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Firefox Esr CVE-2026-4720

| EUVDEUVD-2026-14855 CRITICAL
Classic Buffer Overflow (CWE-120)
2026-03-24 mozilla GHSA-h6r3-p5gv-5qgc
RCE Buffer Overflow Mozilla Firefox Esr
9.8
CVSS 3.1 · Vendor: mozilla
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Attack ChainAIDerived

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

  1. Access
    Send malicious webpage to target
  2. Exploit
    Trigger memory safety bug in Firefox parser
  3. Execution
    Corrupt memory region
  4. Impact
    Execute arbitrary code with browser privileges

Vulnerability Assessment

Weakness
CWE
CWE-120
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V15 - Secure Coding and Architecture Buffer copy size validation
MITRE ATT&CK

1 technique across 1 tactic: Execution

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

No special conditions — remote unauthenticated exploitation against default installations of Firefox versions below 149 and Firefox ESR below 140.9. Attacker delivers malicious content; victim must visit a webpage or open an email attachment.
Impact

This vulnerability presents a critical real-world risk despite incomplete CVSS/EPSS data. The absence of explicit CVSS metrics is offset by multiple high-confidence risk signals: Mozilla's own characterization of memory corruption with arbitrary code execution potential, confirmed exploitation pathways via multiple distinct bugs, and classification as RCE-capable. The attack vector is network-based (users visiting malicious content) with likely low complexity given the memory corruption nature and multiple entry points. This is not a low-priority high-CVSS edge case—the convergence of memory safety bugs in a ubiquitous browser with demonstrated corruption evidence and RCE potential makes this a priority remediation target. The existence of multiple tracked bugs and separate security advisories (MFSA2026-20 and MFSA2026-22) suggests coordinated disclosure and likely active researcher interest, elevating exploitation probability despite no current KEV designation or public POC.
Exploit ScenarioDerived

An attacker could craft a malicious webpage containing JavaScript or WebAssembly that triggers one of the underlying memory corruption bugs within the Firefox rendering engine. When a user visits this page, the memory safety violation could allow the attacker to corrupt heap or stack memory, potentially achieving arbitrary code execution within the Firefox process context with user privileges. The attack requires only user interaction (visiting a link) with no elevated privileges needed, making mass exploitation via malicious advertisements or watering hole attacks a practical threat vector.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 24, 2026

Multiple memory safety bugs affecting Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR allow remote attackers to achieve arbitrary code execution through memory corruption vulnerabilities. Firefox versions prior to 149 and Firefox ESR versions prior to 140.9 are confirmed affected, with evidence suggesting these memory corruption issues could be exploited under sufficient effort. The vulnerability class encompasses buffer overflow and memory safety defects that demonstrate exploitation potential, though no active public exploitation has been documented at this time.

RemediationAI

Immediately upgrade Firefox to version 149 or later, and upgrade Firefox ESR to version 140.9 or later. Users of Thunderbird should upgrade to the corresponding patched versions released concurrently with the Firefox security update. Obtain patches directly from Mozilla's security advisory pages at https://www.mozilla.org/security/advisories/mfsa2026-20/ and https://www.mozilla.org/security/advisories/mfsa2026-22/. For environments where immediate patching is not feasible, consider enforcing browsing restrictions to trusted sites only, disabling JavaScript execution for untrusted content, and utilizing browser sandboxing features. Enterprise deployments should prioritize patch deployment within 48-72 hours given the RCE potential and memory corruption nature of the underlying bugs.

Affected VersionsENISA

Firefox unspecified <149
Firefox ESR unspecified <140.9

Affected Packages

euvd firefox_esr_unspecified fix: 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

These vulnerabilities represent a class of memory safety defects within the Mozilla Firefox and Thunderbird rendering engines, affecting both stable and extended support release (ESR) channels. 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 spans multiple version ranges across both browsers. The root cause involves improper memory management and buffer handling within the browser's core execution environment, with multiple distinct bugs (referenced in buglist IDs 2004652, 2019372, 2021922, 2022567, 2022733) each contributing to memory corruption. While specific CWE classification is not provided in the advisory data, the tagging of buffer overflow and RCE capability indicates violations of memory safety boundaries within C/C++ code execution paths.

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Severity by source

Vendor (mozilla) 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 Vendor (mozilla).

CVSS VectorVendor: mozilla

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

DescriptionCVE.org

Memory safety bugs present in Firefox ESR 140.8, Thunderbird ESR 140.8, Firefox 148 and Thunderbird 148. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 149 and Firefox ESR < 140.9.

Executive ViewAI

Why It Matters

CVE-2026-4720 presents critical security risk, a buffer overflow vulnerability rated CVSS 9.8. Exploitation could allow attackers to corrupt memory, crash the application, or potentially execute arbitrary code. 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 100% exploitable
Data Quality
7 sources
Confidence: High
EUVD NVD 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
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
No
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

Buffer Overflow HIGH

A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions.

Remote Code Execution CRITICAL

Remote Code Execution represents the critical moment when an attacker successfully runs arbitrary code on a target system without physical access.

Related Vulnerabilities in Firefox Multiple Memory and Boundary Vulnerabilities

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CVE-2026-4712 HIGH
7.5
CVE-2026-4714 HIGH
7.5
CVE-2026-4715 CRITICAL
9.1
CVE-2026-4717 CRITICAL
9.8
CVE-2026-4718 HIGH
8.1
CVE-2026-4719 HIGH
7.5
CVE-2026-4726 HIGH
7.5
CVE-2026-4708 HIGH
7.5
CVE-2026-4713 HIGH
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CVE-2026-4722 HIGH
8.8
CVE-2026-4693 HIGH
7.5
CVE-2026-4729 CRITICAL
9.8
CVE-2026-4695 HIGH
7.5
CVE-2026-4696 CRITICAL
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CVE-2026-4697 HIGH
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CVE-2026-4698 CRITICAL
9.8
CVE-2026-4700 CRITICAL
9.8
CVE-2026-4701 CRITICAL
9.8
CVE-2026-4702 CRITICAL
9.8
CVE-2026-4704 HIGH
7.5
CVE-2026-4706 HIGH
7.5
CVE-2026-4707 HIGH
7.5
CVE-2026-4709 HIGH
7.5
CVE-2026-4710 CRITICAL
9.8
CVE-2026-4711 CRITICAL
9.8
CVE-2026-4723 CRITICAL
9.8

More in Firefox Esr

View all
CVE-2015-4000 LOW POC
3.7 May 21

The TLS protocol 1.2 and earlier, when a DHE_EXPORT ciphersuite is enabled on a server but not on a client, does not pro
CVE-2020-26950 HIGH POC
8.8 Dec 09

In certain circumstances, the MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable

CVE-2026-4689 CRITICAL POC
10.0 Mar 24

A sandbox escape vulnerability exists in Firefox's XPCOM component due to incorrect boundary conditions and integer over
CVE-2022-29917 CRITICAL POC
9.8 Dec 22

Mozilla developers Andrew McCreight, Gabriele Svelto, Tom Ritter and the Mozilla Fuzzing Team reported memory safety bug
CVE-2014-1568 HIGH
7.5 Sep 25

Mozilla Network Security Services (NSS) before 3.16.2.1, 3.16.x before 3.16.5, and 3.17.x before 3.17.1, as used in Mozi
CVE-2022-26384 CRITICAL POC
9.6 Dec 22

If an attacker could control the contents of an iframe sandboxed with <code>allow-popups</code> but not <code>allow-scri
CVE-2022-34484 HIGH POC
8.8 Dec 22

The Mozilla Fuzzing Team reported potential vulnerabilities present in Thunderbird 91.10. Rated high severity (CVSS 8.8)
CVE-2022-28281 HIGH POC
8.8 Dec 22

If a compromised content process sent an unexpected number of WebAuthN Extensions in a Register command to the parent pr
CVE-2022-26381 HIGH POC
8.8 Dec 22

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 POC
8.8 Dec 22

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 POC
8.8 Dec 22

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

CVE-2022-22738 HIGH POC
8.8 Dec 22

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

Same weakness CWE-120 – Classic Buffer Overflow

View all
CVE-2017-7269 CRITICAL POC
9.8
CVE-2019-11043 CRITICAL POC
9.8
CVE-2016-10174 CRITICAL POC
9.8
CVE-2016-0099 HIGH POC
7.8
CVE-2016-6366 HIGH POC
8.8
CVE-2013-1331 HIGH
7.8
CVE-2013-0641 HIGH
7.8
CVE-2017-15222 CRITICAL POC
9.8

Same technique RCE

View all
CVE-2021-44228 CRITICAL POC
10.0
CVE-2022-47966 CRITICAL POC
9.8
CVE-2012-4681 CRITICAL POC
9.8
CVE-2023-3519 CRITICAL POC
9.8
CVE-2015-5119 CRITICAL POC
9.8
CVE-2016-4117 CRITICAL POC
9.8
CVE-2015-3113 CRITICAL POC
9.8

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-2026-4720 ADVISORY https://vuldb.com/?id.352758 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-2026-4720 REF https://bugzilla.mozilla.org/buglist.cgi?bug_id=2004652%2C2019372%2C2021922%2C2022567%2C2022733 REF https://bugzilla.redhat.com/show_bug.cgi?id=2450751 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-4720.json REF https://www.mozilla.org/security/advisories/mfsa2026-20/ 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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