Summary

• A vulnerability in the classic access control list (ACL) compression feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to bypass the protection that is offered by a configured ACL on an affected device.

  This vulnerability is due to incorrect destination address range encoding in the compression module of an ACL that is applied to an interface of an affected device. An attacker could exploit this vulnerability by sending traffic through the affected device that should be denied by the configured ACL. A successful exploit could allow the attacker to bypass configured ACL protections on the affected device, allowing the attacker to access trusted networks that the device might be protecting.

  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-comp3acl-vGmp6BQ3

  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 Cisco products if they were running a vulnerable release of Cisco IOS XR Software and had a classic IPv4 ACL with either level 2 or level 3 compression applied. Hybrid ACLs with the use of object-groups were not affected.

  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 the Device Configuration

  To determine if an IPv4 ACL has either level 2 or 3 compression applied, use the show running-config | include "compress level 2|compress level 3" CLI command. If the command returns output, the device is affected by this vulnerability. However, the impact depends on the configuration of the ACL. See details for more information.

  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

• Hybrid ACLs are not affected by this vulnerability. Classic ACLs are affected by this vulnerability if they meet both of the following conditions:

  • A compression level of either Level 2 or Level 3 (depending on hardware platform)
  • Access Control Entries (ACE) with the same destination address, but with different masks

  When using level 2 or level 3 compression internally on a classic ACL, the compression code checks for duplicate IP addresses in the ACE. If two or more entries share the same destination address, the destination mask from the first one is applied to all further duplicate entries, as shown in the following examples: 

  Example 1:

  ipv4 access-list CSCwe08950
   10 deny icmp any 198.51.100.0 0.0.0.7 time-exceeded
   20 deny icmp any 198.51.100.0 0.0.0.3
   40 permit ipv4 any any 

  In Example 1, an ICMP echo request to 198.51.100.7 should hit ACE line 40 and be permitted. Because ACE 20 and 10 use the same destination address, when compression is applied, only the first destination mask is applied to the compressed ACL, resulting in the ICMP echo request being denied.

  Example 2: 

  ipv4 access-list CSCwe08950
   10 deny icmp any 198.51.100.0 0.0.0.7 time-exceeded
   20 permit ipv4 any 198.51.100.0 0.0.0.3
   30 permit ospf any any

  In Example 2, we would expect telnet traffic to permitted by the ACL to host 198.51.100.3, but not permitted to host 198.51.100.7. However, because ACE 20 is a duplicate destination address of ACE 10, the ACL compression code has used ACE 10 mask and is permitted.

  Example 3: 

  ipv4 access-list CSCwe08950
   10 deny icmp host 1.1.1.1 198.51.100.0 0.0.0.7 time-exceeded
   20 permit icmp host 1.1.1.1 198.51.100.0 0.0.0.255
   40 permit ipv4 host 1.1.1.1 198.51.100.0 0.0.0.15
   50 permit ospf any any

  In Example 3, we would expect an ICMP echo-response sourced from 1.1.1.1 to 198.51.100.30 to be permitted. However, because ACE 20 is a duplicate destination address of ACE 10, the compression applies the ACE 10 mask, and the ICMP echo-response sourced from 1.1.1.1 to 198.51.100.30 is not permitted. The telnet session from 1.1.1.1 to 198.51.100.15 should also have been permitted, but it was dropped because ACE 40 is a duplicate of ACE 10, so the ACE 10 mask was used and dropped.

Workarounds

• There is a workaround that addresses this vulnerability.

  Customers are advised to review their classic ACLs if using level 2 or 3 compression for duplicate destination addresses and adjust accordingly.

  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 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 described in this advisory and which release included the fix for this vulnerability.

  Cisco IOS XR Release	First Fixed Release
  7.2 and earlier	Migrate to a fixed release.
  7.3	7.3.5
  7.4	Migrate to a fixed release.
  7.5	7.5.4
  7.6	Migrate to a fixed release.
  7.7	Migrate to a fixed release.
  7.8	7.8.2
  7.9	7.9.1

  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-comp3acl-vGmp6BQ3

Revision History

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

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