Use After Free in Mozilla Firefox
CVE-2026-84120
Use-after-free in the Audio/Video component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
Vulnerability class: Use-After-Free
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.002 (11.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 11th percentile — 11.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000
Affected products
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2026-82/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-83/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-84/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-85/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-86/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-87/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-88/ (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-84120?
- CVE-2026-84120 is a medium-severity vulnerability in Mozilla Firefox, classified under Use After Free. CVSS score: 5.4/10. Published 2026-09-01.
- How severe is CVE-2026-84120?
- Medium severity. CVSS v3 base score is 5.4 out of 10.