---
title: "CVE-2026-20135: Cisco Secure Firewall Threat Defense - Denial of Service"
description: "Cisco Secure Firewall Threat Defense Software contains a denial of service vulnerability caused by improper buffer management in TLS 1.3 implementation, letting unauthenticated remote attackers crash the LINA process and reload the device, exploit requires sending crafted TLS 1.3 packets."
url: https://panop.io/threat-heartbeat/CVE-2026-20135/
---

# CVE-2026-20135 Cisco Secure Firewall Threat Defense - Denial of Service

Cisco Secure Firewall Threat Defense Software contains a denial of service vulnerability caused by improper buffer management in TLS 1.3 implementation, letting unauthenticated remote attackers crash the LINA process and reload the device, exploit requires sending crafted TLS 1.3 packets.

CVSS base score 8.6 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.46% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-20135)

## Details

Type

Denial of Service

Impact

Denial Of Service

CVSS vector

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

Published

16 September 2026

Updated

15 October 2026

Weakness

[CWE-415](https://cwe.mitre.org/data/definitions/415.html)

## Vendor fix

Update to the latest version of Cisco Secure Firewall Threat Defense Software.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-20135](https://nvd.nist.gov/vuln/detail/CVE-2026-20135)
- [https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls1.3-dos-dLxwFWgF](https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls1.3-dos-dLxwFWgF)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

Panop answers these inside your environment, for this CVE and every other finding.

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