Summary

• A vulnerability in the implementation of Intermediate System–to–Intermediate System (IS–IS) routing protocol functionality in Cisco IOS XR Software could allow an unauthenticated attacker who is in the same IS-IS area to cause a denial of service (DoS) condition.

  The vulnerability is due to incorrect processing of IS–IS link-state protocol data units (PDUs). An attacker could exploit this vulnerability by sending specific link-state PDUs to an affected system to be processed. A successful exploit could allow the attacker to cause incorrect calculations used in the weighted remote shared risk link groups (SRLG) or in the IGP Flexible Algorithm. It could also cause tracebacks to the logs or potentially cause the receiving device to crash the IS–IS process, resulting in a DoS condition.

  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-20190807-iosxr-isis-dos-1918

  A companion advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190807-iosxr-isis-dos-1910

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco devices if they are running Cisco IOS XR Software releases later than 6.5.1 and earlier than 6.6.3.
  
  Cisco has confirmed that this vulnerability affects both Cisco IOS XR 32-bit Software and Cisco IOS XR 64-bit Software.

  Determining Whether the Device Has a Vulnerable Configuration

  To determine if the device is configured for IS–IS routing, administrators can use the show running-config router isis EXEC command in the CLI. If the router is configured with IS–IS routing, it will return output. The following example shows the output of the show running-config router isis command on a device that is configured as a level 1 IS–IS router.

  The device is affected if it is configured as one of the following: is-type level-1, level-1-2 or level-2-only.

  RP/0/RSP1/CPU0:IOSXR#show running-config router isis
  Sun May 26 05:20:23.088 UTC
  router isis core
  is-type level-1
  net 49.0001.1921.6825.5091.00
  address-family ipv4 unicast
  !
  interface TenGigE0/0/0/1
    address-family ipv4 unicast
   !

  For the device to be exploitable, it would need at least one formed IS–IS adjacency.

  RP/0/RSP1/CPU0:IOSXR#show isis adjacency
  
  IS-IS p Level-1 adjacencies:
  System Id      Interface        SNPA           State Hold     Changed  NSF      BFD
  12a4           PO0/1/0/1        *PtoP*         Up    23       00:00:06 Capable  Init
  12a4           TenGigE0/0/0/1   0004.2893.f2f6 Up    56       00:04:01 Capable  Up
  
  Total adjacency count: 2
   !

  Products Confirmed Not Vulnerable

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

Details

• Cisco recommends that customers ensure the authentication of routers within the IS–IS area for adjacency establishment (Hello packets) and for link-state PDUs when implementing IS–IS. If this best practice is followed, then only an authenticated adjacency within the same IS-IS area that sends the specific IS–IS link-state PDU could exploit this vulnerability.

  Cisco also recommends that customers enable Non-Stop Forwarding and/or Non-Stop Routing.

Workarounds

• There are no workarounds that address this vulnerability. However, following Cisco best practices and ensuring that IS–IS authentication is enabled for adjacencies (Hello packets) and link-state PDUs would restrict this vulnerability to only authenticated link-state PDUs. For example:

  router isis core
  lsp-password hmac-md5 encrypted 13061E010803
  interface TenGigE0/0/0/6
    hello-password hmac-md5 encrypted 094F471A1A0A
  !
  

  For more information on IS–IS Authentication, see the IS–IS Authentication chapter of Implementing IS–IS on Cisco IOS XR Software.

Fixed Software

• Cisco has released free software updates that address the vulnerability described in this advisory. Customers may only install and expect support for software versions and feature sets for which they have purchased a license. By installing, downloading, accessing, or otherwise using such software upgrades, customers agree to follow the terms of the Cisco software license:
  https://www.cisco.com/c/en/us/products/end-user-license-agreement.html

  Additionally, customers may only download software for which they have a valid license, procured from Cisco directly, or through a Cisco authorized reseller or partner. In most cases this will be a maintenance upgrade to software that was previously purchased. Free security software updates do not entitle customers to a new software license, additional software feature sets, or major revision upgrades.

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories and Alerts 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.

  Customers Without Service Contracts

  Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC:
  https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html

  Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade.

  Fixed Releases

  At time of publication, Cisco had fixed this vulnerability in Cisco IOS XR Software Release 6.6.3.

  At time of publication, the SMUs in the following table were available. This table will be updated if any new SMUs are released. Customers who require SMUs for other platforms and releases are advised to contact their support organization.

  IOS XR Release	SMU ID	Platform	SMU Name
  6.5.3	AA15778	ASR9000	asr9k-px-6.5.3.CSCvq451181
  6.6.1	AA15881	NC56K	ncs6k-6.6.1.CSCvq45118.tar1

  ¹The SMUs contain the fixes for the vulnerabilities described in cisco-sa-20190807-iosxr-isis-dos-1910 and cisco-sa-20190807-iosxr-isis-dos-1918 advisories.

  Customers can download the SMUs from the Software Center on Cisco.com by doing the following:

  1. Click Browse All.
  2. Choose IOS and NX-OS Software > IOS XR > IOS XR Software > Optical Networking or Routers.
  3. Choose the appropriate product and model.
  4. Access releases by using the left pane of the appropriate product page.

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-20190807-iosxr-isis-dos-1918

Revision History

• Version	Description	Section	Status	Date
  1.1	Added details about available SMUs.	Fixed Software	Interim	2019-August-21
  1.0	Initial public release.	—	Final	2019-August-07

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