Cisco IOS and IOS XE Software Crafted Network Time Protocol Packets Denial of Service Vulnerability

Haythem Elmir
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  • A vulnerability in the processing of Network Time Protocol (NTP) packets by Cisco IOS and Cisco IOS XE could allow an unauthenticated, remote attacker to cause an interface wedge and an eventual denial of service (DoS) condition on the affected device.

    The vulnerability is due to insufficient checks on clearing the invalid NTP packets from the interface queue. An attacker could exploit this vulnerability by sending a number of crafted NTP packets to be processed by an affected device. An exploit could allow the attacker to cause an interface wedge and an eventual denial of service (DoS) condition on the affected device.

    Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability; however, there is a mitigation for this vulnerability.

    Affected Products

  • Vulnerable Products

    The following releases of Cisco IOS Software and the corresponding releases of IOS XE Software are affected by this vulnerability:

    • 15.5(3)S3 – 3.16.3S
    • 15.6(1)S2 – 3.17.2S
    • 15.6(2)S1 – 3.18.1S
    • 15.6(2)T1

    Cisco devices running an affected version of IOS or IOS XE Software are vulnerable if they are configured for NTP operations. NTP is not enabled in Cisco IOS or IOS XE Software by default.

    To see if a device is configured with NTP, log in to the device and issue the command-line interface (CLI) command show running-config | include ntp. If the output returns any of the following commands, the device is vulnerable:

    ntp master <any following commands>
    ntp peer <any following commands>
    ntp server <any following commands>
    ntp broadcast client
    ntp multicast client

    The following example identifies a Cisco device that is configured with NTP:

    router#show running-config | include ntp
    ntp peer

    The following example identifies a Cisco device that is not configured with NTP:

    router#show running-config | include ntp

    This vulnerability can be exploited using both IPv4 and IPv6 packets. The vulnerability can be triggered by crafted NTP packets destined to UDP listening port 123 and using an IPv4 or IPv6 unicast address of any interface configured on a device or a network address.

    This vulnerability can be triggered only by traffic destined to an affected device and cannot be exploited with traffic transiting an affected device.

    Determining the Cisco IOS or IOS XE Software Release

    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 command-line interface (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 versioncommand or may provide different output.

    The following example identifies a Cisco product that is running Cisco IOS Software Release 15.5(2)T1 with 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:
    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.

    Products Confirmed Not Vulnerable

    No other Cisco products are currently known to be affected by this vulnerability.
Indicators of Compromise
  • On devices where this vulnerability is exploited, crafted NTP packets will get stuck in the ingress input queue of the receiving interface and eventually wedge the queue. Once this interface is wedged, it will stop receiving traffic until the router is reloaded.
  • There are no workarounds that address this vulnerability; however, there is a mitigation for this vulnerability.

    As a mitigation measure, customers can use Control Plane Policing (CoPP) with an interface access control list (ACL), limiting NTP traffic to that coming from known NTP peers. Detailed knowledge and careful configuration is required by the network administrators in order to avoid dropping valid NTP traffic by implementing such mitigation measures. Because the NTP protocol in this vulnerability uses UDP as a transport, it is possible to spoof the sender’s IP address, which may defeat ACLs that permit communication to these ports from trusted IP addresses.

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

    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 consult the Cisco Security Advisories and Responses archive at and review subsequent advisories 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 Technical Assistance Center (TAC):

    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.

    Fixed Releases

    This vulnerability has been corrected in the following releases of Cisco IOS Software and the corresponding releases of Cisco IOS XE Software:

    • 15.6(3)M
    • 15.6(2)SP – 3.18.0SP

    Cisco will release fixed software for remaining affected releases as soon as it is available.

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


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