CVE-2026-20135: Double Free (CVSS 8.6)

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.

Is your site exposed to CVE-2026-20135?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

8.6

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0046

Probability of exploitation

0.38%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-415 CWE-415

References

Other References

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

Frequently Asked Questions

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. It has a CVSS v3.1 base score of 8.6 (HIGH).

How severe is CVE-2026-20135? +

CVE-2026-20135 has a CVSS v3.1 score of 8.6 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0046, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-20135? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-12659

A denial-of-service security issue exists in the affected products. The security issue stems from improper handling of exceptional conditions when …

CVE-2025-2027

A double free vulnerability has been identified in the ASUS System Analysis service. This vulnerability can be triggered by sending …

CVE-2025-8058

The regcomp function in the GNU C library version from 2.4 to 2.41 is subject to a double free if …

CVE-2026-11388

Double Free vulnerability in RTI Connext Professional (Core Libraries) allows File Manipulation. This issue affects Connext Professional: from 7.4.0 before …

CVE-2026-19316

A double-free vulnerability in the WatchGuard Fireware OS iked process allows a remote unauthenticated attacker to create a Denial of …

CVE-2026-17573

A double free vulnerability was discovered in the HDF5 library. Processing a crafted HDF5 file containing an oversized chunk size …