High-severity RCE use-after-free in widely used Firefox/Thunderbird products that can enable drive-by compromise; merits broad IT/security distribution.
CVE-2026-100831
Use-after-free in Firefox/Thunderbird UI events allowing potential remote code execution.
Is CVE-2026-100831 being exploited?
Not confirmed. CVE-2026-100831 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-2026-100831?
CVE-2026-100831 is rated High with a CVSS score of 8.8. 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-2026-100831?
Yes. A fix has been recorded for CVE-2026-100831. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-100831 affect?
CVE-2026-100831 affects Firefox 157, Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-100831?
Update to the fixed Firefox/Thunderbird versions immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote code execution via crafted web content or email; attacker can execute arbitrary code in browser/process and compromise the host.
Immediate action required