High [CVE-2026-20135] Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability
This high-severity Cisco advisory covers CVE-2026-20135 affecting Cisco Secure Firewall Threat Defense (FTD) Software.
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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 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 versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
Mitigation checklist
- 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.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
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