A vulnerability in the implementation of the 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 crafted IS-IS link-state protocol data units (PDUs). An attacker could exploit this vulnerability by sending a crafted link-state PDU to an affected system to be processed. A successful exploit could allow the attacker to cause all routers within the IS-IS area to unexpectedly restart 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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190807-iosxr-isis-dos-1910 A companion advisory is available at the following link: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190807-iosxr-isis-dos-1918
A vulnerability in the implementation of the 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 crafted IS-IS link-state protocol data units (PDUs). An attacker could exploit this vulnerability by sending a crafted link-state PDU to an affected system to be processed. A successful exploit could allow the attacker to cause all routers within the IS-IS area to unexpectedly restart 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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190807-iosxr-isis-dos-1910
This vulnerability affects Cisco devices if they are running a vulnerable release of Cisco IOS XR Software earlier than Release 6.6.3 and are configured with the IS-IS routing protocol.
Cisco has confirmed that this vulnerability affects both Cisco IOS XR 32-bit Software and Cisco IOS XR 64-bit Software.
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.
Vulmon
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
!
The following output includes two adjacencies. 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
!
Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.
If exploitation occurs, the routers in the IS-IS level can be recovered by removing the corrupt link-state PDU from all IS-IS routers in the area. Customers who experience an outage due to this vulnerability are advised to contact their support organization.
Cisco recommends that customers ensure the authentication of routers within the IS-IS area for adjacency establishment (Hello packets) and for link-state PDUs before implementing IS-IS. If this best practice is followed, then only an authenticated attacker within the same IS-IS area who sends the crafted IS-IS link-state PDU could exploit this vulnerability.
Cisco also recommends that customers enable Non-Stop Forwarding and/or Non-Stop Routing.
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.
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.
At time of publication, Cisco had fixed this vulnerability in Cisco IOS XR Software Releases 6.6.25 and 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:
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.
This vulnerability was found during internal security testing.
To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
| Version | Description | Section | Status | Date |
|---|---|---|---|---|
| 1.1 | Added information about SMU availability. | Fixed Software | Interim | 2019-August-21 |
| 1.0 | Initial public release. | - | Final | 2019-August-07 |
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