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.