CVE-2026-8959

Description

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.

Overview

State: PUBLISHED
Assigner (CNA): mozilla
CVSS severity: critical
CVSS score: 9.6 / 10
CVSS vector: AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
Effective score: 9.6 / 10 CRITICAL source: CNA overview
CWE(s): —
Reserved: 2026-05-19
Published: 2026-05-19 12:29 UTC
Last updated: 2026-05-19 17:10 UTC
Source: https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/8xxx/CVE-2026-8959.json
Linked Threat: CVE-2026-8959 — CVE-2026-8959

NVD triage scoring NVD CVE 2.0

Layer NVD adds on top of the CNA's CVE record — published / last-modified timestamps, exploitability / impact subscores, and the FIRST.org EPSS probability that this CVE will be exploited in the wild in the next 30 days.

NVD published: 2026-05-19 14:16:52 UTC
NVD last modified: 2026-05-20 14:28:29 UTC
NVD CVSS v3.1: 9.6 / 10 CRITICAL source: 134c704f-9b21-4f2e-91b3-4a467353bcc0
NVD CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
Exploitability subscore: 2.8 / 10
Impact subscore: 6.0 / 10
EPSS score: 0.0008 (probability of exploitation in next 30 days)
EPSS percentile: 24.14% vs all CVEs — higher = more likely to be exploited, as of 2026-05-24

NVD-assigned CWE(s): CWE-20, CWE-119, CWE-693 (differs from the CNA list above)

European Union Vulnerability Database ENISA EUVD

ENISA's official EU repository for curated vulnerability intelligence. Carries a separate identifier (EUVD-YYYY-NNNN) and frequently exposes an earlier-published description + CVSS than NVD does.

EUVD ID: EUVD-2026-30910
Assigner: mozilla
Published: May 19, 2026, 12:29:55 PM
Updated: May 19, 2026, 5:10:50 PM
EUVD base score (CVSS 3.1): 9.6 / 10
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
EUVD-reported EPSS: 0.1600
Aliases: GHSA-3x92-cxc7-h2pw

ENISA description: 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.

EUVD references (5)

Affected products (2)

Vendor	Product	Versions	Platforms
Mozilla	Firefox	`140.11` (unaffected), `151` (unaffected)	—
Mozilla	Thunderbird	`140.11` (unaffected), `151` (unaffected)	—

Affected products — CPE 2.3 (4) NVD

NVD's normalized CPE 2.3 matchers, used by vendor tools (vulnerability scanners, asset managers) for automated detection. Compare with the CNA's free-text "Affected products" section above.

cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

Vendor references (5)

References embedded in the original CVE record by the assigning CNA.

Web references (16)

DuckDuckGo results ranked by threat-intel / vendor advisory domains. Generated by the 🔎 Find references (web) button above — same flow as the Remediations search.

http://www.mozilla.org/security/announce/2026/mfsa2026-51.html rapid7:www.mozilla.org
http://www.mozilla.org/security/announce/2026/mfsa2026-48.html rapid7:www.mozilla.org
http://cwe.mitre.org/data/definitions/693.html rapid7:cwe.mitre.org
http://www.mozilla.org/security/announce/2026/mfsa2026-46.html rapid7:www.mozilla.org
https://attackerkb.com/topics/CVE-2026-8959 rapid7:attackerkb.com
http://cwe.mitre.org/data/definitions/119.html rapid7:cwe.mitre.org
http://cwe.mitre.org/data/definitions/20.html rapid7:cwe.mitre.org
https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-30910 rapid7:euvd.enisa.europa.eu
https://www.mozilla.org/security/advisories/mfsa2026-51/ tenable:www.mozilla.org
https://nvd.nist.gov/vuln/detail/CVE-2026-8959 tenable:nvd.nist.gov
https://www.cve.org/CVERecord?id=CVE-2026-8959 tenable:www.cve.org
https://www.first.org/epss/ tenable:www.first.org
https://www.mozilla.org/security/advisories/mfsa2026-46/ tenable:www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-48/ tenable:www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-50/ tenable:www.mozilla.org
https://bugzilla.mozilla.org/show_bug.cgi?id=2034754 tenable:bugzilla.mozilla.org

NVD-tagged references (5)

Reference list NVD curates from the CNA record, vendor advisories, and third-party reports. The tag chips below are NVD's analyst-assigned categories.

https://bugzilla.mozilla.org/show_bug.cgi?id=2034754 security@mozilla.org Permissions Required
https://www.mozilla.org/security/advisories/mfsa2026-46/ security@mozilla.org Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-48/ security@mozilla.org Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-50/ security@mozilla.org Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-51/ security@mozilla.org Vendor Advisory

Remediations (15)

Remediations are stored against the linked Threat row; the list below is deduplicated across both pages.

CVE-2026-8959 - Vulnerability Details - OpenCVE

web:app.opencve.io

A flaw in the Widget: Win32 component allows a malicious page to bypass the browser sandbox by exploiting incorrect boundary checks. This is an instance of buffer overrun and improper input validation (CWE-119) and insecure code path selection (CWE-693). The bug can lead to execution with higher privileges than the sandbox grants, potentially compromising the confidentiality, integrity, and ...

2026-05-23 22:18 UTC
Zero-Day Threat Report May 2026 - CVEs, Exploits & Remediation ...

web:carthageelectronics.com

Status: Actively exploited — CISA KEV listed — patch by May 12, 2026 What Happened On April 28, 2026 , CISA added CVE - 2026 -32202 to its Known Exploited Vulnerabilities (KEV) catalog with a federal remediation deadline of May 12, 2026 . This vulnerability is the result of an incomplete patch Microsoft released in February for CVE - 2026 -21510.

2026-05-23 22:18 UTC
CVE-2026-8959 - Sandbox escape due to incorrect boundary conditions in ...

web:cvefeed.io

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.

2026-05-23 22:18 UTC
CVE-2026-8959 - CRITICAL Severity Vulnerability | CVEInfo.com

web:cveinfo.com

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

2026-05-23 22:18 UTC
Exchange Emergency Mitigation (EM) service - learn.microsoft.com

web:learn.microsoft.com

The EM service checks the issuer, the Extended Key Usage, and the certificate chain. After successful validation, the EM service applies the mitigation . Each mitigation is a temporary, interim fix until you can apply the Security Update that fixes the vulnerability. The EM service isn't a replacement for Exchange SUs.

2026-05-23 22:18 UTC
Nvd - Cve-2026-8959

web:nvd.nist.gov

An official website of the United States government Here's how you know

2026-05-23 22:18 UTC
CVE-2026-8959 : Sandbox Escape in Firefox: Mozilla Vulnerability Alert

web:securityvulnerability.io

Critical sandbox escape vulnerability affecting Mozilla Firefox. Learn more about CVE-2026-8959 and mitigate risks.

2026-05-23 22:18 UTC
CVE-2026-8959 | Ubuntu

web:ubuntu.com

Ubuntu is an open source software operating system that runs from the desktop, to the cloud, to all your internet connected things.

2026-05-23 22:18 UTC
CVE-2026-8959 - Critical Vulnerability - TheHackerWire

web:www.thehackerwire.com

CVE-2026-8959 is a Critical severity vulnerability (CVSS 9.6). Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox...

2026-05-23 22:18 UTC
Patch Tuesday May 2026: Security Updates & CVE Analysis

web:zecurit.com

Get the complete breakdown of Microsoft's May 2026 Patch Tuesday. We analyze the latest security updates and all critical CVEs .

2026-05-23 22:18 UTC
SecuriTricks - CVE-2026-8959 - 9.6/10

web:securitricks.com

CVE CVE-2026-8959 - Score : 9.6 - Source : security@mozilla.org - Description : Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component.

2026-05-26 02:58 UTC
Microsoft Confirms Active 0-Day Exploit—Check Emergency Mitigation

web:www.forbes.com

Updated May 17: This article, originally published May 16, has been updated to include further details on the emergency mitigation process recommended after the CVE - 2026 -42897 Microsoft Exchange ...

2026-05-26 02:58 UTC
Security Vulnerabilities fixed in Firefox 151 - Mozilla

web:www.mozilla.org

Description Memory safety bugs present in Firefox ESR 115.35, Firefox ESR 140.10 and Firefox 150. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.

2026-05-26 02:58 UTC
Rapid7 Vulnerability Database

web:www.rapid7.com

vulnerability MFSA2026-46 Firefox: Security Vulnerabilities fixed in Firefox 151 ( CVE-2026-8959 )

2026-05-26 02:58 UTC
CVE-2026-8959 | Tenable®

web:www.tenable.com

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.

2026-05-26 02:58 UTC

AI Forensic Analysis

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Raw JSON

The full cvelistV5 record. Download as CVE-2026-8959.json.

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