Medium [CVE-2026-20290] Cisco Secure Firewall Threat Defense Software Snort 2 SSL/TLS Denial of Service Vulnerability

A vulnerability in SSL/TLS certificate parsing in the Snort 2 Detection Engine of Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the Snort 2 Detection Engine to restart.

This vulnerability is due to incomplete validation of the SSL certificate. An attacker could exploit this vulnerability by sending a crafted SSL connection setup request to be parsed by Snort 2.

A successful exploit could allow the attacker to cause the Snort 2 Detection Engine to restart unexpectedly, resulting in a denial of service (DoS) condition.

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 products named by the advisory: Firepower 2100 Series; Firepower 1000 Series; ASA 5500-X Series Firewalls; 3000 Series Industrial Security Appliances (ISA); and 3 more.