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Mozilla Firefox CVE-2026-74986

| EUVDEUVD-2026-60698 CRITICAL
Information Exposure (CWE-200)
2026-08-18 mozilla GHSA-r8r4-h9f4-p8gc
Information Disclosure 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
    Serve crafted CSS content
  3. Exploit
    Bypass site-isolation in CSS engine
  4. Execution
    Read cross-origin data
  5. Impact
    Exfiltrate leaked information to attacker

Vulnerability Assessment

Weakness
CWE
CWE-200
OWASP Top 10:2025
A01:2025 - Broken Access Control
ASVS 5.0
V14 - Data Protection Data exposure prevention
MITRE ATT&CK

2 techniques across 2 tactics: Collection, Discovery

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

Exploitation requires the victim to load attacker-controlled web content that triggers the vulnerable CSS parsing/computation path in Firefox - the specific prerequisite is rendering a malicious page or embedded resource in an unpatched Firefox (before 154) or Firefox ESR (before 153.1). The supplied vector lists UI:N, but as a browser rendering flaw the realistic limiting factor is that a user must navigate to or load the malicious content; no authentication is required (PR:N). Exploitation is bounded to information disclosure across the site-isolation boundary rather than code execution, and is limited to the Firefox desktop product on affected versions.
Impact

Signals are mixed and lean toward 'patch promptly but not a fire drill.' The supplied CVSS is 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H), but two elements deserve scrutiny: for a CWE-200 information-disclosure bug the A:H (high availability impact) is inconsistent with the described 'site isolation / information disclosure' behavior - verify with Mozilla, as this looks like an over-scored availability metric. Likewise, UI:N is optimistic for a browser flaw that in practice requires the victim to load malicious web content, so effective attack complexity is higher than the raw score implies. Countervailing the high CVSS, EPSS is 0.15% (5th percentile), it is not on CISA KEV, and no POC is referenced - so this is a high-severity-on-paper issue with low evidence of real-world exploitation. That said, browser info-leaks defeating site isolation are attractive for chaining and credential/secret theft, so any organization running Firefox should treat browser updates here as urgent-but-routine rather than emergency.
Exploit ScenarioDerived

An attacker hosts a crafted page containing malicious CSS and lures a Firefox user to visit it (via link, ad, or compromised site); when the browser parses and computes the styles, the site-isolation boundary is bypassed and cross-origin data the page should not see is exposed back to the attacker. Given AV:N/AC:L, delivery is trivial once the victim loads the content, but no public POC is currently known and no active exploitation has been reported.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.1% exploitation probability (EPSS)

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AnalysisAIv1 – Aug 20, 2026

Cross-origin information disclosure in Mozilla Firefox's CSS Parsing and Computation component allows a malicious web page to bypass site isolation and read data it should not have access to, affecting Firefox before 154 and Firefox ESR before 153.1. The flaw is remotely reachable over the network with no privileges required per the supplied CVSS vector, though realistic exploitation hinges on a victim loading attacker-controlled web content. There is no public exploit identified at time of analysis, and EPSS is very low (0.15%, 5th percentile), indicating no observed weaponization.

RemediationAI

Vendor-released patch: upgrade to Firefox 154 or Firefox ESR 153.1 or later, per Mozilla's MFSA advisories (https://www.mozilla.org/security/advisories/mfsa2026-74/ and mfsa2026-77/78/80). This is the primary and complete fix; browsers auto-update by default, so ensure managed/enterprise deployments are not pinned to an older ESR point release and push the update through your software-distribution channel. There is no documented feature-level workaround for a CSS-engine site-isolation flaw, so if immediate patching is not possible the practical compensating control is to reduce exposure to untrusted web content - enforce strict browsing policies or route users through a controlled/isolated browsing environment - accepting the usability trade-off that this restricts general web access rather than surgically disabling the vulnerable code path.

Affects your stack

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Technical ContextAI · show details

The issue lives in Firefox's CSS engine (parsing and computed-style handling), where 'site isolation' is the security boundary that keeps content from one origin from reading resources or state belonging to another origin. Classified as CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor), the root cause is a boundary failure in how CSS parsing/computation handles cross-origin data, permitting information that should be siloed per-origin to leak. The single affected CPE is cpe:2.3:a:mozilla:firefox - the desktop browser - and the fix spans both the rapid-release train (154) and the extended-support train (ESR 153.1), which is typical for a defect present in shared Gecko/Stylo CSS code.

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Severity by source

Sources disagree (Low–Critical)
Vendor (mozilla) PRIMARY 9.1 CRITICAL
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
vuln.today AI 6.5 MEDIUM

Reachable via malicious web content (AV:N, AC:L, PR:N) but requires the victim to load the page (UI:R); a CWE-200 site-isolation leak yields C:H with no integrity or availability impact (I:N/A:N).

3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
4.0 AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
SUSE HIGH
Red Hat 3.4 LOW

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
None
Availability
High

Lifecycle Timeline

3
CVSS changed
Aug 20, 2026 - 18:22 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

Site isolation issue in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 154 and Firefox ESR 153.1.

Executive ViewAI

Why It Matters

Mozilla Firefox contains a critical cross-origin information disclosure vulnerability (CVSS 9.1) that allows any malicious website to bypass security boundaries and access data from other sites a user is visiting, creating a confidentiality breach risk across all Firefox users. No vendor patch has been released to date, and while public exploit code is not available, this flaw affects the foundational security architecture protecting user data across website boundaries.

Business Impact: High
Compensating Controls

Disable JavaScript execution on untrusted websites; implement browser isolation or sandboxing solutions, particularly for users accessing high-risk content; restrict browsing to pre-approved trusted domains; monitor for abnormal data exfiltration patterns or signs of exploitation attempts.

Threat Intelligence Overview

EPSS Score
0.1%
Top 95% exploitable
Prevalence
High
Firefox is a top-tier desktop browser deployed across many enterprise and consumer environments
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 0.1%
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
Scheduled
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
Yes
Human Impact
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

Information Disclosure MEDIUM

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

Affected Products (CPE)CVE.org

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

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Same weakness CWE-200 – Information Exposure

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CVE-2016-6415 HIGH POC
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CVE-2025-31125 MEDIUM POC
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CVE-2017-11165 CRITICAL POC
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Same technique Information Disclosure

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CVE-2019-2725 CRITICAL POC
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CVE-2021-34523 CRITICAL POC
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CVE-2013-2251 CRITICAL POC
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CVE-2017-5638 CRITICAL POC
9.8
CVE-2025-24813 CRITICAL POC
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CVE-2025-68613 HIGH POC
8.8
CVE-2015-1701 HIGH POC
7.8
CVE-2017-0147 HIGH POC
7.5

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-2026-74986 ADVISORY https://vuldb.com/vuln/391638 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3658/ REF https://bugzilla.mozilla.org/show_bug.cgi?id=2060048 REF https://www.mozilla.org/security/advisories/mfsa2026-74/ 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/mfsa2026-80/

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