Summary

• A vulnerability in the implementation of the IPv4 Virtual Fragmentation Reassembly (VFR) feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.

  This vulnerability is due to improper reassembly of large packets that occurs when VFR is enabled on either a tunnel interface or on a physical interface that is configured with a maximum transmission unit (MTU) greater than 4,615 bytes. An attacker could exploit this vulnerability by sending fragmented packets through a VFR-enabled interface on an affected device. A successful exploit could allow the attacker to cause the 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-ipv4-vfr-dos-CXxtFacb

  This advisory is part of the March 2023 release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: March 2023 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco IOS XE Software and have the VFR feature enabled:

  • 1000 Series Integrated Services Routers
  • 4000 Series Integrated Services Routers
  • Catalyst 8000V Edge Software Routers
  • Catalyst 8200 Series Edge Platforms
  • Catalyst 8300 Series Edge Platforms
  • Catalyst 8500L Series Edge Platforms
  • Cloud Services Router 1000V Series

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

  Determine the Device Configuration

  To determine whether a device is configured for a non-default MTU, use the show running-config | include interface | mtu CLI command. If the MTU has been changed, this command will return mtu under the enabled interfaces, as shown in the following example:

  Router# show running-config | include interface | mtu 
  interface GigabitEthernet0/0/1
   mtu 9000

  To determine whether the VFR feature is enabled, use the show ip virtual-reassembly CLI command, as shown in the following example:

  Router# show ip virtual-reassembly
  GigabitEthernet0/0/1:
     Virtual Fragment Reassembly (VFR) is ENABLED [in]
     Concurrent reassemblies (max-reassemblies): 16
     Fragments per reassembly (max-fragments): 32
     Reassembly timeout (timeout): 3 seconds
     Drop fragments: OFF
  
  [Output omitted for readability]
  
  Tunnel1:
     Virtual Fragment Reassembly (VFR) is ENABLED [in]
     Concurrent reassemblies (max-reassemblies): 16
     Fragments per reassembly (max-fragments): 32
     Reassembly timeout (timeout): 3 seconds
     Drop fragments: OFF

  In these two examples, the physical interface GigabitEthernet0/0/1 has an MTU configured at greater than 4,615 bytes and has the VFR feature enabled. This means the interface is affected by this vulnerability. Interface Tunnel1, which has VFR enabled with the default MTU, is also affected by this vulnerability.

  Note: VFR is automatically enabled by features such as Network Address Translation (NAT), Zone-Based Firewall, and IPSec to process Layer 4 or Layer 7 information. Disabling VFR is not recommended. The configuration command ip virtual-reassembly will not be displayed in the output of show running-config unless the VFR feature was manually enabled. For more information, see the IP Addressing: Fragmentation and Reassembly Configuration Guide.

  Products Confirmed Not Vulnerable

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • IOS Software
  • IOS XR Software
  • Meraki products
  • NX-OS Software

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 with service contracts that entitle them to regular software updates should obtain security fixes through their usual update channels.

  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.

  The Cisco Support and Downloads page on Cisco.com provides information about licensing and downloads. This page can also display customer device support coverage for customers who use the My Devices tool.

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories 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 the Cisco Software Checker. This tool identifies any Cisco security advisories that impact a specific software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).

  To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to determine whether a release is affected by any Cisco Security Advisory. To use the form, follow these steps:

  1. Choose which advisories the tool will search-only this advisory, only advisories with a Critical or High Security Impact Rating (SIR), or all advisories.
  2. Enter a release number-for example, 15.9(3)M2 or 17.3.3.
  3. Click Check.

  
  

  Only this advisory All Critical and High advisories All advisories

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 Cisco TAC support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ipv4-vfr-dos-CXxtFacb

Revision History

• Version	Description	Section	Status	Date
  1.1	Updated the method for determining whether VFR is enabled.	Vulnerable Products	Final	2023-MAR-23
  1.0	Initial public release.	-	Final	2023-MAR-22

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