CVE-2026-16355
A critical bug in Firefox and Thunderbird’s JavaScript engine can let attackers run arbitrary code on a victim’s machine. The flaw is caused by a miscompilation in the JIT compiler, which can be exploited without any user interaction. The issue has been fixed in recent releases.
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, ESR 115.38, and ESR 140.13, and Mozilla Thunderbird versions earlier than 153, ESR 140.13.
Real-world impact
An attacker could execute arbitrary code with the privileges of the user, potentially taking full control of the victim’s computer.
Why this severity
The CVSS score of 9.8 reflects that the vulnerability is network reachable, requires no privileges or user interaction, and gives an attacker complete compromise of confidentiality, integrity, and availability.
What to do about it
- 01Update Firefox to version 153 or later, or to ESR 115.38 or later.
- 02Update 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
- mozilla.org/security/advisories/mfsa202…vendor advisory