Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software for Secure Firewall 3100 and 4200 Series DTLS Denial of Service Vulnerability

A vulnerability in Datagram TLS (DTLS) message handling of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Secure Firewall 3100 Series and 4200 Series devices could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to improper resource management when processing certain DTLS messages. An attacker could exploit this vulnerability by sending a crafted stream of DTLS traffic to 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 workarounds that address this vulnerability. This advisory is available at the following link: 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 .

A vulnerability in Datagram TLS (DTLS) message handling of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Secure Firewall 3100 Series and 4200 Series devices could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.

This vulnerability is due to improper resource management when processing certain DTLS messages. An attacker could exploit this vulnerability by sending a crafted stream of DTLS traffic to 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 workarounds that address this vulnerability.

This advisory is available at the following link:

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 .

Vulnerable Products This vulnerability affects Cisco Secure Firewall 3100 Series and 4200 Series devices if they are running a vulnerable release of Cisco Secure Firewall ASA or Secure FTD Software and have DTLS flow offload enabled. DTLS flow offload is enabled by default. For information which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. Determine Whether DTLS Flow Offload Is Enabled To determine whether DTLS flow offload is enabled on a Cisco Secure Firewall 3100 Series or 4200 Series device, use the show flow-offload-dtls info CLI command, as shown in the following example: # show flow-offload-dtls info DTLS offload : Enabled Egress Optimization: Enabled 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 Secure Firewall Management Center (FMC) Software.

This vulnerability affects Cisco Secure Firewall 3100 Series and 4200 Series devices if they are running a vulnerable release of Cisco Secure Firewall ASA or Secure FTD Software and have DTLS flow offload enabled. DTLS flow offload is enabled by default.

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

Determine Whether DTLS Flow Offload Is Enabled

To determine whether DTLS flow offload is enabled on a Cisco Secure Firewall 3100 Series or 4200 Series device, use the show flow-offload-dtls info CLI command, as shown in the following example:

# show flow-offload-dtls info DTLS offload : Enabled Egress Optimization: Enabled

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 Secure Firewall Management Center (FMC) Software.

There is a that addresses this vulnerability. Use the no flow-offload-dtls CLI command to disable DTLS flow offload. Note: For devices that are running Cisco FTD Software, the no flow-offload-dtls command can be pushed using FlexConfig. Disabling DTLS flow offload will cause DTLS-encrypted traffic to be processed in software rather than hardware, which may reduce throughput and increase CPU utilization on the affected device. This impact will be more significant in deployments with a high volume of DTLS sessions. 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.

There is a that addresses this vulnerability. Use the no flow-offload-dtls CLI command to disable DTLS flow offload.

Note: For devices that are running Cisco FTD Software, the no flow-offload-dtls command can be pushed using FlexConfig.

Disabling DTLS flow offload will cause DTLS-encrypted traffic to be processed in software rather than hardware, which may reduce throughput and increase CPU utilization on the affected device. This impact will be more significant in deployments with a high volume of DTLS sessions.

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.

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 for vulnerabilities that affect a specific software release. To use the form, follow these steps: Choose which advisories the tool will -all advisories, only advisories with a Critical or High Security Impact Rating (SIR) , or only this advisory. Choose the appropriate software. Choose the appropriate platform. 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. Click Check . 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

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 for vulnerabilities that affect a specific software release. To use the form, follow these steps:

Choose which advisories the tool will -all advisories, only advisories with a Critical or High Security Impact Rating (SIR) , or only this advisory.

Choose the appropriate software.

Choose the appropriate platform.

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.

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.

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 Cisco Technical Assistance Center (TAC) support case.

This vulnerability was found during the resolution of a Cisco Technical Assistance Center (TAC) support case.

Cisco Security Vulnerability Policy

To learn Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy . This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco. For additional information on Cisco's vulnerability management, disclosure cadence, and software patching strategy, see Cisco's Transition to a Risk-Based Vulnerability Disclosure Model .

To learn Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy . This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.

For additional information on Cisco's vulnerability management, disclosure cadence, and software patching strategy, see Cisco's Transition to a Risk-Based Vulnerability Disclosure Model .

Related to This Advisory

Version Description Section Status Date 1.0 Initial public release. - Final 2026-SEP-16 Show Less

SOFTWARE DOWNLOADS AND TECHNICAL SUPPORT The Cisco Support and Downloads page on Cisco.com provides information licensing and downloads. This page can also display customer device support coverage for customers who use the My Devices tool. Please note that customers may download only software that was procured from Cisco directly or through a Cisco authorized reseller or partner and for which the license is still valid. 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. When considering software upgrades , customers are advised to regularly consult the advisories for the relevant Cisco products 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 the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers. LEGAL DISCLAIMER DETAILS CISCO DOES NOT MAKE ANY EXPRESS OR IMPLIED GUARANTEES OR WARRANTIES OF ANY KIND, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. WITHOUT LIMITING THE GENERALITY OF THE FOREGOING, CISCO DOES NOT GUARANTEE THE ACCURACY OR COMPLETENESS OF THIS INFORMATION. THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME. Copies or summaries of the information contained in this Security Advisory may lack important information or contain factual errors. Customers are advised to visit the Cisco Security Advisories page for the most recent version of this Security Advisory. The Cisco Product Security Incident Response Team (PSIRT) assesses only the affected and fixed release information that is documented in this advisory. See the Cisco Security Vulnerability Policy for more information.

SOFTWARE DOWNLOADS AND TECHNICAL SUPPORT

The Cisco Support and Downloads page on Cisco.com provides information licensing and downloads. This page can also display customer device support coverage for customers who use the My Devices tool. Please note that customers may download only software that was procured from Cisco directly or through a Cisco authorized reseller or partner and for which the license is still valid.

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.

When considering software upgrades , customers are advised to regularly consult the advisories for the relevant Cisco products 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 the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

LEGAL DISCLAIMER DETAILS

CISCO DOES NOT MAKE ANY EXPRESS OR IMPLIED GUARANTEES OR WARRANTIES OF ANY KIND, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. WITHOUT LIMITING THE GENERALITY OF THE FOREGOING, CISCO DOES NOT GUARANTEE THE ACCURACY OR COMPLETENESS OF THIS INFORMATION. THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

Copies or summaries of the information contained in this Security Advisory may lack important information or contain factual errors. Customers are advised to visit the Cisco Security Advisories page for the most recent version of this Security Advisory. The Cisco Product Security Incident Response Team (PSIRT) assesses only the affected and fixed release information that is documented in this advisory. See the Cisco Security Vulnerability Policy for more information.

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