Summary

• A vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS Software and Cisco IOS XE Software on Catalyst 4500 Series Switches could allow an authenticated, remote attacker to cause a denial of service (DoS) condition.

  The vulnerability is due to insufficient input validation when the software processes specific SNMP object identifiers. An attacker could exploit this vulnerability by sending a crafted SNMP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition.

  Note: To exploit this vulnerability by using SNMPv2c or earlier, the attacker must know the SNMP read-only community string for an affected system. To exploit this vulnerability by using SNMPv3, the attacker must know the user credentials for the affected system.

  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://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snmp-dos-USxSyTk5

  This advisory is part of the June 3, 2020, release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication, which includes 23 Cisco Security Advisories that describe 25 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: June 2020 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco Catalyst 4500 Series Switches that are running a vulnerable release of Cisco IOS or IOS XE Software, are configured for SNMP polling, and have Power over Ethernet (PoE) line-card modules installed.

  For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.

  Assess the SNMP Configuration

  To determine whether a device is configured for SNMP, administrators can use the show run | include snmp command in the CLI. If any output is returned, SNMP is enabled on the device. The following example shows the output of the show run | include snmp command on a device that is configured with both read-only and read-write community strings:

  Router# show run | include snmp
  
  snmp-server community public RO
  snmp-server community write RW

   

  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 Cisco IOS XR Software or Cisco NX-OS Software.

Details

• An attacker could exploit this vulnerability by sending a crafted SNMP packet to an affected device by using IPv4 or IPv6. Only traffic that is directed to an affected system can be used to exploit this vulnerability.

  To exploit this vulnerability by using SNMPv2c or earlier, the attacker must know the SNMP read-only community string for an affected system. A community string is a password that is applied to a device to restrict both read-only and read-write access to SNMP data on a device. These community strings, as with all passwords, should be chosen carefully to ensure that they are not trivial. They should be changed at regular intervals. They should also be changed in accordance with network security policies, such as when a network administrator changes roles or leaves the organization.

  To exploit this vulnerability by using SNMPv3, the attacker must know the user credentials for an affected system.

Workarounds

• There are no workarounds that address this vulnerability.

  However, administrators can mitigate this vulnerability by disabling the affected MIB, in this case CISCO-POWER-ETHERNET-EXT-MIB. To create or update a view entry and disable CISCO-POWER-ETHERNET-EXT-MIB, administrators can log in to the device and use the snmp-server view global configuration command in the CLI, as shown in the following example:

  snmp-server view SNMP_VIEW_NAME ciscoPowerEthernetExtMIB excluded

  To apply the configuration to an SNMPv2 community string, administrators can use the following command:

  snmp-server community mycomm view SNMP_VIEW_NAME RO

  To apply the configuration to an SNMPv3 community string, administrators can use the following command:

  snmp-server group v3group v3 auth read SNMP_VIEW_NAME write SNMP_VIEW_NAME

   

  SNMP Access Control List

  Administrators can reduce the attack surface by using an access control list (ACL) to restrict SNMP queries from specific IP addresses. However, administrators are advised to also use Unicast Reverse Path Forwarding (uRPF).

  Since the vulnerable feature uses UDP as a transport, an attacker could spoof the IP address and bypass an ACL that permits communication to these UDP ports from trusted IP addresses. The following is an example of an ACL that allows only hosts at 192.0.2.1 to query the device with SNMP requests:

  access-list 1 permit 192.0.2.1
  snmp-server community example RO 1

   

Fixed Software

• 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:

  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

• 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

• This vulnerability was found during the resolution of a Cisco TAC support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snmp-dos-USxSyTk5

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2020-JUN-03

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