Mozilla Firefox — CVE-2026-84142 (Critical)

Date Sep 1, 2026

Type Vendor advisory

Signal Critical vendor advisory

Vendor / Product Mozilla · Firefox

CVE CVE-2026-84142

Critical vendor advisory CVE-2026-84142

Summary

Internally found bugs present in Thunderbird 154. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155 and Thunderbird 155.

Why it matters

This CVE carries a CRITICAL severity rating (CVSS 9.8) in Mozilla Firefox. Patch or mitigate promptly. EPSS percentile: 4%.

Operator check

Review CVE-2026-84142 in your asset inventory. Apply patches per vendor guidance and verify Firefox is not exposed. CVSS score: 9.8. EPSS probability: 0.1%; percentile: 4%.

Sources

NVD — CVE-2026-84142 Source reference Source reference

PatchBrief uses public sources. It does not scan environments, verify exposure, or replace vendor guidance.

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