Medium [CVE-2026-20011] Multiple Cisco Products Snort 3 SMTP MIME Denial of Service Vulnerability

Multiple Cisco products are affected by a vulnerability in the Snort 3 SMTP MIME Decoder that could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to restart.

This vulnerability is due to a lack of error checking when the MIME fields of the SMTP header are parsed. An attacker could exploit this vulnerability by sending crafted SMTP packets through an established connection to be parsed by Snort 3.

A successful exploit could allow the attacker to cause a denial of service (DoS) condition when the Snort 3 Detection Engine unexpectedly restarts.

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 October 21, 2026, Security Advisories.

In addition, for further documentation of improvements and fixes in Snort 3, see Multiple Cisco Products Snort 3 Security Hardening Release: October 2026.

Affected products named by the advisory: Secure Firewall Threat Defense (FTD) Software; Firepower 2100 Series; Firepower 1000 Series; ASA 5500-X Series Firewalls; and 4 more.