CVE-2021-23998: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 0.6% chance of exploitation in the next 30 days.

Through complicated navigations with new windows, an HTTP page could have inherited a secure lock icon from an HTTPS page. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

Affected products

• Mozilla Firefox: before 88.0 (fixed in 88.0)
• Mozilla Firefox ESR: before 78.10 (fixed in 78.10)
• Mozilla Thunderbird: before 78.10 (fixed in 78.10)

Published 2021-06-24. Last modified 2026-06-17.