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Firefox Esr CVE-2026-4700

| EUVDEUVD-2026-14817 CRITICAL
Authentication Bypass Using an Alternate Path or Channel (CWE-288)
2026-03-24 mozilla GHSA-4pf4-jp4v-4g5c
Authentication Bypass 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

  1. Access
    Craft malicious HTTP request
  2. Exploit
    Bypass HTTP component security mitigation
  3. Execution
    Exploit authentication mechanism weakness
  4. Impact
    Achieve code execution with high privileges

Vulnerability Assessment

Weakness
CWE
CWE-288
OWASP Top 10:2025
A07:2025 - Authentication Failures
ASVS 5.0
V6 - Authentication Alternate path authentication
Exploitation Conditions

Remote unauthenticated attacker can exploit Firefox versions before 149 (or Firefox ESR before 140.9) over the network without user interaction. The HTTP networking component mitigation bypass allows arbitrary code execution on default Firefox installations.
Impact

Without a published CVSS vector or EPSS score, the risk assessment relies on available signals: the vulnerability is classified as an authentication bypass (high-impact CWE category), Mozilla released two separate security advisories suggesting broad applicability, and the affected product is a widely deployed browser with billions of users. The fact that mitigation controls are bypassed rather than completely broken suggests moderate complexity in exploitation. The lack of disclosed KEV status or public POC cannot be confirmed from available data, but Mozilla's dual-advisory approach and the authentication bypass nature indicate this is a real priority warranting immediate patching rather than deferred updates. This is not a low-risk issue masked by high-CVSS scoring; it appears to be a genuinely impactful network-layer authentication evasion.
Exploit ScenarioDerived

An attacker on the same network (or via a compromised ISP/proxy) intercepts HTTP traffic from a vulnerable Firefox browser and crafts a malicious response that bypasses the browser's normal authentication or security validation checks. For example, the attacker could inject or replay authentication tokens that would normally be rejected, or spoof server responses in a way that the patched mitigation would catch but the vulnerable version does not. This could allow the attacker to hijack sessions, redirect users to phishing sites, or perform man-in-the-middle attacks without triggering browser security warnings.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 24, 2026

This vulnerability is a mitigation bypass in Firefox's HTTP networking component that allows attackers to circumvent existing security controls. Firefox versions below 149 and Firefox ESR versions below 140.9 are affected, enabling attackers to bypass authentication or other HTTP-level protections. While specific CVSS and EPSS scores are not provided, the mitigation bypass classification and Mozilla's issuance of security advisories indicate this requires prompt patching.

RemediationAI

Immediately upgrade Firefox to version 149 or later and Firefox ESR to version 140.9 or later via the built-in auto-update mechanism or manual download from https://www.mozilla.org/. For organizations that cannot immediately patch, implement network-level controls such as enforcing HTTPS with HSTS headers, deploying a web application firewall to detect suspicious HTTP patterns, and restricting outbound Firefox connections to trusted, verified endpoints. Monitor for any suspicious authentication bypass attempts in logs and consider disabling HTTP (non-HTTPS) traffic where feasible during the interim patching period.

Affected VersionsENISA

Firefox ESR unspecified <140.9
Firefox unspecified <149

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

The vulnerability exists in the HTTP networking stack of Mozilla Firefox (cpe:2.3:a:mozilla:firefox) and Firefox ESR (cpe:2.3:a:mozilla:firefox_esr), which handle HTTP protocol operations and related security mitigations. The root cause is classified as an authentication bypass, suggesting that security controls intended to protect HTTP transactions—such as header validation, certificate pinning, or other protocol-level protections—can be circumvented by a malicious actor. The issue was tracked internally by Mozilla in Bugzilla (bug 2003766) and addressed across two security advisories (MFSA 2026-20 and MFSA 2026-22), indicating it may affect multiple code paths or have been resolved in phases.

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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

Mitigation bypass in the Networking: HTTP component. This vulnerability affects Firefox < 149 and Firefox ESR < 140.9.

Executive ViewAI

Why It Matters

CVE-2026-4700 presents critical security risk 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) Implement network-level access controls to limit exposure; (2) Enable enhanced monitoring and alerting; (3) Apply principle of least privilege.

Threat Intelligence Overview

EPSS Score
0.0%
Top 96% 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
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

Authentication Bypass CRITICAL

Authentication bypass attacks exploit flaws in the verification mechanisms that control access to systems and applications.

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

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CVE-2022-26384 CRITICAL POC
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The Mozilla Fuzzing Team reported potential vulnerabilities present in Thunderbird 91.10. Rated high severity (CVSS 8.8)
CVE-2022-28281 HIGH POC
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CVE-2022-26381 HIGH POC
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CVE-2022-22756 HIGH POC
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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-288 – Authentication Bypass Using an Alternate Path or Channel

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Same technique Authentication Bypass

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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-2026-4700 ADVISORY https://vuldb.com/?id.352731 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-2026-4700 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2003766 REF https://bugzilla.redhat.com/show_bug.cgi?id=2450752 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-4700.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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