Summary

• A vulnerability in the Border Gateway Protocol (BGP) additional paths feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to prevent authorized users from monitoring the BGP status and cause the BGP process to stop processing new updates, resulting in a denial of service (DOS) condition.

  The vulnerability is due to an incorrect calculation of lexicographical order when displaying additional path information within Cisco IOS XR Software, which causes an infinite loop. An attacker could exploit this vulnerability by sending a specific BGP update from a BGP neighbor peer session of an affected device; an authorized user must then issue a show bgp command for the vulnerability to be exploited. A successful exploit could allow the attacker to prevent authorized users from properly monitoring the BGP status and prevent BGP from processing new updates, resulting in outdated information in the routing and forwarding tables.  

  Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-ErKEqAer

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected Cisco IOS XR Software releases earlier than 7.1.2, 7.2.1, and 7.3.1.

  See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  To be vulnerable, the device needs to be configured to receive additional paths under any of the supported address families.

  Determine the Device Configuration

  To determine whether the device is configured to receive additional paths, check the output of the show running-config router bgp | include additional-paths receive command. The following example shows the output of the show running-config router bgp | include additional-paths receive command for a device that is vulnerable: 

  RP/0/RSP1/CPU0:9904#show run router bgp | include additional-paths receive
  Wed Aug 5 10:38:42.316 UTC
    additional-paths receive
  RP/0/RSP1/CPU0:9904#
  
  

  Products Confirmed Not Vulnerable

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

Details

• The information in the BGP update that causes this vulnerability is locally generated by the BGP neighbor peer session of the affected device. Cisco IOS Software, Cisco IOS XR Software, and Cisco NX-OS Software would never generate a BGP update that would result in the show bgp commands incorrectly calculating the lexicographical order when displaying additional path information. The BGP update would need to come from a BGP implementation other than one done by a Cisco network operating system.

  For a device to be affected by this vulnerability, both of the following conditions must be met:

  • The BGP update that causes this vulnerability is processed by the device and the data is inserted into the BGP table.
  • An authorized user executes show bgp commands that incorrectly calculate lexicographical order when displaying additional path information.

  Once show bgp commands are executed (as long as the offending data is in the BGP table), the affected device will return an error message that is similar to the following example:

  % Failed to retrieve data from EDM server: 'sysdb' detected the 'warning' condition 'An 
  EDM took too long to process a request and was timed out'

  Once the device is affected, further BGP updates will not be processed. While there is no immediate data plane loss, if there is network churn, information in future BGP updates will fail to update the BGP table and result in outdated routing and forwarding tables.

Workarounds

• There are no workarounds that address this vulnerability.

  Once the device is in the state that is caused by the vulnerability, restarting the BGP process will allow the device to continue to process BGP correctly. However, any additional show bgp commands will return the device to the vulnerable state.

Fixed Software

• When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

  Fixed Releases

  At the time of publication, Cisco IOS XR Software releases 7.1.2, 7.2.1, and 7.3.1 contained the fix for this vulnerability.

  At the time of publication, the SMUs in the following table were available. Customers who require SMUs for other platforms and releases are advised to contact their support organization.

  IOS XR Release	Platform	SMU ID
  6.5.3	ASR9K	AA17486

  See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

Source

• This vulnerability was found during the resolution of a Cisco TAC support case.

URL

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

Revision History

• Version	Description	Section	Status	Date
  1.1	Changed the first fixed release in the 7.1 train to 7.1.2 instead of 7.1.3.	Vulnerable Products, Fixed Software	Final	2020-AUG-05
  1.0	Initial public release.	—	Final	2020-AUG-05

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