CVE-2026-4699
Published: 24 March 2026
Summary
CVE-2026-4699 is a high-severity Improper Check for Unusual or Exceptional Conditions (CWE-754) vulnerability in Mozilla Firefox. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 7.3th 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 SI-2 (Flaw Remediation) and RA-5 (Vulnerability Monitoring and Scanning).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Directly mitigates the CVE by requiring timely installation of patches to fixed Firefox and Thunderbird versions addressing the boundary condition flaw causing DoS crashes.
Enables scanning and monitoring to identify systems running vulnerable browser versions affected by this text and fonts layout flaw.
Ensures receipt and review of security advisories like MFSA 2026-20 detailing this CVE and necessary updates.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Remote network-reachable boundary condition flaw in client application (Firefox/Thunderbird) directly enables crafted content to trigger application crash, matching application exploitation for endpoint DoS.
NVD Description
Incorrect boundary conditions in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
Deeper analysisAI
CVE-2026-4699 is a vulnerability involving incorrect boundary conditions (CWE-754) in the Layout: Text and Fonts component of Mozilla Firefox and Thunderbird. Published on 2026-03-24, it affects Firefox versions prior to 149, Firefox ESR prior to 115.34 and 140.9, Thunderbird prior to 149, and Thunderbird prior to 140.9. The issue has a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating high severity primarily due to availability impact.
Remote attackers can exploit this vulnerability over the network with low complexity, requiring no privileges, authentication, or user interaction. Successful exploitation leads to a denial-of-service condition, such as application crashes, without impacting confidentiality or integrity.
Mozilla security advisories (MFSA 2026-20 through 2026-23) and the associated Bugzilla entry (bug 2021863) confirm the vulnerability was addressed in the listed fixed releases. Security practitioners should prioritize updating affected Firefox and Thunderbird installations to mitigate the risk.
Details
- CWE(s)