TYPO3 CMS Insecure Deserialization

It has been discovered that the Form Framework (system extension `form`) is vulnerable to Insecure Deserialization when being used with the additional PHP PECL package `yaml`, which is capable of unserializing YAML contents to PHP objects. A valid backend user account as well as having PHP setting `yaml.decode_php` enabled is needed to exploit this vulnerability (which is the default value according to PHP documentation).