CVE-2026-4721
Published: 24 March 2026
Summary
CVE-2026-4721 is a critical-severity Classic Buffer Overflow (CWE-120) vulnerability in Mozilla Firefox. Its CVSS base score is 9.8 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Drive-by Compromise (T1189); ranked at the 34.1th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SC-39 (Process Isolation) and SI-16 (Memory Protection).
Deeper analysis
CVE-2026-4721 encompasses multiple memory safety bugs classified under CWE-120 (Buffer Copy without Checking Size of Input) affecting Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird ESR 140.8, Firefox 148, and Thunderbird 148. Some of these bugs exhibited evidence of memory corruption, which Mozilla presumes could be leveraged with sufficient effort to enable arbitrary code execution.
The vulnerability carries a CVSS v3.1 base score of 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H), making it remotely exploitable over the network by unauthenticated attackers with low complexity and no user interaction required. Successful exploitation could grant attackers high-impact control over confidentiality, integrity, and availability, potentially allowing arbitrary code execution within the affected browser or email client processes.
Mozilla has mitigated this issue in updated releases including Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Security practitioners should consult Mozilla Security Advisories MFSA2026-20, MFSA2026-21, MFSA2026-22, and MFSA2026-23, along with the linked Bugzilla entries, for patch details and additional technical analysis.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-14857
Vulnerability details
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Memory corruption RCE in client applications (browser/email) with AV:N/UI:N directly enables drive-by compromise via malicious web/email content and exploitation for client execution to achieve arbitrary code execution.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Requires timely remediation of identified flaws, such as applying patches for memory safety bugs in Firefox and Thunderbird to eliminate the vulnerability.
Implements memory protection mechanisms like ASLR and DEP to directly mitigate exploitation of memory corruption from buffer copy without size checks.
Enforces process isolation, such as browser sandboxing, to contain arbitrary code execution within affected processes like Firefox renderers.