CVE-2026-76505

This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco IOS XR 64-bit Software and have IPv4 enabled for any interface on any of the installed line cards:

• ASR 9000 Series Aggregation Services Routers if they have a Lightspeed- or Lightspeed-Plus-based line card installed

• ASR 9900 Series Aggregation Services Routers if they have a Lightspeed- or Lightspeed-Plus-based line card installed

• ASR 9902 Compact High-Performance Routers

• ASR 9903 Compact High-Performance Routers

For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.

Determine Which Line Cards Are Installed

To determine which line cards are installed in a device, use the show platform CLI command.

Cisco ASR 9902 and ASR 9903 Compact High-Performance Routers integrate a Lightspeed-Plus-based line card.

The following line cards are Lightspeed-based:

• A9K-16X100GE-TR

• A99-16X100GE-X-SE

• A99-32X100GE-TR

• A9K-16X100GE-CM

• A99-32X100GE-CM

The following line cards are Lightspeed-Plus-based:

• A9K-4HG-FLEX-SE

• A9K-4HG-FLEX-TR

• A9K-8HG-FLEX-SE

• A9K-8HG-FLEX-TR

• A9K-20HG-FLEX-SE

• A9K-20HG-FLEX-TR

• A99-4HG-FLEX-SE

• A99-4HG-FLEX-TR

• A99-10X400GE-X-SE

• A99-10X400GE-X-TR

• A99-32X100GE-X-SE

• A99-32X100GE-X-TR

• A9K-20HG-FLEX-FC

• A9K-8HG-FLEX-FC

• A9K-4HG-FLEX-FC

• A99-4HG-FLEX-FC

• A99-32HG-FC

• A99-4T-FC

For more information about line card type identification, see Understand ASR 9000 Series Line Card Types.

Note: The Cisco Lightspeed-Plus list of product identifiers was accurate at the time of publication. For specific questions and further clarification about a product identifier, contact the Cisco Technical Assistance Center (TAC).

Determine Whether a Device Has a Vulnerable Configuration

A device is considered vulnerable if it has an IPv4 address configured. The following example shows a device that has a single interface configured with IPv4:

Router# show ipv4 vrf all interface brief
Wed Oct 21 16:00:00.000 UTC
TenGigE0/0/0/12                198.51.100.1      Up              Up       default 
Router#  

IPv6-only configurations are not affected by this vulnerability.

There are no workarounds that address this vulnerability. However, there is a mitigation.

To mitigate this vulnerability, filter UDP traffic that is destined to the infrastructure address space with UDP destination port 1701.

Configure an ingress infrastructure access control list (iACL) on any edge or external interfaces to drop UDP port 1701 traffic that is destined for local router addresses while continuing to forward transit traffic.

The following is an example snippet that should be incorporated into existing iACL statements:

ipv4 access-list ACL_INFRA_PROTECTION
 ! Permit required routing and management protocols
 permit ospf any any
 permit bgp any host 
 permit icmp any any echo-reply
 !
 ! Deny UDP 1701 destined ONLY to router interface/loopback IPs
 deny udp any host  eq 1701
 deny udp any host  eq 1701
 deny udp any   eq 1701
 !
 ! Permit all customer transit traffic
 permit ipv4 any any
!
interface 
 ipv4 access-group ACL_INFRA_PROTECTION ingress

While this mitigation has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations.

Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

Cisco considers any workarounds and mitigations (if applicable) to be temporary solutions until an upgrade to a fixed software release is available. To remediate this vulnerability and avoid future exposure as described in this advisory, Cisco strongly recommends that customers upgrade to the fixed software indicated in this advisory.

Fixed Releases

In the following table, the left column lists Cisco software releases or trains. The right column indicates whether a release (train) is affected by the vulnerability that is described in this advisory and the first release that includes the fix for this vulnerability.

SMUs are also available to address this vulnerability. Customers who require SMUs for platforms or releases that are not available are advised to contact their support organization. For more information on finding which SMUs may have been published for this vulnerability, see the Download section of Understand Cisco IOS XR Software Maintenance Updates (SMUs).

Note: These SMUs are In-Service Software Upgrade (ISSU)/Reload SMU install impact type.

The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.