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Mozilla Firefox CVE-2026-74959

| EUVDEUVD-2026-60758 CRITICAL
Protection Mechanism Failure (CWE-693)
2026-08-18 mozilla GHSA-hq5h-hpvx-rq86
Authentication Bypass Mozilla
Critical
Disputed · 9.1 Vendor: mozilla
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Attack ChainAIDerived

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

  1. Access
    Lure victim to malicious page
  2. Delivery
    Load crafted Cache API script
  3. Exploit
    Bypass Cache API mitigation
  4. Execution
    Access or alter cross-boundary data
  5. Impact
    Exfiltrate sensitive information

Vulnerability Assessment

Weakness
CWE
CWE-693
OWASP Top 10:2025
A06:2025 - Insecure Design
ASVS 5.0
V15 - Secure Coding and Architecture Protection mechanism completeness
Exploitation Conditions

Exploitation targets the Storage: Cache API code path, so the concrete prerequisite is that the victim loads attacker-controlled web content in a vulnerable Firefox build (any version before Firefox 154 / ESR 140.14 / ESR 153.1) that exercises CacheStorage/Cache - commonly reached through a Service Worker or page script. The supplied CVSS marks UI:N (no interaction), but for browser web-content flaws the realistic limiting factor is that the user must visit or render the malicious page; there is no authentication requirement (PR:N) and the flaw affects default configurations, while the mitigation-bypass nature (CWE-693) means impact is bounded to whatever protection the Cache API mitigation was enforcing. Verify the UI requirement against the Mozilla advisory, as it materially changes realistic risk.
Impact

Signals here conflict and warrant scrutiny. The supplied CVSS is 9.1 (Critical) with vector AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N, implying fully remote, unauthenticated, no-interaction exploitation - but for a browser Cache API flaw that model is questionable, since browser web-content vulnerabilities almost always require the victim to visit or load attacker-controlled content, i.e. UI:R; verify the UI metric with Mozilla. Against that high CVSS, EPSS is only 0.16% (6th percentile), there is no CISA KEV listing and no public exploit identified, so real-world exploitation pressure is currently low. This looks like a legitimately high-severity mitigation bypass by CVSS impact but not an imminent mass-exploitation threat today; it should be patched on the normal browser-update cadence rather than treated as an active-incident emergency, while recognizing browser bugs can be weaponized quickly once details or a POC circulate.
Exploit ScenarioDerived

An attacker lures a Firefox user to a malicious or compromised web page whose script abuses the Cache API to bypass the intended protection mechanism, reading data or influencing content across a security boundary it should not cross. No public exploit code has been identified, and given the CWE-693 nature the attack relies on the victim loading attacker-controlled web content rather than any pre-existing foothold.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.2% exploitation probability (EPSS)

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AnalysisAIv1 – Aug 21, 2026

Protection-mechanism bypass in the Storage: Cache API of Mozilla Firefox lets remote web content defeat a built-in security mitigation, exposing sensitive data and allowing unauthorized modification (CVSS 9.1, C:H/I:H). All Firefox builds prior to Firefox 154, ESR 140.14, and ESR 153.1 are affected. Reported by Mozilla with no public exploit identified at time of analysis; EPSS is low at 0.16% (6th percentile).

RemediationAI

Vendor-released patch: upgrade to Firefox 154, or to Firefox ESR 140.14 or ESR 153.1 if you run an Extended Support Release channel; these are the exact fixed versions and represent the primary and recommended fix. Because Firefox auto-updates by default, ensure automatic updates are enabled and force a restart of the browser to apply the fix, and in managed enterprise environments push the updated package via your software-distribution or policy tooling. No standalone configuration workaround is documented; as an interim compensating control for unpatched fleets you can restrict exposure by limiting use of untrusted sites and, where feasible, disabling Service Workers/site data storage via enterprise policy, accepting that this can break legitimate offline-capable and progressive web applications. Consult the Mozilla advisories at https://www.mozilla.org/security/advisories/mfsa2026-74/ (and mfsa2026-76 through mfsa2026-80) for release-specific detail.

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Technical ContextAI · show details

The affected component is the browser's Storage: Cache API - the standardized CacheStorage/Cache interface exposed to web pages (typically via Service Workers) for persisting HTTP request/response pairs. CWE-693 (Protection Mechanism Failure) indicates the root cause is not a memory-corruption or injection bug but the circumvention of a security control that the Cache API is supposed to enforce (for example same-origin/partitioning or another hardening mitigation), allowing behavior the mitigation was designed to prevent. The single CPE, cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*, scopes this to the Firefox application across versions up to the fixed releases; the parallel ESR fix lines (140.14 and 153.1) confirm the code path exists in both the rapid-release and Extended Support Release branches.

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Severity by source

Sources disagree (Medium–Critical)
Vendor (mozilla) PRIMARY 9.1 CRITICAL
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
vuln.today AI 8.1 HIGH

Remote web-content bug exploitable without auth (PR:N) but realistically requires the victim to load a malicious page (UI:R); high confidentiality/integrity, no availability impact.

3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
4.0 AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
SUSE HIGH
Red Hat 6.1 MEDIUM

vuln.today treats the vendor’s rating as authoritative. A higher third-party CVSS (e.g. CISA-ADP) is shown for transparency but does not drive the headline severity.

CVSS VectorVendor: mozilla

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

Lifecycle Timeline

3
CVSS changed
Aug 21, 2026 - 20:22 NVD
9.1 (CRITICAL)
CVE Published
Aug 18, 2026 - 12:23 cve.org
UNKNOWN (no severity yet)
CVE Published
Aug 18, 2026 - 12:23 cve.org
CRITICAL 9.1

DescriptionCVE.org

Mitigation bypass in the Storage: Cache API component. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, and Firefox ESR 153.1.

Executive ViewAI

Why It Matters

A critical protection-mechanism bypass in Mozilla Firefox's Cache API (CVE-2026-74959, CVSS 9.1) exposes sensitive user data and enables unauthorized content modification across all affected Firefox installations. Organizations with Firefox users face immediate data confidentiality and integrity risks until systems are updated to Firefox 154, ESR 140.14, or ESR 153.1.

Business Impact: Severe
Compensating Controls

Apply vendor patch (Firefox 154, ESR 140.14, or ESR 153.1).

Threat Intelligence Overview

EPSS Score
0.2%
Top 94% exploitable
Prevalence
High
major mainstream web browser with hundreds of millions of users
Data Quality
6 sources
Confidence: High
EUVD 🇯🇵JVN NVD CVE.org EPSS 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 0.2%
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· derived
Out-of-Cycle
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
Yes
Human Impact
Very High

Priority Score

46
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +46
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.

Affected Products (CPE)CVE.org

cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*

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Same weakness CWE-693 – Protection Mechanism Failure

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CVE-2013-2465 CRITICAL POC
9.8
CVE-2024-21412 HIGH POC
8.1
CVE-2019-1003030 CRITICAL POC
9.9
CVE-2013-0431 LOW POC
3.7
CVE-2025-40536 HIGH POC
8.1
CVE-2024-38217 MEDIUM POC
5.4
CVE-2026-21513 HIGH
8.8
CVE-2026-21510 HIGH POC
8.8

Same technique Authentication Bypass

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CVE-2015-3306 CRITICAL POC
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CVE-2023-22515 CRITICAL POC
9.8
CVE-2012-4681 CRITICAL POC
9.8
CVE-2011-3544 CRITICAL POC
9.8
CVE-2022-40684 CRITICAL POC
9.8
CVE-2025-61882 CRITICAL POC
9.8
CVE-2024-0012 CRITICAL POC
9.3

Vendor StatusVendor

SUSE

Severity: Important
Product Status
Container suse/kiosk/firefox-esr:140.14-75.12 Affected
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise High Performance Computing 15 SP7 SUSE Linux Enterprise Module for Desktop Applications 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS Fixed
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Fixed

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-74959 ADVISORY https://vuldb.com/vuln/391607 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3658/ REF https://bugzilla.mozilla.org/show_bug.cgi?id=2047853 REF https://www.mozilla.org/security/advisories/mfsa2026-74/ REF https://www.mozilla.org/security/advisories/mfsa2026-76/ REF https://www.mozilla.org/security/advisories/mfsa2026-77/ REF https://www.mozilla.org/security/advisories/mfsa2026-78/ REF https://www.mozilla.org/security/advisories/mfsa2026-79/ REF https://www.mozilla.org/security/advisories/mfsa2026-80/

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