Critical mitigation-bypass in widely used Mozilla products (Firefox/Thunderbird); high-impact browser vulnerability that should be broadly communicated.
CVE-2026-16380
Mitigation bypass in Firefox/Thunderbird networking, fixed in 153
Is CVE-2026-16380 being exploited?
Not confirmed. CVE-2026-16380 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 0.31% probability of exploitation in the next 30 days.
How severe is CVE-2026-16380?
CVE-2026-16380 is rated Critical with a CVSS score of 9.1. 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-2026-16380?
Yes. A fix has been recorded for CVE-2026-16380. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16380 affect?
CVE-2026-16380 affects Mozilla Firefox < 153, Mozilla Thunderbird < 153. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16380?
Upgrade to Firefox/Thunderbird 153 or later immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypasses networking mitigations allowing remote attackers to deliver crafted responses that can lead to remote code execution or data exfiltration.
Immediate action required