Summary

• A vulnerability in the access control list (ACL) processing on MPLS interfaces in the ingress direction of Cisco IOS XR Software could allow an unauthenticated, remote attacker to bypass a configured ACL.

  This vulnerability is due to incomplete support for this feature. An attacker could exploit this vulnerability by attempting to send traffic through an affected device. A successful exploit could allow the attacker to bypass an ACL on the affected device.

  Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-dnx-acl-PyzDkeYF

  This advisory is part of the September 2023 release of the Cisco IOS XR Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: September 2023 Semiannual Cisco IOS XR Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected the following Cisco products if they were running a vulnerable release of Cisco IOS XR Software and had MPLS packet filtering enabled with the explicit-null or de-aggregation label on the ingress direction:

  • IOS XR White box (IOSXRWBD)
  • Network Convergence Series (NCS) 540 Series Routers
  • NCS 560 Series Routers
  • NCS 5500 Series
  • NCS 5700 Series

  For information about which Cisco software releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Determine Whether Filtering of MPLS Packets on the Ingress Direction is Enabled

  To determine whether filtering of MPLS packets on the ingress direction is enabled, follow this two-step process:

  1. Identify all MPLS interfaces.
  2. Check the configuration to see if an IPv4 or IPv6 ACL is configured for the ingress direction.

  The following example shows an MPLS interface TenGigE0/0/0/0 that has both an IPv4 and an IPv6 ACL configured on the ingress direction. In the show mpls interfaces, if the Enabled column has a Yes present, then MPLS is enabled on that interlace:

  RP/0/RP0/CPU0:NCS5501-1##show mpls interfaces
  Thu Mar 16 02:47:56.142 UTC
  Interface                  LDP      Tunnel   Static   Enabled
  -------------------------- -------- -------- -------- --------
  TenGigE0/0/0/0             No       No       No       Yes
  TenGigE0/0/0/1             No       No       No       Yes
  RP/0/RP0/CPU0:NCS5501-1#
  
  

  The device is affected by this vulnerability if any of the interfaces in the preceding example have either an IPv4 or IPv6 ingress ACL applied, as shown in the following example:

  RP/0/RP0/CPU0:NCS5501-1#show run interface TenGigE0/0/0/0
  !
  interface TenGigE0/0/0/0
   description ** Example where IPv4 and IPv6 ACL ingress applied **
   ipv4 address 192.168.12.1 255.255.255.0
   ipv4 access-group CVE-2023-20191 ingress
   ipv6 access-group CVE-2023-20191 ingress
  !
  RP/0/RP0/CPU0:NCS5501-1#

  IP ingress ACL filtering on MPLS interfaces is not currently supported on any other Cisco IOS XR Platforms.

  Products Confirmed Not Vulnerable

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

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • IOS Software
  • IOS XE Software
  • NX-OS Software

Details

• Exploitation of this vulnerability could allow an attacker to bypass protections that are provided by an ACL that is applied on an affected device. The overall impact of exploitation is organization-specific because it depends on the importance of the assets that the ACL was supposed to protect. Customers should evaluate how exploitation of this vulnerability would impact their network and proceed according to their own vulnerability-handling and remediation processes.

  The vulnerability exists because filtering of MPLS packets with the explicit-null or de-aggregation label on the ingress direction is not supported on any Cisco IOS XR platforms. However, support was added for products listed in the Vulnerable Products section of this advisory. When the affected device receives the labeled packet and pops the label, it then attempts to process the configured ingress ACL. If an ACL deny entry is hit, the packet is punted to generate an ICMP unreachable packet that is to be returned. Rather than dropping the packet, this path then forwards the denied packet to the destination.

Workarounds

• There is a workaround that addresses this vulnerability.

  Customers could remove the ingress ACLs from the MPLS-enabled interfaces and use egress ACLs in their environment. Egress hybrid ACLs are supported in Cisco IOS XR releases 7.6.2 and later.

  While this workaround 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.

Fixed Software

• 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

  At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability that is described in this advisory and which release included the fix for this vulnerability.

  Cisco IOS XR Software Release	First Fixed Release
  6.3 and earlier	Not Affected.
  6.4 - 6.6	Migrate to a fixed release.
  7.0 - 7.6	Migrate to a fixed release or apply available SMU.
  7.7	7.7.21
  7.8	Migrate to a fixed release.
  7.9	7.9.2
  7.10	7.10.1

  Cisco has released the following SMUs to address this vulnerability:

  Cisco IOS XR Software Release	Platform	SMU Name
  7.0.1	NCS5500	ncs5500-7.0.1.CSCwe63504
  7.2.1	IOSXRWBD	iosxrwbd-7.2.1.CSCwe63504
  7.4.15	IOSXRWBD	iosxrwbd-7.4.15.CSCwe63504
  7.7.2	IOSXRWBD	iosxrwbd-7.7.2.CSCwe63504
  7.7.2	NCS540L	ncs540l-aarch64-7.7.2.CSCwe63504
  7.7.2	NCS5500	ncs5500-7.7.2.CSCwe63504

  Note: Customers who require SMUs for releases that are not listed in the following table are advised to contact their support organization.

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

Exploitation and Public Announcements

• The Cisco 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-dnx-acl-PyzDkeYF

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2023-SEP-13

  Show Less

---