CVE Intelligence
Medium Memory corruption

CVE-2026-20221 — Multiple Cisco Products Snort 3 Generic Routing Encapsulation Denial of Service Vulnerabilities

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.

Published Oct 21, 2026 · Updated Oct 22, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: cisco

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • EPSS 0.5% (30-day exploitation probability)+6
  • Technical severity (medium)+3
  • Automatable exploitation (SSVC)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
0.45%
ahead of 38% of scored CVEs · Oct 22, 2026
Affects
Cisco
Cisco Open Source Snort3 and 3 more
CVSS base
5.8
Medium · 3 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation
None
none · proof-of-concept · active
Automatable
Yes
can an attacker script the kill-chain steps
Technical impact
Partial
partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Cisco Cisco Open Source Snort3, Cisco Cisco Secure Firewall Threat Defense (FTD) Software, Cisco Cisco Umbrella Insights Virtual Appliance…).
  2. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
  3. No fix documented in the sources: reduce exposure and monitor the vendor advisory.
  4. Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Cisco Cisco Open Source Snort3 version 3.5.0.0version 3.9.6.0version 3.7.0.0version 3.7.2.0version 3.9.3.0version 3.6.2.0version 3.6.0.0version 3.8.1.0
and 97 more version 3.5.1.0version 3.10.0.0version 3.3.7.0version 3.3.5.0version 3.3.4.0version 3.3.3.0version 3.3.2.0version 3.3.1.0version 3.3.0.0version 3.2.2.0version 3.10.2.0version 3.7.4.0version 3.9.0.0version 3.5.2.0version 3.12.1.0version 3.6.1.0version 3.9.2.0version 3.6.3.0version 3.9.5.0version 3.7.1.0version 3.9.7.0version 3.7.3.0version 3.10.1.0version 3.9.1.0version 3.11.1.0version 3.1.58.0version 3.1.66.0version 3.1.65.0version 3.1.64.0version 3.1.63.0version 3.1.62.0version 3.1.61.0version 3.1.60.0version 3.1.59.0version 3.1.67.0version 3.1.57.0version 3.1.56.0version 3.1.55.0version 3.1.53.0version 3.1.52.0version 3.1.51.0version 3.1.50.0version 3.1.48.0version 3.1.69.0version 3.1.70.0version 3.1.71.0version 3.1.72.0version 3.1.73.0version 3.1.74.0version 3.1.75.0version 3.1.76.0version 3.1.77.0version 3.1.78.0version 3.1.81.0version 3.1.82.0version 3.1.83.0version 3.1.84.0version 3.2.1.0version 3.12.2.0version 3.1.2.0version 3.1.3.0version 3.1.4.0version 3.1.5.0version 3.1.6.0version 3.1.7.0version 3.1.8.0version 3.1.9.0version 3.1.10.0version 3.1.12.0version 3.1.13.0version 3.1.14.0version 3.1.15.0version 3.1.16.0version 3.1.17.0version 3.1.18.0version 3.1.19.0version 3.1.20.0version 3.1.21.0version 3.1.22.0version 3.1.23.0version 3.1.24.0version 3.1.25.0version 3.1.27.0version 3.1.28.0version 3.1.29.0version 3.1.30.0version 3.1.31.0version 3.1.32.0version 3.1.36.0version 3.1.38.0version 3.1.39.0version 3.1.40.0version 3.1.41.0version 3.1.42.0version 3.1.43.0version 3.1.45.0version 3.1.47.0
Vulnerable CVE Program (cve.org) e CISA ADP
Cisco Cisco Secure Firewall Threat Defense (FTD) Software version 7.0.0version 7.0.0.1version 7.0.1version 7.0.1.1version 7.0.2version 7.2.0version 7.0.2.1version 7.0.3
and 52 more version 7.2.0.1version 7.0.4version 7.2.1version 7.0.5version 7.3.0version 7.2.2version 7.2.3version 7.3.1version 7.2.4version 7.0.6version 7.2.5version 7.2.4.1version 7.3.1.1version 7.4.0version 7.0.6.1version 7.2.5.1version 7.4.1version 7.2.6version 7.0.6.2version 7.4.1.1version 7.2.7version 7.2.5.2version 7.3.1.2version 7.2.8version 7.6.0version 7.4.2version 7.2.8.1version 7.0.6.3version 7.4.2.1version 7.2.9version 7.0.7version 7.7.0version 7.4.2.2version 7.2.10version 7.6.1version 7.4.2.3version 7.0.8version 7.6.2version 7.7.10version 7.7.11version 7.0.8.1version 7.6.2.1version 7.7.10.1version 7.4.2.4version 7.2.10.2version 7.4.3version 7.6.4version 10.0.0version 7.4.4version 7.0.9version 7.2.11version 7.4.7
Vulnerable CVE Program (cve.org) e CISA ADP
Cisco Cisco Umbrella Insights Virtual Appliance version 2.6.0version 2.5.6version 2.5version 2.4.12version 2.7version 2.6.2version 2.5.5version 2.5.4
and 65 more version 2.8version 2.6.1version 2.5.7version 1.5.4version 1.5.5version 1.5.6version 2.0.0version 2.0.2version 2.0.3version 2.1.0version 2.1.2version 2.1.4version 2.1.5version 2.2version 2.2.1version 2.3version 2.3.1version 2.4version 2.4.4version 2.4.6version 2.8.9version 3.0version 3.1version 3.2version 2.8.1version 2.8.2version 2.8.3version 2.8.4version 2.8.5version 3.0.1version 3.0.2version 3.0.4version 3.0.5version 3.1.1version 3.1.2version 3.1.3version 3.1.4version 3.2.1version 3.2.2version 3.2.3version 3.3version 3.3.1version 3.3.2version 3.3.3version 3.3.4version 3.4version 3.4.1version 3.4.2version 3.4.3version 3.4.4version 3.4.5version 3.4.6version 3.5version 2.7.1version 2.7.2version 2.7.6version 2.7.9version 2.7.10version 3.5.1version 3.5.2version 3.6.1version 3.6.2version 3.7version 3.7.1version 3.8.3
Vulnerable CVE Program (cve.org) e CISA ADP
Cisco Cisco UTD SNORT IPS Engine Software version 17.15.6version 17.18.4version 26.1.2version 17.9.10version 3.17.1Sversion 16.12.3version Fuji-16.9.5version 16.12.4
and 54 more version 17.3.1aversion 16.6.6version 16.12.2version Fuji-16.9.6version 3.17.0Sversion Fuji-16.9.3version Denali-16.3.7version Fuji-16.9.2version Fuji-16.9.4version Everest-16.6.4version Everest-16.6.3version 16.6.5version Denali-16.3.5version 17.2.1rversion 17.1.1version Everest-16.6.2version 16.6.7aversion Denali-16.3.4version 16.6.1version Denali-16.3.9version Denali-16.3.3version 16.12.1aversion 17.3.2version 17.4.1aversion 16.12.5version 17.5.1version Fuji-16.9.7version 16.6.9version 17.3.3version 17.5.1aversion 17.3.4version 17.3.4aversion 17.4.2version 17.4.1bversion 17.6.1aversion 16.6.10version 17.7.1aversion 16.12.6version Fuji-16.9.8version 17.6.2version 17.8.1aversion 16.12.7version 17.3.5version 17.6.3version 17.6.3aversion 17.7.2version 17.9.1aversion 17.6.4version 17.10.1aversion 17.3.6version 16.12.8version 17.3.7version 17.9.2aversion 17.6.5
Vulnerable CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
Low

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
5.8 CVSS 3.1 Medium 3.9 1.4 psirt@cisco.com via NIST National Vulnerability Database
5.8 CVSS 3.1 Medium — — GitHub, Inc. via GitHub Security Advisories
5.8 CVSS 3.1 Medium — — cisco via CVE Program (cve.org) e CISA ADP
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L (score shown at the top: psirt@cisco.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-476 · NULL Pointer Dereference

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
  • Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.
web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)crash dumps

Timeline

What happened to this CVE, newest first.

  • 2026
  • SSVC assessment updated
    Oct 22, 2026 · CVE Program (cve.org) e CISA ADP
  • Source status changed · Received → Awaiting Analysis
    Oct 21, 2026 · NIST National Vulnerability Database
  • Affected products added · cisco/cisco cyber vision, cisco/cisco open source snort3, cisco/cisco secure firewall threat defense (ftd) software, cis
    Oct 21, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 21, 2026
  • CVSS changed · 5.8
    Oct 21, 2026 · NIST National Vulnerability Database
  • Source status changed · → Received
    Oct 21, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 21, 2026 · Alertas do Telegram
  • CVE published
    Oct 21, 2026 · cisco

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 21, 2026 – Oct 22, 2026
  • 1× Official description updated · Oct 21, 2026 – Oct 21, 2026

References

3 on the record

Elsewhere on this site

Cisco
every CVE for this vendor
Memory corruption
same class
CWE-476
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-76524 Open Source Snort 3 Control Channel Socket Aribitrary Command Execution Vulnerability 1d
  • CVE-2026-76489 Multiple Cisco Products Snort 3 Generic File Processing Memory Exhaustion Denial of Service Vulnerability 1d
  • CVE-2026-76445 Multiple Cisco Products Snort 3 Denial of Service Vulnerability 1d
  • CVE-2026-20356 Multiple Cisco Products Snort 3 Software Hardening Release: October 2026 1d
  • CVE-2026-20236 Multiple Cisco Products Snort 3 Software Hardening Release: October 2026 1d
  • CVE-2026-20291 Multiple Cisco Products Snort 3 Software Hardening Release: October 2026 1d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No fixed version, patch or mitigation documented.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-20221 is "Multiple Cisco Products Snort 3 Generic Routing Encapsulation Denial of Service Vulnerabilities", rated Medium. It affects Cisco Open Source Snort3 (Cisco); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.8 describes technical severity. EPSS 0.45% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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