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AUSCERT External Security Bulletin Redistribution
Cisco ASR 900 and ASR 920 Series Aggregation Services
Routers Access Control List Bypass Vulnerability
23 September 2021
AusCERT Security Bulletin Summary
Product: ASR 900 Series Routers
ASR 920 Series Routers
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Unauthorised Access -- Remote/Unauthenticated
CVE Names: CVE-2021-34696
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Cisco ASR 900 and ASR 920 Series Aggregation Services Routers Access Control
List Bypass Vulnerability
Advisory ID: cisco-sa-asr900acl-UeEyCxkv
First Published: 2021 September 22 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvw57495
CVE Names: CVE-2021-34696
o A vulnerability in the access control list (ACL) programming of Cisco ASR
900 and ASR 920 Series Aggregation Services Routers could allow an
unauthenticated, remote attacker to bypass a configured ACL.
This vulnerability is due to incorrect programming of hardware when an ACL
is configured using a method other than the configuration CLI. 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 no workarounds that address this vulnerability.
This advisory is available at the following link:
This advisory is part of the September 2021 release of the Cisco IOS and
IOS XE Software Security Advisory Bundled Publication. For a complete list
of the advisories and links to them, see Cisco Event Response: September
2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled
o Vulnerable Products
At the time of publication, this vulnerability affected the following Cisco
products if they were running a vulnerable release of Cisco IOS XE Software
and had an IPv4 ACL configured:
ASR 900 Series Aggregation Services Routers with Route Switch
Processors (RSPs) 1, 2, or 3
ASR 920 Series Aggregation Services Routers
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
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
IOS XR Software
o There are no workarounds that address this vulnerability. However,
administrators can reload the device to fix the ACL programming.
This vulnerability has not been found in ACLs that are manually configured
using the CLI. However, there is no mechanism to determine whether an ACL
has been correctly programmed on a device. Reloading the device will apply
the correct programming to all ACLs and cause them to operate as
configured, regardless of the method that was used for their initial
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
o 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
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
Cisco IOS and IOS XE Software
To help customers determine their exposure to vulnerabilities in Cisco IOS
and IOS XE Software, Cisco provides the Cisco Software Checker to identify
any Cisco Security Advisories that impact a specific software release and
the earliest release that fixes the vulnerabilities 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
Choose the software 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, a specific advisory, or all advisories in
the most recent bundled publication.
Customers can also use the following form to determine whether a release is
affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE
Software release-for example, 15.1(4)M2 or 3.13.8S :
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 on Cisco.com and check the Medium check box in the
drop-down list under Impact Rating when customizing a search.
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 Cisco would like to thank Gert Doring of SpaceNet for reporting this
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.
Related to This Advisory
o Cisco Event Response: September 2021 Semiannual Cisco IOS and IOS XE
Software Security Advisory Bundled Publication
| Version | Description | Section | Status | Date |
| 1.0 | Initial public release. | - | Final | 2021-SEP-22 |
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