Summary

• A vulnerability in Simple Network Management Protocol (SNMP) subsystem of Cisco IOS XE Software could allow an authenticated, remote attacker to cause a denial of service (DoS) condition.
  
  The vulnerability is due to improper management of memory resources, referred to as a double free. An attacker could exploit this vulnerability by sending crafted SNMP packets to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition.
  
  To exploit this vulnerability via SNMP Version 2c or earlier, the attacker must know the SNMP read-only community string for an affected system. To exploit this vulnerability via SNMP Version 3, the attacker must know the user credentials for the affected system.

  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://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-snmp-dos

  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.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco devices that are running a vulnerable release of Cisco IOS XE Software, have been configured to be queried over SNMP, and have Network Address Translation (NAT) enabled.

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

  Assessing the SNMP Configuration

  To determine whether a device is configured for SNMP, administrators can use the show run | include snmp command in the CLI. 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

  Determining the Cisco IOS XE Software Release

  To determine which Cisco IOS XE 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 XE Software, the system banner displays Cisco IOS Software, Cisco IOS XE Software, or similar text.

  The following example shows the output of the command for a device that is running Cisco IOS XE Software Release 16.2.1 and has an installed image name of CAT3K_CAA-UNIVERSALK9-M:

  ios-xe-device# show version
  
  Cisco IOS Software, Catalyst L3 Switch Software (CAT3K_CAA-UNIVERSALK9-M), Version Denali 16.2.1, RELEASE SOFTWARE (fc1)
  Technical Support: http://www.cisco.com/techsupport
  Copyright (c) 1986-2016 by Cisco Systems, Inc.
  Compiled Sun 27-Mar-16 21:47 by mcpre
  .
  .
  .

  For information about the naming and numbering conventions for Cisco IOS XE 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 Software, 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 via IP Version 4 or IP Version 6. Only traffic that is directed to an affected system can be used to exploit this vulnerability.

  To exploit this vulnerability via SNMP Version 2c 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 also be changed at regular intervals and in accordance with network security policies. For example, the strings should be changed when a network administrator changes roles or leaves the organization.

  To exploit this vulnerability via SNMP Version 3, the attacker must know the user credentials for an affected system.

Indicators of Compromise

• A successful exploit of this vulnerability will cause an affected device to reload and generate a crashinfo file.

  A successful exploit of this vulnerability may be confirmed by decoding the stack trace for the device and determining whether the stack trace correlates with this vulnerability.

  Error messages that may be found in the device logs or the crashinfo file when the SNMP process crashes may be similar to the following example:

  Exception to IOS Thread:
  Frame pointer 0x7F48DA5F5300, PC = 0x7F4B736A40EB
  UNIX-EXT-SIGNAL: Segmentation fault(11), Process = SNMP ENGINE

  Contact the Cisco Technical Assistance Center (TAC) to review the crashinfo file and determine whether the device has been compromised by exploitation of this vulnerability.

Workarounds

• Administrators can mitigate this vulnerability by disabling the affected Management Information Base (MIB), in this case the NAT-MIB. To create or update a view entry and disable the NAT-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:

  !Standard VIEW and Security Exclusions
  snmp-server view NO_BAD_SNMP iso included
  snmp-server view NO_BAD_SNMP internet included
  snmp-server view NO_BAD_SNMP snmpUsmMIB excluded
  snmp-server view NO_BAD_SNMP snmpVacmMIB excluded
  snmp-server view NO_BAD_SNMP snmpCommunityMIB excluded
  snmp-server view NO_BAD_SNMP ciscoMgmt.252 excluded
  !End Standard View
   
  !Advisory Specific Mappings
  snmp-server view NO_BAD_SNMP mib-2.123 excluded

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

  snmp-server community mycomm view NO_BAD_SNMP RO

  For SNMP Version 3, administrators can use the following command:

  snmp-server group v3group auth read NO_BAD_SNMP write NO_BAD_SNMP

  SNMP Access Control List

  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 in conjunction with ACLs to offer a better mitigation solution. 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 a tool, the Cisco IOS Software Checker, that identifies 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 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 or Cisco IOS XE Software release—for example, 15.1(4)M2 or 3.13.8S—in the following field:

  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 support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-snmp-dos

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2018-March-28

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