CVE-2021-23999: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.4% chance of exploitation in the next 30 days.

If a Blob URL was loaded through some unusual user interaction, it could have been loaded by the System Principal and granted additional privileges that should not be granted to web content. 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.