Critical severity · published May 19, 2026
CVE-2026-8959
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component
9.6
CVSS
0.4%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Confidentiality
High
Integrity
High
Availability
High
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 36% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FI
Firefox · Mozilla
before 140.11.0; before 151.0.0 · source: NVD
TH
Thunderbird · Mozilla
before 140.11; before 151.0.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-8959
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-8959
Permissions Required
https://bugzilla.mozilla.org/show_bug.cgi?id=2034754
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-46/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-48/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-50/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-51/
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