CVE-2026-16396
A critical flaw in Mozilla Firefox and Thunderbird lets attackers gain full control over the system via WebExtensions. The vulnerability can be exploited remotely without authentication or user interaction. Updating to the patched versions removes the risk.
An official fix is available, so this entry is kept for 30 days and then removed automatically.
Who is affected
Mozilla Firefox versions earlier than 153 and ESR 140.13, and Mozilla Thunderbird versions earlier than 153 and ESR 140.13.
Real-world impact
An attacker could take over the affected system, read, modify, or delete data, install malware, or disrupt services.
Why this severity
The CVSS score of 9.8 reflects that the flaw can be exploited over the network, requires no special privileges or user interaction, and grants complete control over confidentiality, integrity, and availability.
What to do about it
- 01Upgrade Firefox to version 153 or later, or to Firefox ESR 140.13 or later.
- 02Upgrade Thunderbird to version 153 or later, or to Thunderbird ESR 140.13 or later.
- 03Restart the browser to ensure the new version is running.
NVD-referenced vendor advisory
Timeline
- Jul 21, 2026 · 12d agoPublishedDisclosed and added to the National Vulnerability Database.
- Jul 22, 2026 · 10d agoOfficial fix availableA vendor patch or mitigation now exists — see the remediation steps above.
- Jul 24, 2026 · 9d agoAdvisory updatedThe NVD record was last revised.
How it’s attacked
References & advisories
- bugzilla.mozilla.org/show_bug.cgipermissions required
- mozilla.org/security/advisories/mfsa202…vendor advisory
- mozilla.org/security/advisories/mfsa202…vendor advisory
- mozilla.org/security/advisories/mfsa202…vendor advisory
- mozilla.org/security/advisories/mfsa202…vendor advisory