CVE-2026-16387
A critical security flaw in Firefox and Thunderbird allows attackers to bypass site isolation, potentially exposing sensitive data and enabling malicious actions. The issue has been fixed in newer releases of both browsers.
An official fix is available, so this entry is kept for 30 days and then removed automatically.
Who is affected
Firefox versions prior to 153 and ESR 140.13, and Thunderbird versions prior to 153 and ESR 140.13.
Real-world impact
An attacker could read or modify data from other sites, steal credentials, or inject malicious content, effectively compromising user privacy and security.
Why this severity
The CVSS score of 9.8 reflects that the vulnerability is exploitable over the network with no authentication or user interaction, and it can lead to complete compromise of confidentiality, integrity, and availability.
What to do about it
- 01Upgrade Firefox to version 153 or later, or to ESR 140.13 or later.
- 02Upgrade Thunderbird to version 153 or later, or to ESR 140.13 or later.
NVD-referenced vendor advisory
Timeline
- Jul 21, 2026 · 12d agoPublishedDisclosed and added to the National Vulnerability Database.
- Jul 23, 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