The issue is related to host spoofing due to improper validation of the HTTP Host header. TYPO3 uses this header to generate absolute URLs, but since it is provided by the client, it can be forged. A regression during TYPO3 v11 development led to this vulnerability, as the existing setting `$GLOBALS['TYPO3 CONF VARS']['SYS']['trustedHostsPattern']` was not evaluated anymore.