CVE-2026-20135: Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
If TLS 1.3 is not required, prevent exposure of the affected device by disabling TLS 1.3 on any externally accessible TLS 1.3-enabled listening sockets, since exploitation requires sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket.
Event History
Frequently Asked Questions
Which systems are exposed to this issue?
Cisco Secure Firewall Threat Defense Software devices are exposed when they have a TLS 1.3-enabled listening socket. Both TLS 1.3 data traffic and user-management traffic can be relevant.
Does an attacker need credentials or user interaction to trigger the reload?
No. The issue can be exploited remotely by an unauthenticated attacker who sends a crafted TLS 1.3 packet to an affected TLS 1.3-enabled listening socket; the reload may occur before or after connection authentication.
What is the operational impact of successful exploitation?
Successful exploitation can crash the LINA process, causing the affected device to reload unexpectedly. This results in a denial-of-service condition; the provided information does not indicate confidentiality or integrity impact.