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 48.2th 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).
Deeper analysis
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.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-14815
Vulnerability details
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.
- CWE(s)
Related Threats
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.
CVEs Like This One
Affected Assets
Mitigating Controls
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.
Hardening callouts derived
Configuration rules from DISA STIG baselines that reduce the attack surface for weaknesses of the type cited by this CVE. Derived transitively via CVE→CWE→STIG over `controls_xwalks` (authoritative rows only).
Oracle Linux 8 (3 rules)
- V-248574 YUM must be configured to prevent the installation of patches, service packs, device drivers, or OL 8 system components that have not been digitally signed using a certificate that is recognized and approved by the organization. via CWE-754
- V-248573 The OL 8 file integrity tool must notify the system administrator (SA) when changes to the baseline configuration or anomalies in the operation of any security functions are discovered within an organizationally defined frequency. via CWE-754
- V-248575 OL 8 must prevent the installation of software, patches, service packs, device drivers, or operating system components of local packages without verification they have been digitally signed using a certificate that is issued by a Certificate Authority (CA) that is recognized and approved by the organization. via CWE-754
Oracle Linux 9 (1 rule)
- V-271452 OL 9 must use a Linux Security Module configured to enforce limits on system services. via CWE-754