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

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.

Published: 2026-05-19 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-8959 is rated Moderate Risk (50.2/100): CVSS Critical severity, with low exploitation likelihood (EPSS 0.42%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-8959

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.16% 0.42% +0.25%
2 2026-05-25 0.06% 0.16% +0.11%
3 2026-05-20 — 0.06% —

Full EPSS history (3 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-8959

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.6 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.8 6.0 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2026-8959

GitHub Security Advisory for CVE-2026-8959

GHSA-3x92-cxc7-h2pw · Severity: critical — Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This...

OS Trackers for CVE-2026-8959

vendor priority summary link
debian unimportant CVE-2026-8959 unimportant priority: Debian including 3 source packages (firefox, firefox-esr, thunderbird), 11 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 11. https://security-tracker.debian.org/tracker/CVE-2026-8959
redhat medium — https://access.redhat.com/security/cve/CVE-2026-8959
suse high CVE-2026-8959 severity important: SUSE including 4 source package names (firefox-esr-140.11.0-1.1, firefox-esr-branding-upstream-140.11.0-1.1, firefox-esr-translations-common-140.11.0-1.1, firefox-esr-translations-other-140.11.0-1.1), 4 product×package rows across 1 product lines (openSUSE Tumbleweed): Fixed 4. https://www.suse.com/security/cve/CVE-2026-8959/
ubuntu medium CVE-2026-8959 medium priority: Ubuntu including 9 source packages (firefox, mozjs102, …), 49 status rows across 7 suites (bionic, focal, jammy, noble, questing, resolute, upstream): DNE 23, not-affected 9, ignored 8, needs-triage 8, needed 1. https://ubuntu.com/security/CVE-2026-8959

Affected software / configurations for CVE-2026-8959

Vendor Product Version Raw CPE
mozilla firefox < 140.11.0 cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
mozilla firefox < 151.0.0 cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
mozilla thunderbird < 140.11 cpe:2.3:a:mozilla:thunderbird:*:*:*:*:esr:*:*:*
mozilla thunderbird < 151.0.0 cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

References for CVE-2026-8959

cvelogic Threat Intelligence