Widely-used Firefox/Thunderbird components; sandbox escape could enable code execution; urgent patching needed.
CVE-2026-8959
Sandbox escape in Firefox Win32 widget; fixed in Firefox 151/ESR 140.11.
Is CVE-2026-8959 being exploited?
Not confirmed. CVE-2026-8959 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.42% probability of exploitation in the next 30 days.
How severe is CVE-2026-8959?
CVE-2026-8959 is rated Critical with a CVSS score of 9.6. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-8959?
Yes. A fix has been recorded for CVE-2026-8959. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-8959 affect?
CVE-2026-8959 affects Firefox 151, Firefox ESR 140.11, Thunderbird 151, Thunderbird 140.11. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-8959?
Upgrade to Firefox 151/ESR 140.11 or Thunderbird 151/140.11.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Potential remote code execution by escaping the browser sandbox via Win32 Widget.
Immediate action required