Cisco Cyber Vision GRE Tunnel Decapsulation denial of service

Summaryinfo

A vulnerability, which was classified as very critical, was found in Cisco Cyber Vision, Open Source Snort3, Secure Firewall Threat Defense Software, UDT SNORT IPS Engine Software and Umbrella Insights Virtual Appliance. This affects an unknown part of the component GRE Tunnel Decapsulation. Executing a manipulation can lead to denial of service. This vulnerability appears as CVE-2026-20221. The attack may be performed from remote. There is no available exploit. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in Cisco Cyber Vision, Open Source Snort3, Secure Firewall Threat Defense Software, UDT SNORT IPS Engine Software and Umbrella Insights Virtual Appliance. It has been declared as very critical. This vulnerability affects some unknown functionality of the component GRE Tunnel Decapsulation. The manipulation with an unknown input leads to a denial of service vulnerability. The CWE definition for the vulnerability is CWE-404. The product does not release or incorrectly releases a resource before it is made available for re-use. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

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 section of this advisory. The Snort 3 Detection Engine will restart automatically. No manual intervention is required.

The advisory is shared for download at sec.cloudapps.cisco.com. This vulnerability was named CVE-2026-20221 since 10/08/2025. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 10/21/2026). The MITRE ATT&CK project declares the attack technique as T1499.

Upgrading eliminates this vulnerability.

VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Type

• Firewall Software

Vendor

• Cisco

Name

• Cyber Vision
• Open Source Snort3
• Secure Firewall Threat Defense Software
• UDT SNORT IPS Engine Software
• Umbrella Insights Virtual Appliance

License

• commercial

Website

• Vendor: https://www.cisco.com/

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

VulDB is the best source for vulnerability data and more expert information about this specific topic.

Discussion