CVE-2021-23953: Mozilla Firefox

Medium severity, CVSS 4.3. EPSS: 1.1% chance of exploitation in the next 30 days.

If a user clicked into a specifically crafted PDF, the PDF reader could be confused into leaking cross-origin information, when said information is served as chunked data. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

Affected products

• Mozilla Firefox: before 85.0 (fixed in 85.0)
• Mozilla Firefox ESR: before 78.7 (fixed in 78.7)
• Mozilla Thunderbird: before 78.7 (fixed in 78.7)

Published 2021-02-26. Last modified 2026-06-17.