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AUSCERT External Security Bulletin Redistribution
Cisco IOS XR Software Intermediate System-to-Intermediate
System Denial of Service Vulnerability
8 August 2019
AusCERT Security Bulletin Summary
Product: Cisco IOS XR
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Denial of Service -- Remote/Unauthenticated
CVE Names: CVE-2019-1918
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Cisco Security Advisory
Cisco IOS XR Software Intermediate System-to-Intermediate System Denial of
Advisory ID: cisco-sa-20190807-iosxr-isis-dos-1918
First Published: 2019 August 7 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvp90854
CVSS Score: Base 7.4
o 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:
A companion advisory is available at the following link:
o 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
address-family ipv4 unicast
address-family ipv4 unicast
For the device to be exploitable, it would need at least one formed
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.
o 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
o 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
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.
o 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
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
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:
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
Cisco fixed this vulnerability in Cisco IOS XR Software Release 6.6.3.
Cisco has also released the following software maintenance upgrades (SMUs)
that address this vulnerability:
IOS XR Release SMU ID Platform SMU Name
6.5.3 AA15778 ASR9000 asr9k-px-6.5.3.CSCvq45118^1
6.6.1 AA15881 NC56K ncs6k-6.6.1.CSCvq45118.tar^1
^1The SMUs contain the fixes for the vulnerabilities described in
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
o 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.
o This vulnerability was found during the resolution of a Cisco TAC support
Cisco Security Vulnerability Policy
o 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 |
| 1.0 | Initial public release. | -- | Final | 2019-August-07 |
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