CVE-2019-9801: Mozilla Firefox

Medium severity, CVSS 5.3. EPSS: 1.3% chance of exploitation in the next 30 days.

Firefox will accept any registered Program ID as an external protocol handler and offer to launch this local application when given a matching URL on Windows operating systems. This should only happen if the program has specifically registered itself as a "URL Handler" in the Windows registry. *Note: This issue only affects Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

Affected products

• Mozilla Firefox: before 66.0 (fixed in 66.0)
• Mozilla Firefox ESR: before 60.6 (fixed in 60.6)
• Mozilla Thunderbird: before 60.6 (fixed in 60.6)

Published 2019-04-26. Last modified 2026-06-17.