Summary

• A vulnerability in the TLS cryptography functionality of the Snort 3 Detection Engine of Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to unexpectedly restart, resulting in a denial of service (DoS) condition.

  This vulnerability is due to improper implementation of the TLS protocol. An attacker could exploit this vulnerability by sending a crafted TLS packet to an affected system. A successful exploit could allow the attacker to cause a device that is running Cisco Secure FTD Software to drop network traffic, resulting in a DoS condition. 

  Note: TLS 1.3 is not affected by this vulnerability.

  Cisco has released software updates that address this vulnerability. There are 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-tcp-dos-rHfqnwRg

  This advisory is part of the March 2026 release of the Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: March 2026 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected Cisco products if they were running a vulnerable release of Cisco Secure FTD Software or Cisco FirePOWER Services and the following conditions were met:

  • Snort 3 was enabled.
  • An SSL or decryption policy was deployed with a rule that blocked traffic from a specific TLS version. 
  • An SSL or decryption policy was set to not decrypt traffic for unsupported ciphers. 

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

  Determine the Snort Configuration on Cisco Secure FTD Software

  To determine if Snort 3 is running on Cisco Secure FTD Software, see Determine the Active Snort Version that Runs on Firepower Threat Defense (FTD). Snort 3 has to be active for this vulnerability to be exploited.

  Determine the Cisco Secure FTD Software SSL or Decryption Policy Configuration

  To determine whether an SSL or decryption policy is configured on a device that is managed by Cisco Secure Firewall Management Center (FMC) Software, use the following steps:

  1. Log in to the Cisco Secure FMC Software web-based management interface.
  2. From the Policies menu, choose Access Control > SSL or Access Control > Decryption.
     
     • If no policy is listed, the device is not affected by this vulnerability.
     • If a policy name is listed, the device has an SSL or decryption policy applied and could be affected by this vulnerability. Continue to the next configuration check.

   

  Determine Whether a Cisco Secure FTD Software SSL or Decryption Policy Configuration Blocks Traffic from a Specific TLS Version

  To determine if the SSL or decryption policy that was identified in the preceding section blocks traffic from a specific TLS version, use the following steps:

  1. Log in to the Cisco Secure FMC Software web-based management interface.
  2. From the Policies menu, choose Access Control > SSL or Access Control > Decryption.
  3. Choose the appropriate SSL or decryption policy.
  4. Click the Edit pencil icon.
  5. Choose the appropriate rule.
  6. Click the Edit pencil icon.
  7. Click the Version tab.
     
     • If all check boxes are checked, the device is not affected by this vulnerability.
     • If TLS v1.0, TLS v1.1, or TLS v1.2 is unchecked and the action is blocked, the device blocks traffic from a specific TLS version and could be affected by this vulnerability. Continue to the next configuration check.

   

  Determine Whether a Cisco Secure FTD Software SSL or Decryption Policy Configuration Decrypts Traffic for Unsupported Ciphers

  To determine if the SSL or decryption policy is configured to not decrypt traffic for unsupported ciphers, use the following steps:

  1. Log in to the Cisco Secure FMC Software web-based management interface.
  2. From the Policies menu, choose Access Control > SSL or Access Control > Decryption.
  3. Choose the appropriate SSL or decryption policy.
  4. Click the Edit pencil icon.
  5. Click the Undecryptable Actions tab.
     
     • The device is affected by this vulnerability if Unsupported Cipher Suite is Do not decrypt.
     • The device is affected by this vulnerability if Unsupported Cipher Suite is Inherit Default Action and Default Action is Do not decrypt.
     • The device is not affected by this vulnerability if Unsupported Cipher Suite is Inherit Default Action and Default Action is Block or Block with reset.
     • The device is not affected by this vulnerability if Unsupported Cipher Suite is not Do not decrypt or Inherit Default Action.

   

  Determine the Cisco Secure FTD Software SSL or Decryption Policy Default Action Configuration

  To determine the SSL or decryption policy default action, use the following steps:

  1. Log in to the Cisco Secure FMC Software web-based management interface.
  2. From the Policies menu, choose Access Control > SSL or Access Control > Decryption.
  3. Choose the appropriate SSL or decryption policy.
  4. Click the Edit pencil icon.
  5. Click the Rules tab.
  6. View the Default Action.

   

  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 products:

  • Cisco Secure Access
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
  • Cisco Secure FMC Software
  • Cisco Cyber Vision
  • Cisco Meraki products
  • Cisco Secure Access - Secure Internet Access (SIA) Advantage
  • Cisco Secure Access - Secure Private Access (SPA) Advantage
  • Cisco Umbrella Cloud-delivered Firewall (CDFW), formerly Umbrella Secure Internet Gateway (SIG)
  • Cisco Unified Threat Defense (UTD)
  • Open Source Snort 2
  • Open Source Snort 3

Workarounds

• As a workaround for this vulnerability, configure the device so that traffic from a specific TLS version is not blocked. To implement the workaround, complete the following steps:

  1. Log in to the Cisco Secure FMC Software web-based management interface.
  2. From the Policies menu, choose Access Control > SSL or Access Control > Decryption.
  3. Choose the appropriate SSL or decryption policy.
  4. Click the Edit pencil icon.
  5. Choose the appropriate rule.
  6. Click the Edit pencil icon.
  7. Click the Version tab.
  8. Ensure that all version check boxes are checked.
  9. Click Save to deploy the changes.

  While this workaround 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 fully 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 advisoryAll Critical and High advisoriesAll advisoriesCisco ASA SoftwareCisco FMC SoftwareCisco FTD SoftwareAny Platform3000 Series Industrial Security Appliances (ISA)ASA 5500-X Series FirewallsASA Service ModuleAdaptive Security Virtual Appliance (ASAv)Firepower 1000 SeriesFirepower 2100 SeriesFirepower 4100 SeriesFirepower 9000 SeriesSecure Firewall 1200 SeriesSecure Firewall 3100 SeriesSecure Firewall 4200 Series

   

  For instructions on upgrading a Cisco Secure FTD device, see the appropriate Cisco Secure FMC upgrade guide.

  Additional Resources

  For help determining the best Cisco 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 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 internal security testing.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tcp-dos-rHfqnwRg

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2026-MAR-04

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