Firefox Esr EUVD-2026-14806

| CVE-2026-4691 CRITICAL

Use After Free (CWE-416)

2026-03-24 mozilla

GHSA-h895-88fj-h89r

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

Deliver malicious CSS stylesheet
Delivery

Trigger parser memory management error
Exploit

Free CSS object prematurely
Execution

Access freed memory in computation
Impact

Execute arbitrary code with browser privileges

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Weakness	CWE CWE-416 OWASP Top 10:2025 A06:2021 - Vulnerable and Outdated Components ASVS 5.0 V15 - Secure Coding and Architecture Memory lifecycle management MITRE ATT&CK 2 techniques across 2 tactics: Execution, Privilege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	Remote unauthenticated attacker exploits use-after-free in CSS parsing when victim visits a malicious webpage in Firefox < 149, Firefox ESR < 115.34, or Firefox ESR < 140.9. No user interaction beyond visiting the page required.
Impact	Without CVSS vector details, EPSS scores, or explicit KEV (Known Exploited Vulnerability) status in the provided data, a definitive real-world risk assessment is constrained. However, use-after-free vulnerabilities in browser rendering engines are historically high-priority issues due to their potential for reliable exploitation and memory disclosure. The fact that Mozilla issued multiple security advisories (MFSA2026-20, MFSA2026-21, MFSA2026-22) suggests coordinated disclosure affecting multiple Firefox release channels. The vulnerability's presence in both stable and ESR channels indicates broad impact across consumer and enterprise user bases. The lack of published EPSS or CVSS scoring may indicate this was coordinated under embargo, suggesting potential active exploitation risk or advanced threat considerations. Organizations should treat this as a high-priority patch requiring expedited deployment.
Exploit ScenarioDerived	An attacker could craft a malicious webpage containing specially-designed CSS that triggers a use-after-free condition in the browser's CSS parsing engine. When a victim visits the malicious site, the CSS parser would process the stylesheet, causing it to access memory that has already been freed. Through careful heap layout manipulation and repeated exploitation, an attacker could read adjacent memory regions to disclose sensitive information such as authentication tokens, cached credentials, or data from other browser tabs. No public proof-of-concept has been referenced in the provided intelligence, though the coordinated multi-advisory disclosure suggests the vulnerability may have been discovered through internal testing or coordinated vulnerability research rather than in-the-wild exploitation.

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

Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 24, 2026

Critical use-after-free in Mozilla Firefox's CSS parsing engine enables unauthenticated remote code execution with no user interaction required, affecting Firefox versions below 149, ESR 115.34, and ESR 140.9. An attacker can exploit this memory corruption vulnerability by crafting a malicious web page that triggers the vulnerability when rendered, achieving full system compromise. 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 CSS Parsing and Computation component that could allow attackers to achieve information disclosure. The vulnerability affects Firefox versions below 149, Firefox ESR versions below 115.34, and Firefox ESR versions below 140.9. While specific CVSS scoring and EPSS data are not available in the provided intelligence, the use-after-free class of vulnerability typically enables memory corruption attacks that could leak sensitive information from the browser process.

RemediationAI

Users should upgrade Firefox to version 149 or later immediately. Users of Firefox ESR should upgrade to version 115.34 or later, or version 140.9 or later, depending on their deployed ESR track. Detailed patching instructions and availability timelines are provided in the Mozilla security 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 enterprise environments using Firefox ESR, administrators should prioritize deployment of the patched ESR versions through their software distribution infrastructure. Until patches can be applied, limiting browsing to trusted websites and disabling JavaScript execution via extensions may reduce exposure, though these are temporary mitigations only and do not eliminate the underlying vulnerability.

Affected VersionsENISA

Firefox ESR unspecified <115.34

Firefox ESR unspecified <140.9

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 the CSS Parsing and Computation subsystem of the Gecko rendering engine used by Mozilla Firefox. Use-after-free vulnerabilities occur when memory is accessed after it has been deallocated, potentially allowing attackers to read or manipulate freed memory regions. In the context of CSS parsing, this could occur during the processing of malformed or specially-crafted stylesheets where the parser frees a data structure prematurely but continues to reference it. The affected products are identified via CPE strings cpe:2.3:a:mozilla:firefox and cpe:2.3:a:mozilla:firefox_esr, indicating the vulnerability affects both the main Firefox release channel and the Extended Support Release (ESR) variants used in enterprise environments.

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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 CSS Parsing and Computation component. This vulnerability affects Firefox < 149, Firefox ESR < 115.34, and Firefox ESR < 140.9.

Executive ViewAI

Why It Matters

Organizations using the CSS Parsing and Computation component. This vulnerability affects Firefox face critical risk from CVE-2026-4691, 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 93% 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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In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable

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A sandbox escape vulnerability exists in Firefox's XPCOM component due to incorrect boundary conditions and integer over

CVE-2022-29917 CRITICAL Act Now POC

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CVE-2022-26384 CRITICAL Act Now POC

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CVE-2022-34484 HIGH This Week POC

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

CVE-2022-28281 HIGH This Week POC

If a compromised content process sent an unexpected number of WebAuthN Extensions in a Register command to the parent pr

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

CVE-2022-22756 HIGH This Week POC

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

CVE-2022-22740 HIGH This Week POC

Certain network request objects were freed too early when releasing a network request handle. Rated high severity (CVSS

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Applying a CSS filter effect could have accessed out of bounds memory. Rated high severity (CVSS 8.8), this vulnerabilit

Same weakness CWE-416 – Use After Free

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CVE-2025-68613 HIGH POC

CVE-2015-1701 HIGH POC

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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-2026-4691 ADVISORY https://vuldb.com/vuln/352738 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-2026-4691 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2017512 REF https://bugzilla.redhat.com/show_bug.cgi?id=2450738 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-4691.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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