Summary

• A vulnerability in the TLS 1.3 implementation for a specific cipher for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Firepower 3100 and 4200 Series devices could allow an authenticated, remote attacker to consume resources that are associated with incoming TLS 1.3 connections, which eventually could cause the device to stop accepting any new SSL/TLS or VPN requests.

  This vulnerability is due to the implementation of the TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256. An attacker could exploit this vulnerability by sending a large number of TLS 1.3 connections with the specific TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256. A successful exploit could allow the attacker to cause a denial of service (DoS) condition where no new incoming encrypted connections are accepted. The device must be reloaded to clear this condition.

  Note: These incoming TLS 1.3 connections include both data traffic and user-management traffic. After the device is in the vulnerable state, no new encrypted connections can be accepted.

  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-3100_4200_tlsdos-2yNSCd54

  This advisory is part of the August 2025 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: August 2025 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco Secure Firewall ASA Software and Cisco Secure FTD Software if they are running on a Cisco Secure Firewall 3100 or 4200 Series device that has an SSL listen socket and is configured to allow the TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256. This is not the default configuration.

  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 Firewall ASA Software or 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 a Cisco Secure Firewall ASA device with SSL listen sockets on TCP ports 443 and 8443:

  ciscoasa# 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:*   

   

  Identify Vulnerable TLS 1.3 Cipher Software Configuration

  Devices are affected by this vulnerability only if the TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256 is configured. To determine whether a device that is running Cisco Secure Firewall ASA Software or Cisco Secure FTD Software has TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256 configured, use the show running-config all ssl | include TLS_CHACHA20_POLY1305_SHA256 CLI command. If output is returned, as shown in the following example, the device is considered vulnerable: 

  ciscoasa# show running-config all ssl | include TLS_CHACHA20_POLY1305_SHA256
  ssl cipher tlsv1.3 custom "TLS_CHACHA20_POLY1305_SHA256"

  In addition, the TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256 could be configured as a cipher if ssl cipher tlsv1.3 high, ssl cipher tlsv1.3 medium, or ssl cipher tlsv1.3 low is present under show running-config all ssl | include tlsv1.3, as shown in the following example:

  ciscoasa# show running-config all ssl | include tlsv1.3
  ssl cipher tlsv1.3 medium

  In this case, the administrator should use the show ssl ciphers medium | include v1.3 command to verify if TLS_CHACHA20_POLY1305_SHA256 is configured, as shown in the following example:

  ciscoasa# show ssl ciphers medium | include v1.3
   TLS_AES_128_GCM_SHA256 (tlsv1.3)
   TLS_CHACHA20_POLY1305_SHA256 (tlsv1.3)
   TLS_AES_256_GCM_SHA384 (tlsv1.3)

   

  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.

Indicators of Compromise

• This vulnerability causes devices to run out of Security Context Block (SCB) handles, which are used to process all incoming encrypted connections, including SSL, VPN, TLS, and Internet Key Exchange (IKE). Look for the following two indicators of compromise that could be present if this vulnerability is being exploited:

  Logged SSL Errors

  The show ssl errors CLI command could show the following repeating errors:

  error:1424A044:SSL routines:write_state_machine:internal error@libssl_ext_hndshk_accel.c:87
  error:1424A044:SSL routines:write_state_machine:internal error@libssl_ext_hndshk_accel.c:87

  SSL Error Counters

  The show counters | include HANDLE_ALLOC_FAILED counter would be incrementing rapidly, as shown in the following example:

  ciscoasa# show counters | include HANDLE_ALLOC_FAILED
  CRYPTO       HANDLE_ALLOC_FAILED                           56169   Summary

  Note: When a device is in this failed state, a reboot is required to restore connectivity.

Workarounds

• There is a workaround that addresses this vulnerability. Use the no ssl cipher tlsv1.3 custom <cipher list> CLI command to remove the cipher.

  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 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 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

   

  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 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 TAC support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-3100_4200_tlsdos-2yNSCd54

Revision History

• Version	Description	Section	Status	Date
  1.1	Updated the configuration example for the problematic cipher.	Vulnerable Products	Final	2025-SEP-03
  1.0	Initial public release.	—	Final	2025-AUG-14

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