# CVE-2026-20221: Cisco multiple products Snort 3 GRE tunnel denial of service

Source: https://junglewise.ai/threats/cve-2026-20221-multiple-cisco-products-are-affected-by-a-vulnerability-in-the  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-21. Updated: 2026-10-21  
License: CC BY 4.0 with a link back

CVE-2026-20221 · Severity: medium · CVSS 5.8 · Published 2026-10-21

Multiple Cisco security appliances and routers running Snort 3 intrusion detection contain a vulnerability in GRE tunnel processing. An unauthenticated remote attacker can send a crafted network packet to cause the Snort detection engine to restart unexpectedly, either bypassing threat detection or causing a temporary service outage depending on how the device is configured.

Technologies: [Cisco IOS XE Unified Threat Defense](https://junglewise.ai/threats/technologies/ios-xe-unified-threat-defense), [Cisco Meraki MX Security Appliances](https://junglewise.ai/threats/technologies/meraki-mx-security-appliances), [Cisco Secure Firewall Threat Defense](https://junglewise.ai/threats/technologies/firepower-threat-defense). Vendors: [Cisco](https://junglewise.ai/threats/vendors/cisco).

## Quick answers

**Is CVE-2026-20221 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-20221 being exploited in the wild.

**How severe is CVE-2026-20221?** Medium severity, with a CVSS score of 5.8, published 21 October 2026.

**Which products and versions are affected?** Cisco Snort 3 Detection Engine; Cisco Secure Firewall Threat Defense; Cisco IOS XE Unified Threat Defense; Cisco Catalyst 8000 Series; and 1 more.

**Is there a fix for CVE-2026-20221?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-20221 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-20221?** Check whether you run Cisco Catalyst 8000 Series, Cisco IOS XE Unified Threat Defense, Cisco Meraki MX Security Appliances or one of the others listed. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A null pointer dereference (CWE-476) in Snort 3's GRE tunnel decapsulation feature fails to properly validate nested GRE payloads. An unauthenticated, network-based attacker can trigger an unexpected engine restart by sending a malformed GRE packet, resulting in either detection bypass or denial of service. The engine automatically restarts without manual intervention.

## Affected products

- Cisco Snort 3 Detection Engine
- Cisco Secure Firewall Threat Defense
- Cisco IOS XE Unified Threat Defense
- Cisco Catalyst 8000 Series
- Cisco Meraki MX Security Appliances

## Timeline

- 2026-10-21: disclosed

## References

- [https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort3-gre-JNpMHrTv](https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort3-gre-JNpMHrTv)

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