Problem

DOM processing instructions are not handled correctly. This allows bypassing the cross-site scripting mechanism of typo3/html-sanitizer.

Solution

Update to typo3/html-sanitizer versions 1.5.3 or 2.1.4 that fix the problem described.

Credits

Thanks to Yaniv Nizry and Niels Dossche who reported this issue, and to TYPO3 core & security team member Oliver Hader who fixed the issue.

References