Cisco Security Advisory
Cisco IOS Software Simple Network Management Protocol GET MIB Object ID Denial of Service Vulnerability
A vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS Software running on certain models of Cisco Catalyst Switches could allow an authenticated, remote attacker to cause a denial of service (DoS) condition.
The vulnerability is due to a condition that could occur when the affected software processes an SNMP read request that contains a request for the ciscoFlashMIB object ID (OID). An attacker could trigger this vulnerability by issuing an SNMP GET request for the ciscoFlashMIB OID on an affected device. A successful exploit could cause the affected device to restart due to a SYS-3-CPUHOG.
Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.
This advisory is available at the following link:
This advisory is part of the March 28, 2018, release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication, which includes 20 Cisco Security Advisories that describe 22 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: March 2018 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.
This vulnerability affects the following Cisco devices if they are running a vulnerable release of Cisco IOS Software and are configured to use SNMP Version 2 (SNMPv2) or SNMP Version 3 (SNMPv3):
- Cisco Catalyst 2960-L Series Switches
- Cisco Catalyst Digital Building Series Switches - 8P
- Cisco Catalyst Digital Building Series Switches - 8U
For information about which Cisco IOS Software releases are vulnerable, see the Fixed Software section of this advisory.
To determine whether a device is configured to use SNMP, administrators can use the show running-config | include snmp-server privileged EXEC command in the CLI of the device. If the command returns a line that contains snmp-server community or snmp-server user, the device is vulnerable, as shown in the following examples.
snmp-server community example ro
snmp-server user example example-snmp v3 snmp-server group example-snmp v3 noauth
Determining the Cisco IOS Software Release
To determine which Cisco IOS Software release is running on a device, administrators can log in to the device, use the show version command in the CLI, and then refer to the system banner that appears. If the device is running Cisco IOS Software, the system banner displays text similar to Cisco Internetwork Operating System Software or Cisco IOS Software. The banner also displays the installed image name in parentheses, followed by the Cisco IOS Software release number and release name. Some Cisco devices do not support the show version command or may provide different output.
The following example shows the output of the command for a device that is running Cisco IOS Software Release 15.5(2)T1 and has an installed image name of C2951-UNIVERSALK9-M:
Router> show version Cisco IOS Software, C2951 Software (C2951-UNIVERSALK9-M), Version 15.5(2)T1, RELEASE SOFTWARE (fc1) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2015 by Cisco Systems, Inc. Compiled Mon 22-Jun-15 09:32 by prod_rel_team . . .
For information about the naming and numbering conventions for Cisco IOS Software releases, see the Cisco IOS and NX-OS Software Reference Guide.
Products Confirmed Not Vulnerable
No other Cisco products are currently known to be affected by this vulnerability.
Cisco has confirmed that this vulnerability does not affect Cisco IOS XE Software, Cisco IOS XR Software, or Cisco NX-OS Software.
Exploitation of this vulnerability typically results in a crash in the SNMP ENGINE process, as shown in the following example:
%SYS-3-CPUHOG: Task is running for (2012)msecs, more than (2000)msecs (0/0),process = SNMP ENGINE
There are workarounds that address this vulnerability.
Use SNMP Access Control Lists
Note: Because the vulnerable feature uses UDP as a transport, it is possible to spoof the sender's IP address, which may defeat access control lists (ACLs) that permit communication to these ports from trusted IP addresses. Unicast Reverse Path Forwarding (RPF) should be considered for use in conjunction with UDP to offer a better mitigation solution. The following is an example of an ACL that allows only hosts at 192.168.0.1 to query the device with SNMP requests:
access-list 1 permit 192.168.0.1 snmp-server community example RO 1
Exclude the Cisco Flash MIB from the SNMP Query list
snmp-server view example ciscoFlashMIB excluded snmp-server community example view example ro
snmp-server user example example-snmp v3 snmp-server group example-snmp v3 noauth read example snmp-server view example ciscoFlashMIB excluded
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:
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:
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 Software
To help customers determine their exposure to vulnerabilities in Cisco IOS Software, Cisco provides a tool, the Cisco IOS Software Checker, that identifies any Cisco Security Advisories that impact a specific Cisco IOS 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 this tool to perform the following tasks:
- Initiate a search by choosing one or more releases from a drop-down list or uploading a file from a local system for the tool to parse
- Enter the output of the show version command for the tool to parse
- Create a custom search by including all previously published Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication
To determine whether a release is affected by any published Cisco Security Advisory, use the Cisco IOS Software Checker on Cisco.com or enter a Cisco IOS Software release-for example, 15.1(4)M2-in the following field:
In March 2022, the Cisco Product Security Incident Response Team (PSIRT) became aware of additional attempted exploitation of this vulnerability in the wild. Cisco continues to strongly recommend that customers upgrade to a fixed software release to remediate this vulnerability.
This vulnerability was found while handling customer service requests.
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 1.1 Updated exploitation information. Exploitation and Public Announcements Final 2022-DEC-15 1.0 Initial public release. - Final 2018-MAR-28
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
A standalone copy or paraphrase of the text of this document that omits the distribution URL is an uncontrolled copy and may lack important information or contain factual errors. The information in this document is intended for end users of Cisco products.