Given that there are at least two different sites in the same TYPO3 installation - for instance first.example.org and second.example.com - then a session cookie generated for the first site can be reused on the second site without requiring additional authentication.
This vulnerability primarily affects the frontend of the website. It's important to note that exploiting this vulnerability requires a valid user account.
Update to TYPO3 versions 8.7.55 ELTS, 9.5.44 ELTS, 10.4.41 ELTS, 11.5.33, 12.4.8 that fix the problem described above.
Thanks to Rémy Daniel who reported this issue, and to TYPO3 core & security team member Benjamin Franzke who fixed the issue.