Summary

• A vulnerability in the implementation of the Datagram TLS (DTLS) protocol in Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause high CPU utilization, resulting in a denial of service (DoS) condition.

  This vulnerability is due to suboptimal processing that occurs when establishing a DTLS tunnel as part of an AnyConnect SSL VPN connection. An attacker could exploit this vulnerability by sending a steady stream of crafted DTLS traffic to an affected device. A successful exploit could allow the attacker to exhaust resources on the affected VPN headend device. This could cause existing DTLS tunnels to stop passing traffic and prevent new DTLS tunnels from establishing, resulting in a DoS condition.

  Note: When the attack traffic stops, the device recovers gracefully.

  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-vpndtls-dos-TunzLEV

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco products if they are running a vulnerable release of Cisco ASA Software or Cisco FTD Software and were configured for termination of DTLS tunnels for AnyConnect SSL VPN connections.

  For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Note: SSL VPN is disabled by default. When SSL VPN is enabled on an interface, DTLS is enabled by default.

  Determine the Device Configuration

  To determine whether AnyConnect SSL VPN is enabled and configured for DTLS connections, run the show asp table socket | include DTLS command. 

  If this feature is not configured, this command will produce no output. If this command shows any connections, it is configured and could be vulnerable. For example:

  device# show asp table socket | include DTLS
          DTLS   00056858   LISTEN   192.168.1.6:443   0.0.0.0:*

  Note: Cisco FTD Software Remote Access VPN features were first supported as of Release 6.2.2.

  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 Cisco Firepower Management Center (FMC) Software.

Indicators of Compromise

• Exploitation of this vulnerability could cause an affected device to show socket and crypto failures. To check whether these counters are increasing, use the show asp drop | include np-socket-new-conn-failure and show counters | include HANDLE_ALLOC_FAILED commands as shown in the following examples:

  device# show asp drop | include np-socket-new-conn-failure
  Flow drop:
  NP socket new connection failure (np-socket-new-conn-failure) 160299
  
  device# show counters | include HANDLE_ALLOC_FAILED
  CRYPTO      HANDLE_ALLOC_FAILED                               160339
  

  If these counter values increase quickly for a sustained period of time, contact the Cisco Technical Assistance Center (TAC) for further investigation.

Workarounds

• There are no workarounds that address this vulnerability. However, administrators may disable SSL DTLS VPN connections as a mitigation with the enable <interface> tls-only command in webvpn configuration mode. See example below:

  device(config-webvpn)# enable outside tls-only
  

  Note: Disabling DTLS can significantly reduce VPN performance.

  While this mitigation has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

Fixed Software

• 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.

  Fixed Releases

  At the time of publication, the release information in the following tables was accurate. See the Details section in the bug ID at the top of this advisory for the most complete and current information.

  The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability described in this advisory and which release included the fix for this vulnerability.

  ASA Software

  Cisco ASA Software Release	First Fixed Release for This Vulnerability
  9.7 and earlier1	Migrate to a fixed release
  9.8	Migrate to a fixed release.
  9.91	Migrate to a fixed release.
  9.101	Migrate to a fixed release.
  9.12	9.12.4.41 (Jun 2022)
  9.131	Migrate to a fixed release.
  9.14	9.14.4.8 (Jun 2022)
  9.15	Migrate to a fixed release
  9.16	9.16.3.3
  9.17	9.17.1.10 (Jun 2022)

  1. Cisco ASA Software releases 9.7 and earlier as well as releases 9.9, 9.10, and 9.13 have reached end of software maintenance. Customers are advised to migrate to a supported release that includes the fix for this vulnerability.

  FTD Software

  Cisco FTD Software Release	First Fixed Release for This Vulnerability
  6.2.21	Migrate to a fixed release.
  6.2.3	Migrate to a fixed release.
  6.3.01	Migrate to a fixed release.
  6.4.0	6.4.0.15 (May 2022)
  6.5.01	Migrate to a fixed release.
  6.6.0	6.6.7 (Jun 2022)
  6.7.0	Migrate to a fixed release.
  7.0.0	7.0.2 (May 2022)
  7.1.0	7.1.0.3 (Oct 2022)

  1. Cisco FMC and FTD Software releases 6.2.2 and earlier, as well as releases 6.3.0 and 6.5.0, have reached end of software maintenance. Customers are advised to migrate to a supported release that includes the fix for this vulnerability.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is aware that proof-of-concept exploit code is available for the vulnerability described in this advisory.

  The Cisco PSIRT is not aware of any malicious use of the vulnerability that is described in this advisory.

Source

• Cisco would like to thank Fabio Streun of ETH Zurich for reporting this vulnerability.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-vpndtls-dos-TunzLEV

Revision History

• Version	Description	Section	Status	Date
  1.1	Added affected and fixed release information.	Vulnerable Products and Fixed Releases	Final	2022-MAY-02
  1.0	Initial public release.	—	Final	2022-APR-20

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