Affects widely used Mozilla Firefox/Thunderbird PDF viewer; high-severity mitigation bypass enabling code execution risk across many organizations.
CVE-2026-16377
Mitigation bypass in Firefox/Thunderbird PDF viewer allowing potential code execution.
Is CVE-2026-16377 being exploited?
Not confirmed. CVE-2026-16377 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 0.39% probability of exploitation in the next 30 days.
How severe is CVE-2026-16377?
CVE-2026-16377 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-2026-16377?
Yes. A fix has been recorded for CVE-2026-16377. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16377 affect?
CVE-2026-16377 affects Mozilla Firefox <153, Firefox ESR <140.13, Thunderbird <153, Thunderbird ESR <140.13, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16377?
Update Firefox/Thunderbird to 153 or ESR 140.13+ immediately; block/avoid PDFs until patched.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Crafted PDF can bypass mitigations to execute code or escape sandbox, enabling remote compromise via opened PDFs.
Immediate action required