CVE-2026-8959

CRITICALNVD 9.69.6—

9.6

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

CVSS v3: 9.6
EG Score: 9.6(high)
EPSS: 24.2%
KEV: Not listed

Published

May 19, 2026

Last Modified

May 20, 2026

Advisory Details (5)

Auto-updated May 19, 2026

No patch confirmed yet.

generic

Security Vulnerabilities fixed in Thunderbird 140.11 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-51/

generic

Security Vulnerabilities fixed in Thunderbird 151 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-50/

generic

Security Vulnerabilities fixed in Firefox ESR 140.11 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-48/

generic

Security Vulnerabilities fixed in Firefox 151 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-46/

generic

Access Denied

https://bugzilla.mozilla.org/show_bug.cgi?id=2034754

Vendor Advisories for CVE-2026-8959(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

GHSA-3x92-cxc7-h2pw
GitHub Security AdvisoriesCritical
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This...

Frequently asked(5)

What is CVE-2026-8959?

CVE-2026-8959 is a critical vulnerability published on May 19, 2026. Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

When was CVE-2026-8959 disclosed?

CVE-2026-8959 was first published in the National Vulnerability Database on May 19, 2026, with the most recent update on May 20, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2026-8959 actively exploited?

CVE-2026-8959 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 24.2% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.

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

CVE-2026-8959 has a CVSS v3 base score of 9.6 (NVD).

How do I remediate CVE-2026-8959?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-8959, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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CVE-2026-8959

📅 Published

🔄 Last Modified

📋 Advisory Details (5)

Security Vulnerabilities fixed in Thunderbird 140.11 — Mozilla

Security Vulnerabilities fixed in Thunderbird 151 — Mozilla

Security Vulnerabilities fixed in Firefox ESR 140.11 — Mozilla

Security Vulnerabilities fixed in Firefox 151 — Mozilla

Access Denied

📣 Vendor Advisories for CVE-2026-8959(1)

❓ Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2026-8959?

Published

Last Modified

Advisory Details (5)

Vendor Advisories for CVE-2026-8959(1)

Frequently asked(5)