Affects widely used browsers (Firefox/Thunderbird); site-isolation bypass with CVSS 9.8 risks cross-origin data leaks and possible sandbox escape/RCE — org-wide urgency.
CVE-2026-16358
WebRender site-isolation bypass in Firefox/Thunderbird enabling cross-site data leaks and potential RCE.
Is CVE-2026-16358 being exploited?
Not confirmed. CVE-2026-16358 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 0.22% probability of exploitation in the next 30 days.
How severe is CVE-2026-16358?
CVE-2026-16358 is rated Critical with a CVSS score of 9.8. 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-2026-16358?
Yes. A fix has been recorded for CVE-2026-16358. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16358 affect?
CVE-2026-16358 affects Firefox < 153, Firefox 153 (fixed), Firefox ESR 115.38 (fixed), Firefox ESR 140.13 (fixed), and 3 further products or versions. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16358?
Update Firefox/Thunderbird to the fixed releases immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypass site isolation to read cross-origin content, exfiltrate sensitive data, and potentially achieve sandbox escape or remote code execution via crafted web content.
Immediate action required