Summary

• A vulnerability in the TLS 1.3 implementation in Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition.

  This vulnerability is due to improper buffer management during the TLS 1.3 connection. An attacker could exploit this vulnerability by sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket. A successful exploit could allow the attacker to cause the LINA process to crash, which would cause the device to reload. The reload can happen before or after authentication of the connection.
  
  Note: TLS 1.3 connections include both data traffic and user-management traffic.

  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-ftd-tls1.3-dos-dLxwFWgF

  This advisory is part of a group of advisories. For a complete list of the advisories and links to them, see Cisco Advance Notification for Publication of September 16, 2026, Security Advisories. In addition, for further documentation of improvements and fixes in Cisco Secure Firewall products, see Cisco Secure Firewall Adaptive Security Appliance Software, Secure Firewall Threat Defense Software, and Secure Firewall Management Center Software Hardening Release: September 2026.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco devices if they are running a vulnerable release of Cisco Secure FTD Software and have an SSL listening socket that is configured to allow TLS 1.3 connections.

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

  Determine Whether a Device Can Process TLS Packets

  To determine whether a device that is running Cisco Secure FTD Software can process TLS packets, use the show asp table socket | include SSL command and look for an SSL listening socket on any TCP port. The following example shows the output for SSL listening sockets on TCP ports 443 and 8443:

  ftd# show asp table socket | include SSL
  SSL       00185038  LISTEN     172.16.0.250:443    0.0.0.0:*
  SSL       00188638  LISTEN      10.0.0.250:8443    0.0.0.0:*

  Determine TLS Versions in Software Configuration

  To determine whether a device that is running Cisco Secure FTD Software is configured for TLS 1.3, use the show running-config all ssl CLI command. The output will show the maximum TLS version configured. If the version found is TLS 1.3, as shown in the following example, the device may be affected by this vulnerability:

  ftd# show running-config all ssl
  ssl server-max-version tlsv1.3

  Another way to check the TLS 1.3 configuration on Cisco Secure FTD Software is to use the following steps on Cisco Secure Firewall Management Center (FMC) Software:

  1. Log in to Cisco Secure FMC Software.
  2. From the menu bar, choose Devices > Platform Settings.
  3. Add or edit the Platform Settings Policy that is applied to the affected device.
  4. From the navigation menu on the left, choose SSL.
  5. From the TLS version drop-down menu, see if TLSv1.3 is selected.

   

  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:

  • Secure Firewall Adaptive Security Appliance (ASA) Software
  • Secure FMC Software

Workarounds

• There are no workarounds that address this vulnerability. However, as a mitigation, administrators may temporarily remove TLS 1.3 from the configuration of the device. Use Cisco Secure FMC Software and a FlexConfig object to set the minimum and maximum TLS version to TLS 1.2. For more information about FlexConfig objects, see the FlexConfig Policies for FTD chapter of the Firepower Management Center Configuration Guide.

  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

• Cisco considers any workarounds and mitigations (if applicable) to be temporary solutions until an upgrade to a fixed software release is available. To remediate this vulnerability and avoid future exposure as described in this advisory, Cisco strongly recommends that customers upgrade to the fixed software indicated in this advisory.

  Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software

  To help customers determine their exposure to vulnerabilities in Cisco Secure Firewall ASA, Secure FMC, and Secure FTD 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 search for vulnerabilities that affect a specific software release. To use the form, follow these steps:

  1. Choose which advisories the tool will search—all advisories, only advisories with a Critical or High Security Impact Rating (SIR), or only this advisory.
  2. Choose the appropriate software.
  3. Choose the appropriate platform.
  4. Enter a release number—for example, 9.20.3.4 for Cisco Secure Firewall ASA Software or 7.4.2 for Cisco Secure FTD Software.
  5. Click Check.

  Only this advisory All Critical and High advisories All advisories Cisco ASA Software Cisco FMC Software Cisco FTD Software Any Platform 3000 Series Industrial Security Appliances (ISA) ASA 5500-X Series Firewalls ASA Service Module Adaptive Security Virtual Appliance (ASAv) Firepower 1000 Series Firepower 2100 Series Firepower 4100 Series Firepower 9000 Series Secure Firewall 1200 Series Secure Firewall 3100 Series Secure Firewall 4200 Series

   

   

  Additional Resources

  For help determining the best Cisco Secure Firewall ASA, Secure FMC, or Secure FTD Software release, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.

  Cisco Secure Firewall ASA Compatibility
  Cisco Secure Firewall ASA Upgrade Guide
  Cisco Secure Firewall Threat Defense Compatibility Guide

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 Technical Assistance Center (TAC) support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls1.3-dos-dLxwFWgF

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2026-SEP-16

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