Cyber Posture

CVE-2026-4699

High

Published: 24 March 2026

Published
24 March 2026
Modified
13 April 2026
KEV Added
—
Patch
—
CVSS Score 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0003 7.3th percentile
Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

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.

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match
prevent

Directly mitigates the CVE by requiring timely installation of patches to fixed Firefox and Thunderbird versions addressing the boundary condition flaw causing DoS crashes.

RA-5 Vulnerability Monitoring and Scanning partial match
detect

Enables scanning and monitoring to identify systems running vulnerable browser versions affected by this text and fonts layout flaw.

SI-5 Security Alerts, Advisories, and Directives partial match
detect

Ensures receipt and review of security advisories like MFSA 2026-20 detailing this CVE and necessary updates.

Security SummaryAI

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)
CWE-754

Affected Products

mozilla
firefox
≤ 115.34.0 · ≤ 149.0 · 128.0 — 140.9.0

MITRE ATT&CK Enterprise TechniquesAI

T1499.004 Application or System Exploitation Impact
Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.
attack.mitre.org →
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.

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

References