CVE-2021-23984: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 1.1% chance of exploitation in the next 30 days.

A malicious extension could have opened a popup window lacking an address bar. The title of the popup lacking an address bar should not be fully controllable, but in this situation was. This could have been used to spoof a website and attempt to trick the user into providing credentials. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.

Affected products

• Mozilla Firefox: before 87.0 (fixed in 87.0)
• Mozilla Firefox ESR: before 78.9 (fixed in 78.9)
• Mozilla Thunderbird: before 78.9 (fixed in 78.9)

Published 2021-03-31. Last modified 2026-06-17.