Vulnerability · Published May 19, 2026 · Assigned by mozilla
CVE-2026-8959Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component
Critical 9.6
In plain English
A critical-severity memory corruption in Firefox from Mozilla. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 36% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Severity (CVSS 3.1)9.6Critical
Exploit odds (30 days)0.4%top 63.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:10 | thunderbird | 140.11.0-1.el10_2.alma.1 | ALSA-2026:22325 |
| AlmaLinux:8 | thunderbird | 140.11.0-1.el8_10.alma.1 | ALSA-2026:22643 |
| AlmaLinux:9 | thunderbird | 140.11.0-1.el9_8.alma.1 | ALSA-2026:21381 |
| Rocky Linux:10 | thunderbird | 0:140.11.0-1.el10_2 | RLSA-2026:22325 |
| Rocky Linux:8 | thunderbird | 0:140.11.0-1.el8_10 | RLSA-2026:22643 |
| Rocky Linux:9 | thunderbird | 0:140.11.0-1.el9_8 | RLSA-2026:21381 |
| Ubuntu:22.04:LTS | mozjs102 | no fix listed | UBUNTU-CVE-2026-8959 |
| Ubuntu:24.04:LTS | mozjs102 | no fix listed | UBUNTU-CVE-2026-8959 |
| Ubuntu:24.04:LTS | mozjs115 | no fix listed | UBUNTU-CVE-2026-8959 |
| Ubuntu:22.04:LTS | mozjs78 | no fix listed | UBUNTU-CVE-2026-8959 |
| Ubuntu:22.04:LTS | mozjs91 | no fix listed | UBUNTU-CVE-2026-8959 |
| Ubuntu:22.04:LTS | thunderbird | no fix listed | UBUNTU-CVE-2026-8959 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Firefox | see references |
| Mozilla | Thunderbird | see references |
Weakness: CWE-119, CWE-20, CWE-693 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H