CVE-2026-20135 Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability
- Exploited in the wild
- Not known
- Fix
- None listed
What is CVE-2026-20135?
CVE-2026-20135 is a high-severity vulnerability in Cisco Secure Firewall Threat Defense (FTD) Software, published on 16 Sep 2026.
Official description
A vulnerability in the TLS 1.3 implementation in Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper buffer management during the TLS 1.3 connection. An attacker could exploit this vulnerability by sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket. A successful exploit could allow the attacker to cause the LINA process to crash, which would cause the device to reload. The reload can happen before or after authentication of the connection.Note: TLS 1.3 connections include both data traffic and user-management traffic.
Weakness: CWE-415
Which versions are affected by CVE-2026-20135?
How do I fix CVE-2026-20135?
No fix has been published for CVE-2026-20135 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-20135 being exploited?
There is no known exploitation of CVE-2026-20135 in the wild and no public exploit code.
Sources
Also known as GHSA-6f7g-r5ww-w353.