Multiple Cisco Products Snort 3 Generic Routing Encapsulation Denial of Service Vulnerability

Description

Multiple Cisco products are affected by a vulnerability in the generic routing encapsulation (GRE) tunnel decapsulation feature of the Snort 3 Detection Engine that could allow an unauthenticated, remote attacker to cause an unexpected restart of the Snort 3 Detection Engine. This vulnerability is due to improper handling of nested GRE traffic. An attacker could exploit this vulnerability by sending a crafted GRE payload through an affected device. A successful exploit could allow the attacker to trigger an unexpected reload of the Snort 3 Detection Engine, resulting in either a bypass or denial of service (DoS) condition, depending on device configuration. For more information, see the Details section of this advisory. The Snort 3 Detection Engine will restart automatically. No manual intervention is required. Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability. This advisory is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort3-gre-JNpMHrTv 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 Open Source Snort 3, see Multiple Cisco Products Snort 3 Security Hardening Release: October 2026 . Security Impact Rating: Medium CVE: CVE-2026-20221

What to do

Install the fixed versions listed in the official advisory. If you cannot update right away, apply the workarounds or mitigations it describes and limit access to the affected service.

Details

Vendor

Cisco

Product

Snort 3

CVE

CVE-2026-20221

Affected versions

Cisco Cyber Vision 3.0: 7 versions, from 3.0.0 to 3.0.6
Cisco Cyber Vision 3.1: 3.1.0; 3.1.1; 3.1.2
Cisco Cyber Vision 3.2: 5 versions, from 3.2.0 to 3.2.4
Cisco Cyber Vision 4.0: 4 versions, from 4.0.0 to 4.0.3
Cisco Cyber Vision 4.1: 8 versions, from 4.1.0 to 4.1.7
Cisco Cyber Vision 4.2: 7 versions, from 4.2.0 to 4.2.6
Cisco Cyber Vision 4.3: 4 versions, from 4.3.0 to 4.3.3
Cisco Cyber Vision 4.4: 4 versions, from 4.4.0 to 4.4.3
Cisco Cyber Vision 5.0: 5.0.0; 5.0.1; 5.0.2
Cisco Cyber Vision 5.1: 4 versions, from 5.1.0 to 5.1.3
Cisco Cyber Vision 5.2: 5.2.0; 5.2.1
Cisco Cyber Vision 5.3: 4 versions, from 5.3.0 to 5.3.3
Cisco Cyber Vision 5.5: 5.5.1
Cisco Cyber Vision 5.6: 5.6.0; 5.6.1
Cisco Cyber Vision 5.4: 5.4.0; 5.4.1; 5.4.2
Cisco Open Source Snort3 3.12: 3.12.1.0; 3.12.2.0
Cisco Open Source Snort3 3.1: 70 versions, from 3.1.2.0 to 3.1.84.0
Cisco Open Source Snort3 3.2: 3.2.1.0; 3.2.2.0
Cisco Open Source Snort3 3.3: 7 versions, from 3.3.0.0 to 3.3.7.0
Cisco Open Source Snort3 3.5: 3.5.0.0; 3.5.1.0; 3.5.2.0
Cisco Open Source Snort3 3.8: 3.8.1.0
Cisco Open Source Snort3 3.6: 4 versions, from 3.6.0.0 to 3.6.3.0
Cisco Open Source Snort3 3.9: 7 versions, from 3.9.0.0 to 3.9.7.0
Cisco Open Source Snort3 3.7: 5 versions, from 3.7.0.0 to 3.7.4.0
Cisco Open Source Snort3 3.10: 3.10.0.0; 3.10.1.0; 3.10.2.0
Cisco Open Source Snort3 3.11: 3.11.1.0
Cisco Secure Firewall Threat Defense (FTD) Software 7.0: 17 versions, from 7.0.0 to 7.0.9
Cisco Secure Firewall Threat Defense (FTD) Software 7.2: 18 versions, from 7.2.0 to 7.2.11
Cisco Secure Firewall Threat Defense (FTD) Software 7.3: 4 versions, from 7.3.0 to 7.3.1.2
Cisco Secure Firewall Threat Defense (FTD) Software 7.4: 11 versions, from 7.4.0 to 7.4.7 (from the advisory and NVD; check the exact builds in the official advisory)

CVSS score

5.8

Actively exploited

Not known at this time

Source

Cisco PSIRT – original advisory

Published

October 22, 2026

The description and the recommended action are quoted from the public advisory (the action is our generic advice); severity, exploitation status and affected versions are compiled automatically from public sources and may contain errors. The vendor's original advisory prevails: check it before taking action. This information is provided without warranty and does not replace a professional assessment.