High-severity sandbox escape in widely used Mozilla Firefox and Thunderbird allowing sandbox breakout and code execution across broad user base; newsworthy and organization-impacting.
CVE-2026-100758
Sandbox escape in Firefox/Thunderbird DOM Navigation enabling sandbox breakout and code execution.
Is CVE-2026-100758 being exploited?
Not confirmed. CVE-2026-100758 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.
How severe is CVE-2026-100758?
CVE-2026-100758 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-2026-100758?
Yes. A fix has been recorded for CVE-2026-100758. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-100758 affect?
CVE-2026-100758 affects Firefox (fixed in 157; ESR fixed in 153.4, 115.42, 140.17), Thunderbird (fixed in 157, 153.4, 140.17). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-100758?
Update Firefox/Thunderbird to the vendor fixed versions immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypass browser sandbox via DOM Navigation to execute arbitrary code, access local files, or escalate privileges on affected hosts.
Immediate action required