It has been discovered that database fields used as descriptionColumn are vulnerable to cross-site scripting when their content gets previewed in the page module. A valid backend user account is needed to exploit this vulnerability.
Update to TYPO3 versions 10.4.14, 11.1.1 that fix the problem described.
Thanks to Richie Lee who reported this issue and to TYPO3 framework merger Andreas Fernandez who fixed the issue.
github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms-core/CVE-2021-21340.yaml
github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/CVE-2021-21340.yaml
github.com/TYPO3/TYPO3.CMS/security/advisories/GHSA-fjh3-g8gq-9q92
nvd.nist.gov/vuln/detail/CVE-2021-21340
packagist.org/packages/typo3/cms-backend
typo3.org/security/advisory/typo3-core-sa-2021-007