TYPO3 v7 has reached its end-of-life November 30th, 2018 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
There is no further ELTS support. It is recommended that you upgrade your project and use a supported version of TYPO3.
Authentication in TYPO3 CMS is done via the services API. This makes it very flexible. It is easy to add new authentication services via extensions.
Services get executed in order of priority. Services with higher priority gets executed first, which means it is easy to control the order in which services get executed. By default, TYPO3 CMS provides a basic authentication service and two improved ones, which are installed by default: "saltedpasswords" and "rsaauth".
Alternative services are available in the TYPO3 Extension Repository. It is thus possible to find solutions for using LDAP as an authentication server, for example.
You can check which authentication services are installed using the SYSTEM > Reports module, in the Installed Services view: