High-severity use-after-free in Mozilla Firefox/Thunderbird can enable remote compromise of widely used desktop clients; affects many endpoints and warrants broad notification.
CVE-2026-100815
Use-after-free in Firefox/Thunderbird CSS parser enabling potential remote code execution.
Is CVE-2026-100815 being exploited?
Not confirmed. CVE-2026-100815 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-2026-100815?
CVE-2026-100815 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-2026-100815?
Yes. A fix has been recorded for CVE-2026-100815. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-100815 affect?
CVE-2026-100815 affects Firefox < 157, Firefox ESR < 153.4, Thunderbird < 157, Thunderbird ESR < 153.4. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-100815?
Update Firefox/Thunderbird to ESR 153.4, 157 or later immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Crafted web content can trigger a CSS parser use-after-free, causing memory corruption that may allow remote code execution or crash via a visited page.
Patch when possible