# CVE-2026-20135: Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability


- **Severity:** High 8.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 16 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** GHSA-6f7g-r5ww-w353

## 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 (Double Free)

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-20135 and fix it if it is affected.

What it is: CVE-2026-20135 is a high-severity vulnerability in Cisco Secure Firewall Threat Defense (FTD) Software, published on 16 Sep 2026. 
Exploitation: There is no known exploitation of CVE-2026-20135 in the wild and no public exploit code.

Affected product: Cisco Secure Firewall Threat Defense (FTD) Software. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-20135/ (Markdown: https://insights.sbomify.com/CVE-2026-20135/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-20135
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20135
- OSV: https://osv.dev/vulnerability/CVE-2026-20135

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-20135/
Summaries are generated and may contain mistakes; the official description above is the record.