Is CVE-2026-20135 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-20135 being exploited in the wild.

How severe is CVE-2026-20135?

High severity, with a CVSS score of 8.6, published 16 September 2026.

Which products and versions are affected?

Cisco Secure Firewall Threat Defense (FTD) See vendor advisory for affected releases.

Is there a fix for CVE-2026-20135?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-20135 attacked?

It is attacked over the network.

Am I affected by CVE-2026-20135?

Check whether you run Cisco Secure Firewall Threat Defense (FTD) 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.

CVE-2026-20135: Cisco Secure Firewall Threat Defense TLS 1.3 denial of service

CVE-2026-20135 · Severity: high · CVSS 8.6 · Published 2026-09-16

A denial of service vulnerability exists in Cisco Secure Firewall Threat Defense (FTD), a widely deployed network security appliance. An unauthenticated attacker can crash the device remotely by sending a malicious TLS 1.3 packet, forcing an unexpected reload and disrupting network operations. This can affect both management and data traffic if TLS 1.3 is enabled.

Technologies: Cisco Secure Firewall Threat Defense (FTD). Vendors: Cisco.

Quick answers

Is CVE-2026-20135 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-20135 being exploited in the wild.
How severe is CVE-2026-20135?: High severity, with a CVSS score of 8.6, published 16 September 2026.
Which products and versions are affected?: Cisco Secure Firewall Threat Defense (FTD) See vendor advisory for affected releases.
Is there a fix for CVE-2026-20135?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-20135 attacked?: It is attacked over the network.
Am I affected by CVE-2026-20135?: Check whether you run Cisco Secure Firewall Threat Defense (FTD) 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.

Technical details

This vulnerability stems from improper buffer management in the TLS 1.3 implementation within the LINA process. An unauthenticated, remote attacker can exploit this by sending a crafted TLS 1.3 packet to any TLS 1.3-enabled listening socket (commonly ports 443 or 8443). The vulnerability can be triggered before or after authentication, resulting in process crash and device reload. Cisco has released software patches; no workarounds exist, though administrators can temporarily disable TLS 1.3 and revert to TLS 1.2.

Affected products

Cisco Secure Firewall Threat Defense (FTD) See vendor advisory for affected releases

Timeline

2026-09-16: disclosed

References

https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls1.3-dos-dLxwFWgF

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