Firefox Esr EUVD-2026-14811

| CVE-2026-4696 CRITICAL

Use After Free (CWE-416)

2026-03-24 mozilla

GHSA-h22r-8qx2-2mv3

Information Disclosure Use After Free Memory Corruption Mozilla Firefox Esr

9.8

CVSS 3.1 · NVD

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

Access

Send crafted HTML with malformed text layout
Exploit

Trigger use-after-free in Layout component
Execution

Corrupt memory and execute arbitrary code
Impact

Achieve full system compromise

Vulnerability Assessment

Weakness	CWE CWE-416 OWASP Top 10:2025 A06:2021 - Vulnerable and Outdated Components ASVS 5.0 V15 - Secure Coding and Architecture Memory lifecycle management MITRE ATT&CK 2 techniques across 2 tactics: Execution, Privilege Escalation The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. Sign in to read
Exploitation Conditions	The victim must open a crafted web page or document containing malicious content in Mozilla Firefox versions < 149 (or Firefox ESR < 115.34 or < 140.9). Remote unauthenticated attacker can exploit via network without victim interaction required based on CVSS scoring.
Impact	While CVSS and EPSS scores are not provided in the available intelligence, the vulnerability's classification as a use-after-free in a core rendering component suggests high severity. Use-after-free flaws in browser engines are typically exploitable with moderate complexity, requiring an attacker to craft malicious web content that triggers specific text or font rendering paths. The attack vector is network-based (via malicious web pages), requiring no special privileges. The availability of three separate security advisories indicates this was a coordinated, multi-branch fix rather than an isolated issue, suggesting Mozilla considered it sufficiently critical to warrant priority patching. Without explicit KEV or EPSS data, the real-world risk should be assessed as moderate-to-high for users who visit untrusted websites, particularly given the widespread deployment of Firefox and the web-based attack surface. Organizations should prioritize patching, especially for ESR deployments in managed environments.
Exploit ScenarioDerived	An attacker could host a malicious website containing specially crafted fonts or text rendering instructions that trigger the use-after-free condition in Firefox's layout engine. When a user visits the malicious page, the vulnerability is triggered during text measurement or glyph rendering, allowing the attacker to read sensitive data from the browser's memory (such as cached passwords, session tokens, or other sensitive information) or potentially execute arbitrary code with the privileges of the Firefox process. No special user interaction beyond visiting the website is required, making drive-by attacks feasible via compromised legitimate sites, malvertising, or direct phishing.

Act Now Prioritize within 24-48h. Why: CRITICAL severity.

Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 24, 2026

Unauthenticated remote attackers can achieve arbitrary code execution through a use-after-free memory corruption vulnerability in Firefox's text and font rendering engine, affecting Firefox versions below 149, ESR below 115.34, and ESR below 140.9. The vulnerability requires no user interaction or special privileges and allows complete compromise of confidentiality, integrity, and availability. No patch is currently available.

Earlier versions of this analysis (1)

v1 Mar 24, 2026 - 12:45 CVSS 9.8

A use-after-free vulnerability exists in Firefox's Layout: Text and Fonts component, affecting Firefox versions prior to 149, Firefox ESR prior to 115.34, and Firefox ESR prior to 140.9. This memory safety flaw could allow attackers to read sensitive information from memory or potentially achieve code execution through carefully crafted web content. The vulnerability was reported by Mozilla and addressed across three separate security advisories (MFSA2026-20, MFSA2026-21, MFSA2026-22), indicating broad impact across Firefox release channels.

RemediationAI

Immediately upgrade Mozilla Firefox to version 149 or later, and upgrade Firefox ESR installations to version 115.34 or later (for the ESR 115 branch) or version 140.9 or later (for the ESR 140 branch). Automated update mechanisms in Firefox will deploy patched versions; manual updates are available through the Help menu (Firefox menu > Help > About Firefox) or from https://www.mozilla.org/firefox/. For enterprise environments using Firefox ESR, deploy updates via your standard patch management processes and verify update completion across all endpoints. Users who cannot immediately patch should consider temporarily restricting Firefox usage to trusted websites only and disabling JavaScript for untrusted content as a temporary mitigation, though patching remains the only reliable fix. Review the detailed advisories at https://www.mozilla.org/security/advisories/mfsa2026-20/, https://www.mozilla.org/security/advisories/mfsa2026-21/, and https://www.mozilla.org/security/advisories/mfsa2026-22/ for any additional guidance.

Affected VersionsENISA

Firefox ESR unspecified <140.9

Firefox ESR unspecified <115.34

Firefox unspecified <149

Affected Packages

euvd firefox_esr_unspecified fix: 115.34, 140.9

euvd firefox_unspecified fix: 149

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 2 packages in euvd

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Technical ContextAI · show details

The vulnerability resides in Firefox's text and font rendering subsystem, which is responsible for parsing, measuring, and displaying font data and text layout operations. Use-after-free vulnerabilities occur when memory is accessed after it has been deallocated, typically when an object's lifecycle management fails or when reference counting is incorrect. In the context of text rendering, this could manifest when font objects, glyph caches, or text layout structures are freed prematurely while still referenced by active rendering operations. The affected products identified via CPE are Mozilla Firefox (all versions) and Mozilla Firefox ESR (all versions), with the ESR branches maintaining separate version tracks for extended support customers. The root cause falls under CWE categories related to improper memory management and use-after-free conditions common in C/C++ rendering engines.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 9.8 CRITICAL

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

SUSE CRITICAL

Red Hat 7.5 HIGH

Primary rating from NVD.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

2

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

CVE Published

Mar 24, 2026 - 12:30 nvd

CRITICAL 9.8

DescriptionNVD

Use-after-free in the Layout: Text and Fonts component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, and Firefox ESR < 140.9.

Executive ViewAI

Why It Matters

CVE-2026-4696 presents critical security risk, a use-after-free vulnerability rated CVSS 9.8. Successful exploitation could critically impact the confidentiality, integrity, and availability of affected systems. Organizations should apply vendor updates immediately when available and implement compensating controls in the interim.

Business Impact: Severe

Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Enable ASLR and DEP where available; (2) Implement network segmentation; (3) Monitor for exploitation indicators.

Threat Intelligence Overview

EPSS Score

0.0%

Top 95% exploitable

Data Quality

7 sources

Confidence: High

EUVD

CVE.org

EPSS

Vulnrichment Vendor

VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity CRITICAL

CISA KEV No

Public POC No

EPSS 0.0%

Patch available Yes

Active threat No

Deciding rule

CRITICAL severity

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision

Out-of-Cycle

Exploitation

None

System Exposure· derived

Open

Automatable

Yes

Human Impact

Very High

Technical Impact

Total

Priority Score

49

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +49

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Use After Free HIGH

Use-after-free vulnerabilities occur when a program continues to access memory through a pointer after that memory has been deallocated.

Memory Corruption HIGH

Memory corruption occurs when a program writes data beyond the boundaries of allocated memory regions or accesses memory in unintended ways, violating the integrity of the process's address space.

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.

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The Mozilla Fuzzing Team reported potential vulnerabilities present in Thunderbird 91.10. Rated high severity (CVSS 8.8)

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CVE-2022-26381 HIGH This Week POC

An attacker could have caused a use-after-free by forcing a text reflow in an SVG object leading to a potentially exploi

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If a user was convinced to drag and drop an image to their desktop or other folder, the resulting object could have been

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Vendor StatusVendor

Debian

firefox

Release	Status	Fixed Version	Urgency
sid	vulnerable	148.0.2-1	-
(unstable)	fixed	(unfixed)	-

firefox-esr

Release	Status	Fixed Version	Urgency
bullseye	vulnerable	115.14.0esr-1~deb11u1	-
bullseye (security)	vulnerable	140.8.0esr-1~deb11u1	-
bookworm	vulnerable	128.14.0esr-1~deb12u1	-
bookworm (security)	vulnerable	140.8.0esr-1~deb12u1	-
trixie (security), trixie	vulnerable	140.8.0esr-1~deb13u1	-
forky, sid	vulnerable	140.8.0esr-1	-
(unstable)	fixed	(unfixed)	-

SUSE

Severity: Critical

Product	Status
Image SLES15-SP5-SAP-Azure-LI-BYOS Image SLES15-SP5-SAP-Azure-LI-BYOS-Production Image SLES15-SP5-SAP-Azure-VLI-BYOS Image SLES15-SP5-SAP-Azure-VLI-BYOS-Production Image SLES15-SP6-SAP-Azure-LI-BYOS Image SLES15-SP6-SAP-Azure-LI-BYOS-Production Image SLES15-SP6-SAP-Azure-VLI-BYOS Image SLES15-SP6-SAP-Azure-VLI-BYOS-Production Image SLES15-SP7-SAP-Azure-LI-BYOS-Production Image SLES15-SP7-SAP-Azure-VLI-BYOS-Production	Affected
SUSE Liberty Linux 10	Fixed
SUSE Liberty Linux 8	Fixed
SUSE Liberty Linux 9	Fixed
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7	Fixed

References

Advisories & patches

ADVISORY https://access.redhat.com/errata/RHSA-2026:5930 ADVISORY https://access.redhat.com/errata/RHSA-2026:5931 ADVISORY https://access.redhat.com/errata/RHSA-2026:5932 ADVISORY https://access.redhat.com/errata/RHSA-2026:6188 ADVISORY https://access.redhat.com/errata/RHSA-2026:6342 ADVISORY https://access.redhat.com/errata/RHSA-2026:6917 ADVISORY https://access.redhat.com/errata/RHSA-2026:7837 ADVISORY https://access.redhat.com/errata/RHSA-2026:7838 ADVISORY https://access.redhat.com/errata/RHSA-2026:7839 ADVISORY https://access.redhat.com/errata/RHSA-2026:7840 ADVISORY https://access.redhat.com/errata/RHSA-2026:7841 ADVISORY https://access.redhat.com/errata/RHSA-2026:7842 ADVISORY https://access.redhat.com/errata/RHSA-2026:7843 ADVISORY https://access.redhat.com/errata/RHSA-2026:7845 ADVISORY https://access.redhat.com/errata/RHSA-2026:7858 ADVISORY https://access.redhat.com/errata/RHSA-2026:8284 ADVISORY https://access.redhat.com/errata/RHSA-2026:8285 ADVISORY https://access.redhat.com/errata/RHSA-2026:8286 ADVISORY https://access.redhat.com/errata/RHSA-2026:8287 ADVISORY https://access.redhat.com/errata/RHSA-2026:8288 ADVISORY https://access.redhat.com/errata/RHSA-2026:8289 ADVISORY https://access.redhat.com/errata/RHSA-2026:8290 ADVISORY https://access.redhat.com/errata/RHSA-2026:8315 ADVISORY https://access.redhat.com/errata/RHSA-2026:8427 ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-4696 ADVISORY https://vuldb.com/?id.352741 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:1126/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:1127/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:1163/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:20978/ REF https://access.redhat.com/errata/RHSA-2026:8850 REF https://access.redhat.com/security/cve/CVE-2026-4696 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2020190 REF https://bugzilla.redhat.com/show_bug.cgi?id=2450740 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-4696.json REF https://www.mozilla.org/security/advisories/mfsa2026-20/ REF https://www.mozilla.org/security/advisories/mfsa2026-21/ REF https://www.mozilla.org/security/advisories/mfsa2026-22/ REF https://www.mozilla.org/security/advisories/mfsa2026-23/ REF https://www.mozilla.org/security/advisories/mfsa2026-24/

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