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AUSCERT External Security Bulletin Redistribution
Network Convergence System 520 Routers Denial of Service Vulnerability
25 March 2021
AusCERT Security Bulletin Summary
Product: Cisco IOS XE Software for Network Convergence System 520 Routers
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Denial of Service -- Remote/Unauthenticated
CVE Names: CVE-2021-1394
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Cisco IOS XE Software for Network Convergence System 520 Routers Denial of
Advisory ID: cisco-sa-ncs520-tcp-ZpzzOxB
First Published: 2021 March 24 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvm96192
CVE Names: CVE-2021-1394
o A vulnerability in the ingress traffic manager of Cisco IOS XE Software for
Cisco Network Convergence System (NCS) 520 Routers could allow an
unauthenticated, remote attacker to cause a denial of service (DoS)
condition in the web management interface of an affected device.
This vulnerability is due to incorrect processing of certain IPv4 TCP
traffic that is destined to an affected device. An attacker could exploit
this vulnerability by sending a large number of crafted TCP packets to the
affected device. A successful exploit could allow the attacker to cause the
web management interface to become unavailable, resulting in a DoS
Note: This vulnerability does not impact traffic that is going through the
device or going to the Management Ethernet interface of the 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:
o Vulnerable Products
At the time of publication, this vulnerability affected Cisco NCS 520
Routers if they were running a vulnerable release of Cisco IOS XE Software
and had the HTTP server enabled.
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
Determine the HTTP Server Configuration
To determine whether the HTTP Server feature is enabled for a device, log
in to the device and use the show running-config | include ip http server|
secure|active command in the CLI to check for the presence of the ip http
server command or the ip http secure-server command in the global
configuration. If either command is present and configured, the HTTP Server
feature is enabled for the device.
The following example shows the output of the show running-config | include
ip http server|secure|active command for a device that has the HTTP Server
Router# show running-config | include ip http server|secure|active
ip http server
ip http secure-server
Note: The presence of either command or both commands in the device
configuration indicates that the web UI feature is enabled.
If the ip http server command is present and the configuration also
contains ip http active-session-modules none , the vulnerability is not
exploitable over HTTP.
If the ip http secure-server command is present and the configuration also
contains ip http secure-active-session-modules none , the vulnerability is
not exploitable over HTTPS.
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.
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.
For a mapping of Cisco IOS XE Software releases to Cisco IOS Software
releases, refer to the Cisco IOS XE 2 Release Notes , Cisco IOS XE 3S
Release Notes , or Cisco IOS XE 3SG Release Notes , depending on the Cisco
IOS XE Software release.
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.
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.
| Version | Description | Section | Status | Date |
| 1.0 | Initial public release. | - | Final | 2021-MAR-24 |
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