Skip to content
VulniPulse
Sign in
Advisory severityMedium5.8Cisco Updated

Medium [CVE-2026-20221] Multiple Cisco Products Snort 3 Generic Routing Encapsulation Denial of Service Vulnerability

This medium-severity Cisco advisory covers CVE-2026-20221 affecting Secure Firewall Threat Defense (FTD) Software, Firepower 2100 Series, Firepower 1000 Series.

Aggregated and source-linked by VulniPulse. Data sources, validation and limitations.

cisco-sa-snort3-gre-JNpMHrTv Source published Source updated

VulniPulse record published Record updated

CVE-2026-20221
Affected products & platforms
CiscoFirewallASA / FirepowerASA 5500
Open source advisory

Android app · Google Play

Monitor future Cisco CVEs from your phone.

Choose a whole vendor or a precise platform, then receive matching security advisories by phone notification, email, or both. Coverage follows 34 official vendor sources and 160+ reviewed platform categories.

Matching phone alertsOptional email delivery
Get CVE alertsSee app features

Summary

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 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.

Affected versions
  • Scope: For information about which products are affected by this vulnerability at the time of publication, see the following sections. Open Source Snort 3
  • Release 17.12 (first fixed: 17.12.8)
  • Release 17.15 (first fixed: 17.15.6)
  • Release 17.18 (first fixed: 17.18.5)
  • Release 26.1 (first fixed: 26.1.2)
  • Release 26.2 (first fixed: 26.2.1)
  • 5.4 and earlier — Migrate to a fixed release
  • Release 5.5 (first fixed: 5.5.0)
  • Release 5.6 (first fixed: 5.6.0)

Official advisory · high-confidence parse· fetched 16 hours ago·verify at source

Fixed versions
  • 17.12.8
  • 17.15.6
  • 17.18.5
  • 26.1.2
  • 26.2.1
  • 5.5.0
  • 5.6.0

Official advisory · high-confidence parse· fetched 16 hours ago·verify at source

Mitigation checklist

Recommended fix / mitigation
  • Upgrade to the first fixed release for your train per the Fixed Releases table in this advisory.
  • Release 17.12: upgrade to 17.12.8.
  • Release 17.15: upgrade to 17.15.6.
  • Release 17.18: upgrade to 17.18.5.
  • Release 26.1: upgrade to 26.1.2.
  • Release 26.2: upgrade to 26.2.1.
  • Release 5.4 and earlier: migrate to a fixed release.
  • Release 5.5: upgrade to 5.5.0.
  • Release 5.6: upgrade to 5.6.0.
Workaround status
  • There are no workarounds that address this vulnerability.

Official advisory · high-confidence parse· fetched 16 hours ago·verify at source

Discussion(0)

No comments yet. Share field notes, upgrade gotchas, or questions — verify against the vendor advisory before acting on community advice.

Sign in to join the discussion.