Due to a missing file extension in the fileDenyPattern, backend user are allowed to upload *.pht files which can be executed in certain web server setups. The new default fileDenyPattern is the following, which might have been overridden in the TYPO3 Install Tool.

\.(php[3-7]?|phpsh|phtml|pht)(\..*)?$|^\.htaccess$

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2017-09-05-4.yaml
• https://typo3.org/security/advisory/typo3-core-sa-2017-007