CVE-2026-4691
Published: 24 March 2026
Summary
CVE-2026-4691 is a critical-severity Use After Free (CWE-416) 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 38.2th 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-2026-4691 is a use-after-free vulnerability (CWE-416) in the CSS Parsing and Computation component of Mozilla products. 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 vulnerability was published on 2026-03-24 and carries a CVSS v3.1 base score of 9.8, indicating critical severity.
The vulnerability enables remote exploitation over the network (AV:N) with low attack complexity (AC:L), requiring no privileges (PR:N) or user interaction (UI:N) in an unchanged impact scope (S:U). Attackers can achieve high impacts on confidentiality, integrity, and availability (C:H/I:H/A:H), potentially allowing arbitrary code execution or other severe compromises within the affected browser or mail client context.
Mozilla fixed the issue in Firefox 149, Firefox ESR 115.34 and 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 Bugzilla bug 2017512, for detailed patch information and mitigation guidance.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-14806
Vulnerability details
Use-after-free in the CSS Parsing and Computation 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?
Use-after-free in CSS parsing enables remote arbitrary code execution in browser/email client (AV:N, UI:N), directly mapping to drive-by delivery of malicious web content and client-side exploitation for code execution.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly addresses the CVE by requiring timely remediation through patching to the fixed versions of Firefox and Thunderbird.
Provides memory protections like ASLR and non-executable memory to mitigate use-after-free exploitation leading to arbitrary code execution.
Enables vulnerability scanning to identify systems running vulnerable versions of Firefox or Thunderbird affected by this use-after-free flaw.
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 (1 rule)
- V-248592 OL 8 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416
RHEL 8 (1 rule)
- V-230279 RHEL 8 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416
RHEL 9 (1 rule)
- V-257794 RHEL 9 must clear memory when it is freed to prevent use-after-free attacks. via CWE-416