Summary

• A vulnerability in the DHCP client implementation of Cisco IOS and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.

  The vulnerability occurs during the parsing of a crafted DHCP packet. An attacker could exploit this vulnerability by sending crafted DHCP packets to an affected device that is configured as a DHCP client. A successful exploit could allow the attacker to cause a reload of an affected device, resulting in a DoS condition.

  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-20170322-dhcpc

  This advisory is part of the March 22, 2017, release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication, which includes five Cisco Security Advisories that describe five vulnerabilities. All the vulnerabilities have a Security Impact Rating of High. For a complete list of the advisories and links to them, see Cisco Event Response: March 2017 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 or IOS XE Software and using a specific DHCP client configuration.

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

  To determine whether a Cisco device running Cisco IOS or IOS XE Software is affected, perform the following steps:

  1. Confirm whether the device is configured as a DHCP client by using the show running-config | include dhcp privileged EXEC command on the device.
     
     The following is the output of the show running-config | include dhcp command for a Cisco IOS device configured as a DHCP client. The ip address dhcp output confirms that the feature is enabled on one of the interfaces:

     Router# show running-config | include dhcp
       ip address dhcp

     If the device is not configured as a DHCP client, the device is not vulnerable. If the device is configured as a DHCP client, proceed to the next step.
     
     
  2. Confirm whether the device is configured as a DHCP server or DHCP relay agent by using the show running-config | include helper|(ip dhcp pool) privileged EXEC command on the device.
     
     The following is the output of the show running-config | include helper|(ip dhcp pool) command for a Cisco IOS device configured as a DHCP relay agent. The ip helper-address 192.168.0.1 output confirms that the feature is enabled on one of the interfaces:

     Router# show running-config | include helper|(ip dhcp pool)
       ip helper-address 192.168.0.1

     If the device is not configured as a DHCP server or DHCP relay agent, the device is not vulnerable; otherwise, proceed to the next step.
     
     
  3. Confirm whether the device is configured to send DHCP Inform and Discover messages by using the show run | include (ip dhcp-client network-discovery) privileged EXEC command on the device.
     
     The following is the output of the show run | include (ip dhcp-client network-discovery) command for a Cisco IOS device configured to send DHCP Inform and Discover messages. The ip dhcp-client network-discovery informs 1 discovers 1 period 15 output confirms that the feature is enabled:

     Router# show running-config | include (ip dhcp-client network-discovery)        
       ip dhcp-client network-discovery informs 1 discovers 1 period 15
     Router#

     If the device is configured to send DHCP Inform and Discover messages, the device is not vulnerable.

  The following table depicts the preceding information in a tabular format:

  Vulnerable State	Configuration
  	DHCP Client	DHCP Server/Relay	DHCP Inform/Discover
  Not Vulnerable	*
  Not Vulnerable		*
  Not Vulnerable			*
  Vulnerable	*	*
  Not Vulnerable	*		*
  Not Vulnerable		*	*
  Not Vulnerable	*	*	*

  
  

  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 White Paper: Cisco IOS and NX-OS Software Reference Guide.

  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 White Paper: 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 XR or Cisco NX-OS Software.

Indicators of Compromise

• Exploitation of this vulnerability will cause an affected device to reload and generate a crashinfo file. Contact the Cisco Technical Assistance Center (TAC) to review the file and determine whether the device has been compromised by exploitation of this vulnerability. A stack trace may point to Process = DHCPD Receive.

Workarounds

• There are no workarounds that address this vulnerability.

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 menu 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:

  Note: Cisco IOS XE Software trains were patched early via Cisco bug CSCsy76009 (registered customers only). Use the Cisco IOS Software Checker to obtain a list of affected releases.

  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-20170322-dhcpc

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2017-March-22

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