What is CVE-2025-1020?

CVE-2025-1020 is a critical-severity vulnerability in Mozilla Firefox, classified under Out-of-bounds Write. CVSS score: 9.8/10. Published 2025-02-04.

How severe is CVE-2025-1020?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Buffer overflow in Mozilla Firefox

CVE-2025-1020

Memory safety bugs present in Firefox 134 and Thunderbird 134. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 135 and Thunderbird 135.

Vulnerability class: Buffer Overflow

Published 2025-02-04 · last modified 2026-09-30.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.005 (43.4th percentile), scored 2026-10-03.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 43.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: -0.000

Affected products

Mozilla Firefox — versions 135
Mozilla Thunderbird — versions 135

Weakness classification (CWE)

CWE-787 — Out-of-bounds Write

References

bugzilla.mozilla.org/buglist.cgi (Broken Link)
www.mozilla.org/security/advisories/mfsa2025-07/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2025-11/ (Vendor Advisory)

Frequently asked questions

What is CVE-2025-1020?: CVE-2025-1020 is a critical-severity vulnerability in Mozilla Firefox, classified under Out-of-bounds Write. CVSS score: 9.8/10. Published 2025-02-04.
How severe is CVE-2025-1020?: Critical severity. CVSS v3 base score is 9.8 out of 10.