CISCO Security Advisory
Published Date: Not specified
CVE: CVE-2026-20221
Advisory Summary
🗓️ Published: Oct 22, 2026
🔶 Multiple Cisco Products: Snort 3 GRE Tunnel Decapsulation DoS/Bypass Risk
⚠️ What’s happening
Cisco reports a medium severity vulnerability in the Snort 3 Detection Engine related to generic routing encapsulation (GRE) tunnel decapsulation, specifically improper handling of nested GRE traffic. A remote attacker can send a crafted GRE payload that triggers an unexpected restart of the Snort 3 Detection Engine.
- Denial of Service (DoS) conditions, and/or
- Detection bypass scenarios (temporary loss of inspection continuity during restart)
- Unauthenticated, remote attacker
- Exploit path: send crafted GRE payload through an affected device
- No manual intervention required because Snort 3 restarts automatically after the fault
🛠️ Fix status
✅ Cisco has released software updates to address this vulnerability.
❌ No workaround is provided.
- CVE-2026-20221
- Restart frequency under malformed nested GRE test cases
- Any operational impacts (throughput/latency and security event continuity)
- Network segmentation / ACLs limiting inbound GRE exposure
- Monitoring for Snort 3 reload/restart events and correlate with traffic patterns
-2026-20221
Reference: Vendor Advisory