CVE-2026-4724
Mozilla Firefox ≤ 149.0
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-4724 is a critical-severity Reliance on Undefined, Unspecified, or Implementation-Defined Behavior (CWE-758) vulnerability in Mozilla Firefox. Its CVSS base score is 9.1 (Critical).
Operationally, ranked at the 36th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-4724 is a vulnerability involving undefined behavior in the Audio/Video component of Mozilla Firefox and Thunderbird. It affects versions of these browsers prior to 149 and is classified under CWE-758 (Reliance on Undefined, Unspecified, or Implementation-Defined Behavior). The issue carries a CVSS v3.1 base score of 9.1, indicating critical severity due to its potential for high impact on confidentiality and integrity.
Remote attackers can exploit this vulnerability over a network with low complexity, requiring no privileges or user interaction. Successful exploitation allows attackers to achieve high confidentiality impact by accessing sensitive data and high integrity impact by modifying application data, without affecting availability or changing the scope.
Mozilla's security advisories (MFSA 2026-20 and MFSA 2026-23) and the associated Bugzilla entry detail the fix implemented in Firefox 149 and Thunderbird 149. Security practitioners should prioritize updating affected browsers to these versions for mitigation.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-14863
Vulnerability Data
Undefined behavior in the Audio/Video component. This vulnerability was fixed in Firefox 149 and Thunderbird 149.
- CWE(s)
Related Threats
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation can discover reliance on undefined behavior through cross-platform or stress testing.
Mandating a documented development process and coding standards directly discourages dependence on unspecified behavior.
Engineering principles can require use of only well-defined language and API behaviors during development.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
A.8.25 mandates secure SDLC practices (secure coding guidelines, security requirements, checkpoints, testing, developer training) that reach a minority slice of undefined-behavior defects when those practices explicitly govern language-specific rules and vulnerability prevention; most instances arise from unspecified compiler/platform behaviors outside the clause's direct mandate.
Secure system architecture and engineering principles mandate use of well-specified interfaces and avoid undefined constructs.
Secure coding explicitly prohibits constructs whose behavior is undefined, unspecified or implementation-defined.
Security testing can detect some instances of undefined behavior but does not prevent the root weakness.
Hardening callouts derived
DISA STIG baselines are authored against specific weaknesses. Where this CVE cites a broad weakness class, the mapping is not asserted at that abstraction (owner ruling, 2026-09-25: grade only at Base and below), so no rule is shown here rather than a rule shown against the wrong grain.
- No fine-grained STIG row for CWE-758, a class-level weakness, and none of its children carries one either.