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 Layer 2 Tunneling Protocol (L2TP) parsing function of Cisco IOS and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause an affected device to reload.
The vulnerability is due to insufficient validation of L2TP packets. An attacker could exploit this vulnerability by sending a crafted L2TP packet to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (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-l2tpThis 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.
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Vulnerable Products
This vulnerability affects Cisco devices that are running a vulnerable release of Cisco IOS or Cisco IOS XE Software if the L2TP feature is enabled for the device and the device is configured as an L2TP Version 2 (L2TPv2) or L2TP Version 3 (L2TPv3) endpoint. By default, the L2TP feature is not enabled.
For information about which Cisco IOS and Cisco IOS XE Software releases are vulnerable, see the Fixed Software section of this advisory.
Assessing the L2TP Configuration
To determine whether L2TP is enabled and configured for a device, administrators can log in to the device and use the CLI to check for the presence of an L2TPv3 pseudowire configuration or an L2TP virtual private dialup network (VPDN) configuration.
The following example shows the configuration settings for a device with an L2TPv3 pseudowire configuration derived from a pseudowire class configuration named V3Example:
pseudowire-class V3Example encapsulation l2tpv3 Interface GigabitEthernet0/0 xconnect 192.0.2.3 16 encapsulation l2tpv3 pw-class V3Example
The following example shows the configuration settings for a device with a VPDN configuration that is named example and uses L2TP as the tunneling protocol:
vpdn enable vpdn-group example accept-dialin protocol l2tp
To check for the presence of either configuration, administrators can issue the show run | include vpdn|pseudowire|xconnect command in the CLI and assess the output of the command. The following example shows the output of the command on a router configured to use L2TP in both a pseudowire configuration and a VPDN configuration:
Router# show run | i vpdn|pseudowire|xconnect
vpdn enable vpdn-group example pseudowire-class V3Example xconnect 192.0.2.1 45 encapsulation l2tpv3 pw-class V3ExampleAlternatively, administrators can use the show vpdn command in the CLI to determine whether any L2TP tunnels are active. The following example shows the output of the command on a router that has three active L2TP tunnels:
Router# show vpdn L2TP Tunnel and Session Information Total tunnels 3 sessions 3
LocTunID RemTunID Remote Name State Remote Address Sessn L2TP Class/ Count VPDN Group 159845550 1169197789 Router2 est 192.0.2.1 1 R2signal LocID RemID TunID Username, Intf/ State Last Chg Uniq ID Vcid, Circuit 3643993064 0 159845550 16, Et0/3 wtaci 01:15:28 0 . . .If no active tunnels are using L2TP, the command output will contain:
%No active L2TP tunnels
In such cases, administrators should inspect the router configuration and verify that the L2TP feature is not configured.
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 Software or Cisco NX-OS Software.
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Exploitation of this vulnerability will cause an affected device to reload and generate a crashinfo file. Exploitation may be confirmed by decoding the stack trace for the device and determining whether the L2TP mgmt daemon process crashed. If the process crashed, device logs or the crashinfo file will contain an error message similar to the following example:
Exception to IOS Thread:
Frame pointer 0x7F8C17350300, PC = 0x7F8D519B0E03
UNIX-EXT-SIGNAL: Segmentation fault(11), Process = L2TP mgmt daemonContact the Cisco Technical Assistance Center (TAC) to review the crashinfo file and determine whether the device has been compromised by exploitation of this vulnerability.
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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 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:
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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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.
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This vulnerability was found during Cisco internal testing.
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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 1.0 Initial public release. — Final 2017-March-22
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