Phar files (formerly known as “PHP archives”) can act als self extracting archives which leads to the fact that source code is executed when Phar files are invoked. The Phar file format is not limited to be stored with a dedicated file extension - “bundle.phar” would be valid as well as “bundle.txt” would be. This way, Phar files can be obfuscated as image or text file which would not be denied from being uploaded and persisted to a TYPO3 installation. Due to a missing sanitization of user input, those Phar files can be invoked by manipulated URLs in TYPO3 backend forms. A valid backend user account is needed to exploit this vulnerability. In theory the attack vector would be possible in the TYPO3 frontend as well, however no functional exploit has been identified so far.
github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms-core/2018-07-12-2.yaml
github.com/TYPO3-CMS/core
github.com/TYPO3-CMS/core/commit/81c6e9e6e49ee4f924bd340fb007d271fa44aa16
github.com/TYPO3-CMS/core/commit/854d03841b42123ab744c2591199b76ce65a8e45
github.com/TYPO3-CMS/core/commit/aa4e270d04313fe7d8ea508a9c694db83875e981
github.com/TYPO3-CMS/core/commit/dd0487bd8b4ec7204dbfff2b5812d797102a1778
typo3.org/security/advisory/typo3-core-sa-2018-002