# CVE-2026-16405 — Mozilla Firefox

> As of 2026-07-22, CVE-2026-16405 is a HIGH-severity vulnerability in Mozilla Firefox, CVSS v3.1 7.5, EPSS 0.2% (17.3th percentile). Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2026-16405, most recently “Mozilla Firefox / Thunderbird Information Disclosure Vulnerability in Networking: WebSockets (CVE-2026-16405)”.

**Last updated:** 2026-07-22

## What is CVE-2026-16405?

Information disclosure in the Networking: WebSockets component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

The record classifies CVE-2026-16405 under weakness class [CWE-200](https://intel.threadlinqs.com/cwe/CWE-200). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality. 2 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 76 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 7.5 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 17.3% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 4.4/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-07-21, last modified 2026-07-22

## Is CVE-2026-16405 being exploited?

No public exploitation evidence for CVE-2026-16405 is recorded in the sources Threadlinqs tracks — CISA KEV, Exploit-DB, public proof-of-concept repositories and ProjectDiscovery Nuclei — as of 2026-07-22. That is an absence of evidence in those feeds, not a guarantee that the vulnerability is unexploited.

## Affected products and versions

- [Mozilla](https://intel.threadlinqs.com/vendors/mozilla): Firefox, Thunderbird

## How to fix CVE-2026-16405

Vendor advisory: [https://www.mozilla.org/security/advisories/mfsa2026-68/](https://www.mozilla.org/security/advisories/mfsa2026-68/). Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory.

## Threat activity tracking CVE-2026-16405

1 tracked threat in the Threadlinqs corpus references CVE-2026-16405, either in the campaign’s CVE list or as an indicator on the campaign record.

- [Mozilla Firefox / Thunderbird Information Disclosure Vulnerability in Networking: WebSockets (CVE-2026-16405)](https://intel.threadlinqs.com/threat/TL-2026-1878) — HIGH · 2026-08-04

## Sources

Enriched from CVE.org, NVD, FIRST EPSS, GitHub Security Advisories. Last verified by Threadlinqs on 2026-08-17. This product uses the NVD API but is not endorsed or certified by the NVD.

**Other references**

- [bugzilla.mozilla.org](https://bugzilla.mozilla.org/show_bug.cgi?id=2036591)
- [mozilla.org](https://www.mozilla.org/security/advisories/mfsa2026-68/)
- [mozilla.org (mfsa2026 70)](https://www.mozilla.org/security/advisories/mfsa2026-70/)
- [mozilla.org (mfsa2026 71)](https://www.mozilla.org/security/advisories/mfsa2026-71/)
- [mozilla.org (mfsa2026 72)](https://www.mozilla.org/security/advisories/mfsa2026-72/)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-16405
Full detection coverage and IOCs for threats exploiting CVE-2026-16405 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp