CVE-2021-1389: Cisco Security Advisory: Cisco IOS XR and Cisco NX-OS Software IPv6 Access Control List Bypass Vulnerability

Cisco IOS XR Software

After being upgraded to a fixed release of Cisco IOS XR Software, the device is able to detect IPv6 packets that may be improperly processed. Once detected, the device sends these packets to the main CPU for further processing. This activity may result in reduced performance in forwarding IPv6 packets that match specific network traffic patterns.

If the device is not configured to perform IPv6 packets classification, administrators can disable the CPU-based inspection for IPv6 packets. The feature can be disabled by using the CLI command hw-module profile acl ipv6 ext-header permit in global configuration mode. Customers should be aware that this action will expose the device to the vulnerability that is described in the advisory, even if the device is running a fixed Cisco IOS XR Software release.

The vulnerability described in this advisory only applies to IPv6 packets that traverse an affected device. It does not apply to IPv4 traffic or to IPv6 traffic that is destined for an affected device.

Cisco NX-OS Software

To protect a device from this vulnerability, administrators must install a fixed release of Cisco NX-OS Software and apply the rule extension-header deny-all to any IPv6 ACL that is configured on the device. A device should be considered vulnerable until the rule extension-header deny-all has been applied to all IPv6 ACLs that are configured on the device, even if it is running a fixed Cisco NX-OS Software release.

Beginning with Cisco NX-OS Release 9.3(7), Cisco Nexus 3600 Platform Switches and Cisco Nexus 9500 R-Series Switching Platforms include the rule extension-header {permit-all | deny-all} for the disposition of IPv6 packets that include extension headers. The rule is not enabled by default. With the rule extension-header deny-all configured, the device will drop any IPv6 packet with at least one extension header, regardless of any other IPv6 ACL rules that match other fields of the packet.

If the rule extension-header permit-all is configured, then the device is vulnerable.

For detailed information about configuring extension-header {permit-all | deny-all}, see the Cisco Nexus 3600 NX-OS Unicast Routing Configuration Guide or the Cisco Nexus 9000 Series NX-OS Unicast Routing Configuration Guide.

For detailed information about configuring ACLs, see the Cisco Nexus 3600 NX-OS Security Configuration Guide or the Cisco Nexus 9000 Series NX-OS Security Configuration Guide.

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

Cisco IOS XR Software

At the time of publication, Cisco IOS XR Software releases 6.6.3, 6.7.1, 7.1.1, 7.2.1, and later contained the fix for this vulnerability.

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

Cisco NX-OS Software

After upgrading a device to a fixed release of Cisco NX-OS Software, customers must apply the rule extension-header deny-all to any IPv6 ACL that is configured on the device. The rule is not enabled by default. A device should be considered vulnerable until the rule extension-header deny-all has been applied to all IPv6 ACLs that are configured on the device, even if it is running a fixed Cisco NX-OS Software release.

For detailed information about configuring the extension-header deny-all rule, see the Cisco Nexus 3600 NX-OS Unicast Routing Configuration Guide or the Cisco Nexus 9000 Series NX-OS Unicast Routing Configuration Guide.

To help customers determine their exposure to vulnerabilities in Cisco NX-OS Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific Cisco NX-OS Software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities described in all the advisories identified (“Combined First Fixed”).

Customers can use the Cisco Software Checker to search advisories in the following ways:

• Choose the software, platform, and one or more releases
• Upload a .txt file that includes a list of specific releases
• Enter the output of the show version command

After initiating a search, customers can customize the search to include all Cisco Security Advisories or one or more specific advisories.

Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by choosing the Cisco NX-OS Software and platform and then entering a release-for example, 7.0(3)I7(5) for Cisco Nexus 3000 Series Switches or 14.0(1h) for Cisco NX-OS Software in ACI mode:

By default, the Cisco Software Checker includes results only for vulnerabilities that have a Critical or High Security Impact Rating (SIR). To include results for Medium SIR vulnerabilities, customers can use the Cisco Software Checker and check the Medium check box in the drop-down list under Impact Rating when customizing a search.