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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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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
qualitative
Red Hat
3.4 LOW
qualitative

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

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
High

Lifecycle Timeline

4
Analysis Generated
Aug 20, 2026 - 18:31 vuln.today
CVSS changed
Aug 20, 2026 - 18:22 NVD
9.1 (CRITICAL)
CVE Published
Aug 18, 2026 - 12:23 cve.org
CRITICAL 9.1
CVE Published
Aug 18, 2026 - 12:23 cve.org
UNKNOWN (no severity yet)

DescriptionCVE.org

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

AnalysisAI

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. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Access
Lure victim to malicious page
Delivery
Serve crafted CSS content
Exploit
Bypass site-isolation in CSS engine
Execution
Read cross-origin data
Impact
Exfiltrate leaked information to attacker
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Vulnerability AssessmentAI

Exploitation 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). … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment 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. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario 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.
Remediation 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). … Detailed patch versions, workarounds, and compensating controls in full report.
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Recommended ActionAI

Within 24 hours, issue a security alert to all Firefox users and establish daily monitoring of Mozilla security advisories for patch release. …

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Threat intelligence, references, and detailed analysis are available after sign-in.

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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

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CVE-2026-74986 vulnerability details – vuln.today
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