CVE-2019-9794: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 1.8% chance of exploitation in the next 30 days.

A vulnerability was discovered where specific command line arguments are not properly discarded during Firefox invocation as a shell handler for URLs. This could be used to retrieve and execute files whose location is supplied through these command line arguments if Firefox is configured as the default URI handler for a given URI scheme in third party applications and these applications insufficiently sanitize URL data. *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 60.6.0 (fixed in 60.6.0); before 66.0 (fixed in 66.0)
• Mozilla Thunderbird: before 60.6.0 (fixed in 60.6.0)

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