CVE intelligence

CVE-2026-20135

Vulnerability intelligence

Published Sep 16, 2026Sources checked Nov 10, 2026

8.6HIGHCVSS out of 10

What this means

Review the available evidence

CVE-2026-20135 and is rated High severity with a CVSS score of 8.6. It is not in the current CISA KEV record we collected. That does not prove exploitation has not occurred.

What to do next

Remediation and response

Cisco Security Advisories guidance

Vendor revision: Sep 16, 2026

To remediate these vulnerabilities, Cisco strongly recommends that customers upgrade to the fixed software that is indicated in the advisories.

Cisco Security Advisories guidance

Vendor revision: Sep 16, 2026

Cisco has released software updates that address this vulnerability.

CISA KEVNot listedBased on the latest collected catalog

EPSS0.7%Estimated 30-day exploitation probability

Ransomware useNot markedCISA KEV ransomware field

Threat actors0Source-linked actor relationships

Overview

What is CVE-2026-20135?

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.

Source: NVD