CVE-2026-8959: Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component

Published May 19, 2026
·
Updated

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.

Affected Software

8 affected componentsFixes available
Mozilla Firefox<151
151
Mozilla Firefox ESR<140.11
140.11
Mozilla Thunderbird<151
151
Mozilla Thunderbird<140.11
140.11
Mozilla Firefox<140.11.0
Mozilla Firefox<151.0.0
Mozilla Thunderbird<140.11
Mozilla Thunderbird<151.0.0

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 151
  2. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 140.11
  3. Upgrade

    Upgrade Firefox Win32 component (Widget) to a version that resolves this vulnerability.

    Fixed in 151
  4. Upgrade

    Upgrade Firefox ESR Win32 component (Widget) to a version that resolves this vulnerability.

    Fixed in 140.11
  5. Upgrade

    Upgrade Thunderbird Win32 component (Widget) to a version that resolves this vulnerability.

    Fixed in 151
  6. Upgrade

    Upgrade Thunderbird ESR Win32 component (Widget) to a version that resolves this vulnerability.

    Fixed in 140.11

Event History

May 19, 2026
CVE Published
via Mozilla·12:00 AM
Data Sourced
via Mozilla·12:00 AM
DescriptionSeverityAffected Software
Updated
via Mozilla·12:00 AM
Affected Software
CVE Published
via MITRE·12:29 PM
Data Sourced
via MITRE·12:29 PM
Description
Data Sourced
via Red Hat·02:02 PM
DescriptionSeverityAffected Software
Data Sourced
via NVD·02:16 PM
DescriptionSeverityWeaknessAffected Software
Jun 29, 58424
Event
via FIRST·11:07 AM

Parent advisories

This vulnerability appears in the following advisories.

Peer vulnerabilities

Found alongside the following vulnerabilities.

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Frequently Asked Questions

1

What is the severity of CVE-2026-8959?

CVE-2026-8959 is considered a high severity vulnerability due to its potential for sandbox escape.

2

How do I fix CVE-2026-8959?

To fix CVE-2026-8959, update to the latest versions: Firefox 151, Firefox ESR 140.11, Thunderbird 151, or Thunderbird 140.11.

3

Which software is affected by CVE-2026-8959?

CVE-2026-8959 affects Mozilla Firefox versions up to 151 and Thunderbird versions up to 151, among others.

4

What type of vulnerability is CVE-2026-8959?

CVE-2026-8959 is a sandbox escape vulnerability due to incorrect boundary conditions in the Win32 component.

5

Is CVE-2026-8959 fixed in the latest software updates?

Yes, CVE-2026-8959 has been addressed and fixed in the latest software updates of affected products.

Company

Resources

Contact

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