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AUSCERT External Security Bulletin Redistribution
ESB-2020.3305
Cisco IOS XE Software RESTCONF and NETCONF-YANG Access
Control List Denial of Service Vulnerability
25 September 2020
===========================================================================
AusCERT Security Bulletin Summary
---------------------------------
Product: Cisco IOS XE Software
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Denial of Service -- Remote/Unauthenticated
Resolution: Patch/Upgrade
CVE Names: CVE-2020-3407
Original Bulletin:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-confacl-HbPtfSuO
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Cisco IOS XE Software RESTCONF and NETCONF-YANG Access Control List Denial of
Service Vulnerability
Priority: High
Advisory ID: cisco-sa-confacl-HbPtfSuO
First Published: 2020 September 24 16:00 GMT
Version 1.0: Final
Workarounds: YesCisco Bug IDs: CSCvs72434
CVE-2020-3407
CWE-476
CVSS Score:
8.6 AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
Summary
o A vulnerability in the RESTCONF and NETCONF-YANG access control list (ACL)
function of Cisco IOS XE Software could allow an unauthenticated, remote
attacker to cause the device to reload.
The vulnerability is due to incorrect processing of the ACL that is tied to
the RESTCONF or NETCONF-YANG feature. An attacker could exploit this
vulnerability by accessing the device using RESTCONF or NETCONF-YANG. A
successful exploit could allow an attacker to cause the device to reload,
resulting in a denial of service (DoS) condition.
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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-confacl-HbPtfSuO
This advisory is part of the September 24, 2020, release of the Cisco IOS
and IOS XE Software Security Advisory Bundled Publication, which includes
25 Cisco Security Advisories that describe 34 vulnerabilities. For a
complete list of the advisories and links to them, see Cisco Event
Response: September 2020 Semiannual Cisco IOS and IOS XE Software Security
Advisory Bundled Publication .
Affected Products
o Vulnerable Products
This vulnerability affects Cisco devices if they are running a vulnerable
release of Cisco IOS XE Software and have either the RESTCONF or
NETCONF-YANG feature configured with one or both of the following tied to
it:
An IP Version 4 (IPv4) ACL that is exactly 13 entries long
An IP Version 6 (IPv6) ACL that is exactly 13 entries long
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
Determine the Device Configuration
To determine whether the RESTCONF feature is enabled with an ACL for a
device, log in to the device and use the show running-config | include
restconf command in the CLI to check for the presence of the restconf <ipv4
|ipv6> access-list name commands.
The following example shows the output of the show running-config | include
restconf command for a router that has the RESTCONF feature enabled with an
ACL attached to the feature:
Device#show running-config | include restconf
restconf
restconf ipv4 access-list name RESTACL
restconf ipv6 access-list name RESTACL
Device#
To determine whether the NETCONF-YANG feature is enabled with an ACL for a
device, log in to the device and use the show running-config | include
netconf command in the CLI to check for the presence of the netconf-yang
ssh <ipv4|ipv6> access-list name commands.
The following example shows the output of the show running-config | include
netconf command for a router that has the NETCONF feature enabled with an
ACL attached to the feature:
Device#show running-config | include netconf
netconf-yang
netconf-yang ssh ipv4 access-list name NETACL
netconf-yang ssh ipv6 access-list name NETACL
Device#
If either of the preceding commands indicates that either RESTCONF or
NETCONF-YANG is enabled with an ACL entry attached to it, take the ACL name
(in this case RESTACL or NETACL) and issue the command show access-lists
RESTACL or show access-lists NETACL . Count the number of lines in the
output, excluding the first line with the heading Extended IP access list
<name> and any remark lines. If the total is 13, the device is vulnerable.
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-XR Software
NX-OS Software
Workarounds
o There is a workaround that addresses this vulnerability.
Alter the attached ACL so it is not exactly 13 entries long.
Fixed Software
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
software license: https://www.cisco.com/c/en/us/products/
end-user-license-agreement.html
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 and Alerts 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.
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
/en/us/support/web/tsd-cisco-worldwide-contacts.html
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
upgrade.
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
following ways:
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 :
[ ] [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 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.
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.
Related to This Advisory
o Cisco Event Response: September 2020 Semiannual Cisco IOS and IOS XE
Software Security Advisory Bundled Publication
URL
o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-confacl-HbPtfSuO
Revision History
o +----------+---------------------------+----------+--------+--------------+
| Version | Description | Section | Status | Date |
+----------+---------------------------+----------+--------+--------------+
| 1.0 | Initial public release. | - | Final | 2020-SEP-24 |
+----------+---------------------------+----------+--------+--------------+
- --------------------------END INCLUDED TEXT--------------------
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