---
id: "CVE-2026-20221"
kind: "vulnerability"
aliases: ["CVE-2026-20221"]
title: "Cisco Snort 3: crafted nested GRE traffic restarts the detection engine"
severity: "medium"
cvss: 5.8
epss: 0.00454
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["cisco", "infrastructure", "network-appliance", "snort-3"]
not_affected_if: ["no-gre-tunnels"]
published_at: "2026-10-21T17:18:27+00:00"
first_seen_at: "2026-10-23T21:57:29+00:00"
updated_at: "2026-10-25T04:10:01+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-20221", "first_seen_at": "2026-10-23T21:57:29+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-20221", "first_seen_at": "2026-10-24T13:15:54+00:00"}]
---

# Cisco Snort 3: crafted nested GRE traffic restarts the detection engine

Cisco's Snort 3 Detection Engine mishandles nested GRE tunnel traffic, so a crafted GRE packet sent through an affected device makes Snort restart. The restart can cause a denial of service or let traffic bypass inspection, depending on device configuration.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- unknown

## Timeline

- 2026-10-23T21:57:29+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-20221
- 2026-10-24T13:15:54+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-20221

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_