2.1
    remote-as 65534
    update-source Loopback0
    address-family ipv4 unicast
    !
  
  
  !--- output truncated
  
  

  Products Confirmed Not Vulnerable

  The following Cisco products are confirmed not vulnerable:

  • Cisco IOS Software
    
  • Cisco IOS XR Software not configured for BGP routing
    

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• 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)

Workarounds

• 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        
    
    

    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.
    

Fixed Software

• 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.

  Cisco IOS XR Version	SMU ID	SMU Name
  3.2.X	Vulnerable to BGP process crash vulnerabilities; Migrate to 3.4.1 or later.
  3.3.X	Vulnerable to BGP process crash vulnerabilities; Migrate to 3.4.1 or later.
  3.4.0	Vulnerable; Migrate to 3.4.1 or later.
  3.4.1	AA03400 AA03414	hfr-rout-3.4.1.CSCtb18562 c12k-rout-3.4.1.CSCtb18562
  3.4.2	AA03399 AA03413	hfr-rout-3.4.2.CSCtb18562 c12k-rout-3.4.2.CSCtb18562
  3.4.3	AA03398 AA03412	hfr-rout-3.4.3.CSCtb18562 c12k-rout-3.4.3.CSCtb18562
  3.5.2	AA03397 AA03411	hfr-rout-3.5.2.CSCtb18562 c12k-rout-3.5.2.CSCtb18562
  3.5.3	AA03410 AA03396	c12k-rout-3.5.3.CSCtb18562 hfr-rout-3.5.3.CSCtb18562
  3.5.4	AA03409 AA03395	c12k-rout-3.5.4.CSCtb18562 hfr-rout-3.5.4.CSCtb18562
  3.6.0	AA03394 AA03408	hfr-rout-3.6.0.CSCtb18562 c12k-rout-3.6.0.CSCtb18562
  3.6.1	AA03407 AA03393	c12k-rout-3.6.1.CSCtb18562 hfr-rout-3.6.1.CSCtb18562
  3.6.2	AA03406 AA03392	c12k-rout-3.6.2.CSCtb18562 hfr-rout-3.6.2.CSCtb18562
  3.6.3	AA03405 AA03391	c12k-rout-3.6.3.CSCtb18562 hfr-rout-3.6.3.CSCtb18562
  3.7.0	AA03390 AA03404	hfr-rout-3.7.0.CSCtb18562 c12k-rout-3.7.0.CSCtb18562
  3.7.1	AA03389 AA03403	hfr-rout-3.7.1.CSCtb18562 c12k-rout-3.7.1.CSCtb18562
  3.7.2	AA03386	asr9k-rout-3.7.2.CSCtb18562
  3.7.3	AA03385	asr9k-rout-3.7.3.CSCtb18562
  3.8.0	AA03388 AA03402	hfr-rout-3.8.0.CSCtb18562 c12k-rout-3.8.0.CSCtb18562
  3.8.1	AA03401 AA03387	hfr-rout-3.8.1.CSCtb18562 c12k-rout-3.8.1.CSCtb18562

Exploitation and Public Announcements

• 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.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090818-bgp

Revision History

• Revision 2.6	2009-August-27	Minor revision to software table
  Revision 2.5	2009-August-24	Added final SMUs to the Software Table.
  Revision 2.4	2009-August-23	Added newly available SMUs to the Software Table.
  Revision 2.3	2009-August-22	Added newly available SMUs to the Software Table.
  Revision 2.2	2009-August-21	Added newly available SMUs to the Software Table.
  Revision 2.1	2009-August-20	Added currently available SMUs to the Software Table and separated CVSS tables.
  Revision 2.0	2009-August-20	Major update to include all bugs in Umbrella fix.
  Revision 1.0	2009-August-18	Initial public release.

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