TYPO3 is an open source PHP based web content management system released under the GNU GPL. In affected versions failing to properly parse, sanitize and encode malicious rich-text content, the content rendering process in the website frontend is vulnerable to cross-site scripting. Corresponding rendering instructions via TypoScript functionality HTMLparser does not consider all potentially malicious HTML tag & attribute combinations per default. In default scenarios, a valid backend user account is needed to exploit this vulnerability. In case custom plugins used in the website frontend accept and reflect rich-text content submitted by users, no authentication is required. Update to TYPO3 versions 7.6.53 ELTS, 8.7.42 ELTS, 9.5.29, 10.4.19, 11.3.2 that fix the problem described.