Vulnerability Description
Memory safety bugs present in Firefox ESR 115.33, 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 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Firefox | < 115.34.0 |
| Mozilla | Thunderbird | < 140.9.0 |
Related Weaknesses (CWE)
References
- https://bugzilla.mozilla.org/buglist.cgi?bug_id=2013762%2C2015291%2C2016591%2C20Broken Link
- https://www.mozilla.org/security/advisories/mfsa2026-20/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-21/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-22/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-23/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-24/Vendor Advisory
FAQ
What is CVE-2026-4721?
CVE-2026-4721 is a vulnerability with a CVSS score of 9.8 (CRITICAL). Memory safety bugs present in Firefox ESR 115.33, 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...
How severe is CVE-2026-4721?
CVE-2026-4721 has been rated CRITICAL with a CVSS base score of 9.8/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-4721?
Check the references section above for vendor advisories and patch information. Affected products include: Mozilla Firefox, Mozilla Thunderbird.