CVE-2026-20356: Cisco Snort 3 uncontrolled resource consumption

CVE-2026-20356 · Severity: high · CVSS 7.5 · Published 2026-10-21

Snort 3 is an open-source intrusion detection and prevention system used by multiple Cisco security products to monitor and block network threats. A vulnerability in Snort 3 allows uncontrolled resource consumption that could lead to denial of service, affecting Cisco firewalls, security appliances, and cloud-delivered security services. Cisco has released software hardening updates to address this and related vulnerabilities discovered during an internal security review.

Technologies: Cisco Cloud-delivered Firewall, Cisco Cyber Vision, Cisco Meraki MX Security Appliances, Cisco Secure Access, Cisco Secure Firewall Threat Defense, Cisco Snort 3. Vendors: Cisco.

Quick answers

Is CVE-2026-20356 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-20356 being exploited in the wild.
How severe is CVE-2026-20356?
High severity, with a CVSS score of 7.5, published 21 October 2026.
Which products and versions are affected?
Cisco Cyber Vision before 5.6.0; Cisco Snort 3 before 3.12.4.0.
Is there a fix for CVE-2026-20356?
Yes: fixed in Snort 3 3.12.4.0; Cisco Cyber Vision 5.6.0; Cisco CDFW and CSA automatically updated as of October 21, 2026. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-20356 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-20356?
Check whether you run Cisco Cloud-delivered Firewall, Cisco Cyber Vision, Cisco IOS XE Software Unified Threat Defense or one of the others listed at an affected version. 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.

How to fix

  • Fixed in: Snort 3 3.12.4.0; Cisco Cyber Vision 5.6.0; Cisco CDFW and CSA automatically updated as of October 21, 2026

Technical details

This vulnerability (CVE-2026-20356) involves uncontrolled resource consumption (CWE-400) in Snort 3, which processes network traffic within multiple Cisco products and services. The attack vector is network-based with no authentication required, allowing a remote attacker to send specially crafted network traffic that exhausts system resources and causes a denial of service. Fixes are available in Snort 3 version 3.12.4.0 and later, and in updated cloud instances of CDFW and CSA as of October 21, 2026.

Affected products

  • Cisco Snort 3 before 3.12.4.0
  • Cisco Cloud-delivered Firewall
  • Cisco Cyber Vision before 5.6.0
  • Cisco Meraki MX Security Appliances
  • Cisco Secure Access
  • Cisco Secure Firewall Threat Defense
  • Cisco IOS XE Software Unified Threat Defense

Timeline

  • 2026-10-21: disclosed

References

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