Buffer overflow in Mozilla Firefox
CVE-2026-4720
Memory safety bugs present in Firefox ESR 140.8, Thunderbird ESR 140.8, Firefox 148 and Thunderbird 148. 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 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
Vulnerability class: Buffer Overflow
Published · last modified .
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: 0.004 (34.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 34th percentile — 34.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- Mozilla Firefox — versions 140.9, 149
- Mozilla Thunderbird — versions 140.9, 149
Weakness classification (CWE)
References
- bugzilla.mozilla.org/buglist.cgi (Broken Link)
- www.mozilla.org/security/advisories/mfsa2026-20/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-22/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-23/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-24/ (Vendor Advisory)
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Frequently asked questions
- What is CVE-2026-4720?
- CVE-2026-4720 is a critical-severity vulnerability in Mozilla Firefox, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2026-03-24.
- How severe is CVE-2026-4720?
- Critical severity. CVSS v3 base score is 9.8 out of 10.