Cyber Posture

CVE-2026-4723

Critical

Published: 24 March 2026

Published
24 March 2026
Modified
13 April 2026
KEV Added
—
Patch
—
CVSS Score 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS Score 0.0002 6.2th percentile
Risk Priority 20 60% EPSS · 20% KEV · 20% CVSS

Description

Use-after-free in the JavaScript Engine component. This vulnerability was fixed in Firefox 149 and Thunderbird 149.

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match
prevent

Timely flaw remediation ensures application of patches like Firefox 149, directly eliminating the use-after-free vulnerability in the JavaScript engine.

SI-16 Memory Protection partial match
prevent

Memory protection safeguards such as ASLR and DEP hinder exploitation of use-after-free memory corruption in the JavaScript engine.

SC-39 Process Isolation partial match
prevent

Process isolation via browser sandboxing confines potential arbitrary code execution from the JavaScript engine vulnerability.

Security SummaryAI

CVE-2026-4723 is a use-after-free vulnerability (CWE-416) in the JavaScript Engine component of Mozilla Firefox and Thunderbird. This flaw allows memory corruption after an object is freed but continues to be referenced, and it was fixed in Firefox version 149 and Thunderbird version 149.

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), marking it as critical. Remote attackers require no privileges or user interaction to exploit it over the network with low complexity, potentially achieving high-impact effects on confidentiality, integrity, and availability, such as arbitrary code execution within the browser sandbox.

Mozilla's security advisories MFSA2026-20 and MFSA2026-23 describe the vulnerability and confirm the patches in Firefox 149 and Thunderbird 149 as the primary mitigation. Additional technical details are available in Bugzilla entry 2013573.

Details

CWE(s)
CWE-416

Affected Products

mozilla
firefox
≤ 149.0

MITRE ATT&CK Enterprise TechniquesAI

T1189 Drive-by Compromise Initial Access
Adversaries may gain access to a system through a user visiting a website over the normal course of browsing.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
Why these techniques?

UAF in browser JS engine directly enables RCE via malicious web content (drive-by) and client app exploitation without auth/UI.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

References