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.
858578.778.7Exploitability
AV:NAC:LPR:NUI:RScope
S:UImpact
C:LI:NA:N4.3/CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N