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AUSCERT External Security Bulletin Redistribution
ESB-2021.0395
Cisco IOS XR and Cisco NX-OS Software IPv6 Access Control
List Bypass Vulnerability
4 February 2021
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AusCERT Security Bulletin Summary
---------------------------------
Product: Cisco IOS XR Software
Cisco NX-OS Software
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Unauthorised Access -- Remote/Unauthenticated
Reduced Security -- Remote/Unauthenticated
Resolution: Patch/Upgrade
CVE Names: CVE-2021-1389
Original Bulletin:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ipv6-acl-CHgdYk8j
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Cisco IOS XR and Cisco NX-OS Software IPv6 Access Control List Bypass
Vulnerability
Priority: Medium
Advisory ID: cisco-sa-ipv6-acl-CHgdYk8j
First Published: 2021 February 3 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvm55638 CSCvv45698
CVE Names: CVE-2021-1389
CWEs: CWE-284
Summary
o A vulnerability in the IPv6 traffic processing of Cisco IOS XR Software and
Cisco NX-OS Software for certain Cisco devices could allow an
unauthenticated, remote attacker to bypass an IPv6 access control list
(ACL) that is configured for an interface of an affected device.
The vulnerability is due to improper processing of IPv6 traffic that is
sent through an affected device. An attacker could exploit this
vulnerability by sending crafted IPv6 packets that traverse the affected
device. A successful exploit could allow the attacker to access resources
that would typically be protected by the interface ACL.
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:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ipv6-acl-CHgdYk8j
Affected Products
o Vulnerable Products
At the time of publication, this vulnerability affected the following Cisco
devices if they were running a vulnerable release of Cisco IOS XR Software
or Cisco NX-OS Software and had IPv6 ACL configured:
Network Convergence System (NCS) 540 Series Routers
NCS 560 Series Routers
NCS 5500 Series
Nexus 3600 Platform Switches
Nexus 9500 R-Series Switching Platforms
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
Determine the Configuration for Cisco IOS XR Software
Use the show running-config | include ipv6 access-list command to view the
configured ACLs for IPv6. If this command produces an output, the device
should be considered vulnerable. The following example shows the output on
a device that has the IPv6 ACL configured:
Router# show running-config | include ipv6 access-list
ipv6 access-list <acl_name>
This does not apply to IPv4 ACLs.
Determine the Configuration for Cisco NX-OS Software
Use the show ipv6 access-lists command to view the configured ACLs for
IPv6. If this command produces an output, the device should be considered
vulnerable. The following example shows the output on a device that has the
IPv6 ACL configured:
Nexus# show ipv6 access-lists
IPv6 access list <acl_name>
This does not apply to IPv4 ACLs.
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:
NCS 520 Series Routers
NCS 5000 Series Switches
NCS 6000 Series Routers
IOS XR SW-only
IOS XRv 9000 Routers
ASR 9000 Series Aggregation Services Routers
Carrier Routing System (CRS)
Firepower 1000 Series
Firepower 2100 Series
Firepower 4100 Series
Firepower 9300 Security Appliances
MDS 9000 Series Multilayer Switches
Nexus 1000 Virtual Edge for VMware vSphere
Nexus 1000V Switch for Microsoft Hyper-V
Nexus 1000V Switch for VMware vSphere
Nexus 3000 Series Switches
Nexus 5500 Platform Switches
Nexus 5600 Platform Switches
Nexus 6000 Series Switches
Nexus 7000 Series Switches
Nexus 9000 Series Switches in standalone NX-OS mode
Nexus 9000 Series Fabric Switches in Application Centric Infrastructure
(ACI) mode
UCS 6200 Series Fabric Interconnects
UCS 6300 Series Fabric Interconnects
UCS 6400 Series Fabric Interconnects
Details
o 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 .
Workarounds
o There are no workarounds that address this vulnerability.
Fixed Software
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
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:
[Cisco NX-OS Software ] [MDS 9000 Series Multilayer Switches]
[ ] [Check]
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.
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.
Source
o This vulnerability was found during the resolution of a Cisco TAC support
case.
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.
URL
o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ipv6-acl-CHgdYk8j
Revision History
o +----------+---------------------------+----------+--------+--------------+
| Version | Description | Section | Status | Date |
+----------+---------------------------+----------+--------+--------------+
| 1.0 | Initial public release. | - | Final | 2021-FEB-03 |
+----------+---------------------------+----------+--------+--------------+
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