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AUSCERT External Security Bulletin Redistribution
Cisco IOS XE Software Easy Virtual Switching System
Arbitrary Code Execution Vulnerability
25 March 2021
AusCERT Security Bulletin Summary
Product: Cisco IOS XE Software Easy Virtual Switching System
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Root Compromise -- Remote/Unauthenticated
Execute Arbitrary Code/Commands -- Remote/Unauthenticated
Denial of Service -- Remote/Unauthenticated
CVE Names: CVE-2021-1451
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Cisco IOS XE Software Easy Virtual Switching System Arbitrary Code Execution
Advisory ID: cisco-sa-ios-xe-evss-code-exe-8cw5VSvw
First Published: 2021 March 24 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvv66062
CVE Names: CVE-2021-1451
o A vulnerability in the Easy Virtual Switching System (VSS) feature of Cisco
IOS XE Software for Cisco Catalyst 4500 Series Switches and Cisco Catalyst
4500-X Series Switches could allow an unauthenticated, remote attacker to
execute arbitrary code on the underlying Linux operating system of an
The vulnerability is due to incorrect boundary checks of certain values in
Easy VSS protocol packets that are destined for an affected device. An
attacker could exploit this vulnerability by sending crafted Easy VSS
protocol packets to UDP port 5500 while the affected device is in a
specific state. When the crafted packet is processed, a buffer overflow
condition may occur. A successful exploit could allow the attacker to
trigger a denial of service (DoS) condition or execute arbitrary code with
root privileges on the underlying Linux operating system of the affected
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 March 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: March 2021
Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled
o Vulnerable Products
This vulnerability affects Cisco Catalyst 4500 and 4500-X Series Switches
if they are running a vulnerable release of Cisco IOS XE Software and one
of the following conditions is true:
The switch is being converted from standalone mode to virtual switch
mode using the Easy VSS feature.
Cisco Discovery Protocol is enabled with Application type, length,
value (TLV) information.
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
Determine the Cisco Discovery Protocol Configuration
To determine whether the Cisco Discovery Protocol is enabled on a device,
use the show cdp command in the device CLI. The output of the command
displays global Cisco Discovery Protocol information. The following example
shows the output of the show cdp command if the Cisco Discovery Protocol is
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Sending CDPv2 advertisements is enabled
The following example shows the output of the show cdp command if the
protocol is not enabled:
% CDP is not enabled
Determine the Cisco Discovery Protocol Application TLVs Configuration
Cisco Discovery Protocol Application TLV processing is enabled by default.
To determine if the Application TLVs are disabled, use the show
running-config | include no cdp tlv app CLI command. If the command returns
no output, then processing of Cisco Discovery Protocol Application TLVs is
If the show running-config | include no cdp tlv app command returns output,
that output will indicate how many interfaces have Cisco Discovery Protocol
Application TLV processing disabled. The following example shows output for
the command if one interface is disabled:
Switch#show running-config | include no cdp tlv app
no cdp tlv app
When the Cisco Discovery Protocol is enabled with Application TLVs on a
device, an adjacent attack vector exists through the Cisco Discovery
Protocol handler. An attacker could use this vector to force the device
into a vulnerable state and exploit this vulnerability by targeting UDP
port 5500 of the affected device.
The Cisco Discovery Protocol is a Layer 2 protocol. To exploit the
vulnerability through the Cisco Discovery Protocol, the attacker would need
to be in the same broadcast domain as the affected device that has the
Cisco Discovery Protocol enabled with Application TLVs (Layer 2 adjacent).
Note : If the switch operates as a VSS member, this vulnerability is not
exploitable remotely. Only the adjacent vector through the Cisco Discovery
Protocol is available. Use the show switch virtual command in the device
CLI to determine the status of a VSS. The following example shows the
output of the show switch virtual CLI command for a device that operates as
a VSS member:
Switch#show switch virtual
Executing the command on VSS member switch role = VSS Active, id = 2
Switch mode: Virtual Switch
The following example shows the output of the show switch virtual CLI
command for a device that does not operate as a VSS member:
Switch# show switch virtual
Switch Mode : Standalone
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 By default, Catalyst 4500 Series and 4500-X Series Switches are configured
to operate in standalone mode, with each switch working independently. The
VSS feature enables administrators to combine two switches into a single
network element that operates in virtual switch mode. The Easy VSS feature
simplifies the VSS configuration through a single command.
The switch convert mode easy-virtual-switch exec command configures the
switches to operate as a VSS. As part of this process, UDP port 5500 is
opened until the switches reload as a single VSS. While the port is open,
an attacker can remotely exploit this vulnerability through UDP port 5500.
Once the system reloads as a VSS, UDP port 5500 is closed and this
vulnerability is no longer exploitable remotely. Only the adjacent attack
vector through the Cisco Discovery Protocol is available.
Indicators of Compromise
o Exploitation of this vulnerability can result in a crash of the vss bringup
process on an affected device. When a device has experienced a crash of the
vss bringup process, the following message may be seen in the system logs:
IOSD-EXT-SIGNAL: Segmentation fault(11), Process = vss bringup
This error message indicates that the vss bringup process has crashed.
However, the process may have crashed for a reason other than the
exploitation of this vulnerability. Customers are advised to contact their
support organization to review the error messages and determine whether the
device has been compromised by an exploitation of this vulnerability.
o There are no workarounds for this vulnerability. However, mitigations are
Disable Cisco Discovery Protocol Application TLVs
Cisco Discovery Protocol Application TLVs are enabled by default. Disabling
the Application TLVs will close the adjacent attack vector and reduce the
To disable use of Cisco Discovery Protocol Application TLVs globally for a
device, use the no cdp tlv app command in the global configuration CLI. To
disable use of Application TLVs for a specific interface of a device, use
the no cdp tlv app command in the interface configuration CLI.
To see if Cisco Discovery Protocol Application TLVs are in use on the
device before disabling them, use the show cdp tlv app command. If the
command returns output, then disabling the Application TLVs globally is not
Disable Cisco Discovery Protocol
Customers who do not use the Cisco Discovery Protocol feature may disable
it either globally, which will close the adjacent attack vector, or on
individual interfaces, which will reduce the attack surface.
To disable the protocol globally for a device, use the no cdp run command
in the global configuration CLI. To disable the protocol for a specific
interface of a device, use the no cdp enable command in the interface
Customers who cannot apply the above mitigations can reduce the attack
surface by restricting inbound traffic to UDP port 5500 by implementing an
access control list (ACL) or an infrastructure access control list (iACL).
While these mitigations have been deployed and were 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
o Cisco has released free software updates that address the vulnerability
described in this advisory. Customers may only install and expect support
for software versions and feature sets for which they have purchased a
license. By installing, downloading, accessing, or otherwise using such
software upgrades, customers agree to follow the terms of the Cisco
Additionally, customers may only download software for which they have a
valid license, procured from Cisco directly, or through a Cisco authorized
reseller or partner. In most cases this will be a maintenance upgrade to
software that was previously purchased. Free security software updates do
not entitle customers to a new software license, additional software
feature sets, or major revision upgrades.
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
Customers Without Service Contracts
Customers who purchase directly from Cisco but do not hold a Cisco service
contract and customers who make purchases through third-party vendors but
are unsuccessful in obtaining fixed software through their point of sale
should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c
Customers should have the product serial number available and be prepared
to provide the URL of this advisory as evidence of entitlement to a free
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 This vulnerability was found during internal security testing by X.B. and
J.F.D. of Cisco.
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: March 2021 Semiannual Cisco IOS and IOS XE Software
Security Advisory Bundled Publication
| Version | Description | Section | Status | Date |
| 1.0 | Initial public release. | - | Final | 2021-MAR-24 |
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