Due to missing file extensions in $GLOBALS['TYPO3_CONF_VARS']['BE'][‘fileDenyPattern’], backend users are allowed to upload *.phar, *.shtml, *.pl or *.cgi files which can be executed in certain web server setups. A valid backend user account is needed in order to exploit this vulnerability.

Derivatives of Debian GNU Linux are handling *.phar files as PHP applications since PHP 7.1 (for unofficial packages) and PHP 7.2 (for official packages).

The file extension *.shtml is bound to server side includes which are not enabled per default in most common Linux based distributions. File extension *.pl and *.cgi require additional handlers to be configured which is also not the case in most common distributions (except for /cgi-bin/ location).

References

• TYPO3/typo3@095ae4a
• TYPO3/typo3@9990278
• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2019-01-22-7.yaml
• https://typo3.org/security/advisory/typo3-core-sa-2019-008