cisco-sa-ftd-tls1.3-dos-dLxwFWgFHighCVSS 8.6

Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability

Published
September 16, 2026
Last Modified
September 16, 2026

🔗 CVE IDs covered (1)

CVE-2026-20135 →

📋 Description

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 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 ["https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-notice-jfxK98ZP"]. 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 ["https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-hardening-asaftdfmc-uvpPROhN"].

CVE-2026-20135 — Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability (CVSS 3.1 8.6, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H)

🎯 Affected products69

  • Cisco 3000 Series Industrial Security Appliances (ISA)
  • Cisco ASA 5500-X Series Firewalls
  • Cisco Firepower 1000 Series
  • Cisco Firepower 2100 Series
  • Cisco Firepower 4100 Series
  • Cisco Firepower 9000 Series
  • Cisco Secure Firewall 1200 Series
  • Cisco Secure Firewall 3100 Series
  • Cisco Secure Firewall 4200 Series
  • Cisco Secure Firewall Threat Defense (FTD) Software
  • Cisco Secure Firewall Threat Defense Virtual
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.0
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.0.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.1.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.2
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.2.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.3
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.4
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.5
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.2
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.3
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.7
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.8
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.8.1
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.0.9
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.2.0
  • Cisco Secure Firewall Threat Defense (FTD) Software 7.2.0.1
  • +39 more not shown

✅ Remediation

Cisco has released software updates that address this vulnerability. https://software.cisco.com 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 ["https://www.cisco.com/c/en/us/td/docs/security/firepower/70/configuration/guide/fpmc-config-guide-v70/flexconfig_policies.html"] 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.

🔗 References (8)