What is the CVSS score of CVE-2026-8959?

CVE-2026-8959 has a contested CVSS base score of 9.6 from a third party (e.g. CISA-ADP), while the vendor rates it Critical. vuln.today uses the vendor rating as authoritative.

Which versions of Mozilla Firefox are affected by CVE-2026-8959?

Mozilla Firefox on Windows contains a critical sandbox escape vulnerability (CVE-2026-8959) that could allow host-level compromise; no public exploit identified at time of analysis.. A patch is available.

How to fix or mitigate CVE-2026-8959?

Within 24 hours, identify all Windows systems running Firefox and verify patch management readiness. Within 7 days, deploy Firefox version 151 or ESR 140.11 or later to all affected Windows systems using your standard patch deployment mechanism. Within 30 days, audit patch deployment coverage across all Windows endpoints and implement enforced Firefox auto-update configuration to prevent future sandbox escape vulnerabilities.

Mozilla Firefox CVE-2026-8959

EUVD-2026-30910 CRITICAL

Protection Mechanism Failure (CWE-693)

2026-05-19 mozilla

GHSA-3x92-cxc7-h2pw

Information Disclosure Mozilla

Critical

Disputed · 9.6 NVD

Severity by source

Sources disagree (Medium–Critical)

NVD PRIMARY

9.6 CRITICAL

AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

vuln.today AI

9.6 CRITICAL

Network-delivered content with required user interaction (UI:R); the sandbox escape crosses a security boundary (S:C) yielding total host impact, consistent with Mozilla's rating.

3.1 AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

4.0 AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

SUSE

CRITICAL

qualitative

Red Hat

6.1 MEDIUM

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 VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

Analysis Generated

Jul 23, 2026 - 17:41 vuln.today

CVE Published

May 19, 2026 - 12:29 nvd

UNKNOWN (no severity yet)

CVE Published

May 19, 2026 - 12:29 cve.org

CRITICAL 9.6

DescriptionCVE.org

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151 and Firefox ESR 140.11.

AnalysisAI

Sandbox escape in Mozilla Firefox's Windows-specific Widget: Win32 component lets a content-process compromise break out of the browser sandbox onto the host due to incorrect boundary conditions (CWE-693, protection mechanism failure). All Firefox versions prior to 151 and ESR versions before 140.11 on Windows are affected, and a scope-changing chain can yield full confidentiality, integrity, and availability impact on the host. There is no public exploit identified at time of analysis, EPSS is very low (0.06%), and CISA's SSVC framework rates exploitation as none, indicating this is a high-severity but not yet actively exploited issue.

Technical ContextAI

The flaw resides in Firefox's Widget: Win32 component, the Windows platform layer that bridges the rendering/content engine to native Win32 GUI and windowing APIs. Because Firefox runs untrusted web content inside a restricted sandbox process, this layer is a security boundary between low-privilege content and the host OS. CWE-693 (Protection Mechanism Failure) here reflects incorrect boundary conditions that undermine that sandbox boundary rather than a memory-safety bug in the classic sense - the escape defeats the containment mechanism itself. The CPE cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* confirms the affected product is the Firefox application; the Win32 scope means only the Windows builds are impacted, not Linux or macOS.

RemediationAI

Vendor-released patch: upgrade to Firefox 151 or Firefox ESR 140.11 or later, which close the incorrect-boundary-condition sandbox escape in the Widget: Win32 component; see the Mozilla advisories at https://www.mozilla.org/security/advisories/mfsa2026-46/ and mfsa2026-48/50/51. SUSE-based systems should apply SUSE-SU-2026:2039 (https://www.suse.com/support/update/SUSE-SU-2026:2039/), and Red Hat/other distros should track their respective errata. Because the vulnerability requires the victim to load attacker content, an interim compensating control is to steer users to updated builds via managed auto-update/enterprise policy; where patching lags, reducing exposure to untrusted sites and enforcing browser update policies lowers the chance of the prerequisite content-process compromise, though these do not remove the underlying flaw. No effective feature-level workaround for the Win32 widget boundary is documented, so patching is the definitive fix.

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Vendor StatusVendor

SUSE

Severity: Critical

Product	Status
SUSE Linux Enterprise Server 12 SP5-LTSS	Fixed
SUSE Linux Enterprise Server LTSS Extended Security 12 SP5	Fixed
openSUSE Tumbleweed	Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS	Affected
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS	Affected

CVE-2026-8959 vulnerability details – vuln.today

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Mozilla Firefox CVE-2026-8959

Severity by source

CVSS VectorNVD

Lifecycle Timeline

DescriptionCVE.org

AnalysisAI

Technical ContextAI

RemediationAI

More in Mozilla

Same weakness CWE-693 – Protection Mechanism Failure

Same technique Information Disclosure

Vendor StatusVendor

SUSE

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External POC / Exploit Code