Cisco Security Advisory
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CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
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A vulnerability in the DHCP option 82 encapsulation functionality of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition.
The vulnerability exists because the affected software performs incomplete input validation of option 82 information that it receives in DHCP Version 4 (DHCPv4) packets from DHCP relay agents. An attacker could exploit this vulnerability by sending a crafted DHCPv4 packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, 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-20180328-dhcpr3This 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.
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Vulnerable Products
This vulnerability affects Cisco devices that meet all the following criteria:
- The device is running a vulnerable release of Cisco IOS Software or Cisco IOS XE Software. For information about which Cisco IOS and IOS XE Software releases are vulnerable, see the Fixed Software section of this advisory.
- An interface of the device is configured as a DHCP relay agent.
- The device or an interface of the device is configured to insert DHCP relay agent information (option 82 information) into DHCP packets.
- The device or an interface of the device is configured to encapsulate option 82 information that it receives from other DHCP relay agents.
Assessing the DHCP Relay Agent Configuration
To determine whether any interfaces of a device are configured as a DHCP relay agent, administrators can log in to the device and use the show running-config | include ip helper-address command in the CLI. If the device is a Cisco cBR-8 Converged Broadband Router, administrators should instead use the show running-config | include cable helper-address command in the CLI.
If the command returns output, at least one interface of the device is configured as a DHCP relay agent.
The following example shows the output of the show running-config | include ip helper-address command for a device that is running Cisco IOS Software and has an interface that is configured to act as a DHCP relay agent and forward DHCP packets to the DHCP server address 10.10.10.1:
Router# show running-config | include ip helper-address ip helper-address 10.10.10.1 Router#If the show running-config | include ip helper-address command, or the show running-config | include cable helper-address command on a Cisco cBR-8 Router, does not return any output, none of the interfaces of the device are configured as a DHCP relay agent.
Assessing Support for Option 82 Insertion
To determine whether a device or an interface of a device is configured to insert option 82 information into DHCP packets, administrators can log in to the device and use the show running-config | include ip dhcp relay information option command in the CLI.
If the command output contains any of the following, the device is configured to insert option 82 information into DHCP packets:
- ip dhcp relay information option-insert—Interface configuration, appears under the interface that is configured as a DHCP relay agent
- ip dhcp relay information option server-id-override—Interface configuration, appears under the interface that is configured as a DHCP relay agent
- ip dhcp relay information option—Global configuration
The following example shows the output of the show running-config | include ip dhcp relay information option command for a device that is running Cisco IOS Software and has an interface that is configured to both act as a DHCP relay agent and insert option 82 information into DHCP packets:
Router# show running-config | include ip dhcp relay information option ip dhcp relay information option-insert Router#If the show running-config | include ip dhcp relay information option command does not return any output, neither the device nor any interfaces of the device are configured to insert option 82 information into DHCP packets.
Assessing Support for Option 82 Encapsulation
To determine whether a device or an interface of a device is configured to encapsulate DHCP option 82 information that it receives from other DHCP relay agents, administrators can log in to the device and use the show running-config | include ip dhcp relay information policy.* encapsulate command in the CLI.
If the command output contains either of the following, the device is configured to encapsulate option 82 information that it receives:
- ip dhcp relay information policy-action encapsulate—Interface configuration, appears under the interface that is configured as a DHCP relay agent
- ip dhcp relay information policy encapsulate—Global configuration
The following example shows the output of the command for a device that is running Cisco IOS Software and is configured to both act as a DHCP relay agent and encapsulate option 82 information that it receives from other DHCP relay agents:
Router# show running-config | include ip dhcp relay information policy.* encapsulate ip dhcp relay information policy encapsulate Router#If the show running-config | include ip dhcp relay information policy.* encapsulate command does not return any output, neither the device nor any interfaces of the device are configured to encapsulate option 82 information.
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.
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 XR Software or Cisco NX-OS Software.
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To exploit this vulnerability, exploit packets would need to be received via an interface that meets all the configuration requirements outlined in the Vulnerable Products section of this advisory.
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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.htmlAdditionally, 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.htmlCustomers 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.
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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.
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Cisco would like to thank Tenable for finding and reporting this vulnerability.
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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.
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Show LessVersion 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
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