Keep the TYPO3 core up-to-date¶
As described in chapter TYPO3 CMS versions and lifecycle, a new version of TYPO3 can either be a major update (e.g. from version 7.x.x to version 8.x.x), a minor update (e.g. from version 8.4.x to version 8.5.x) or a maintenance/bugfix/security release (e.g. from version 8.7.11 to 8.7.12).
In most cases, a maintenance/bugfix/security update is a no-brainer, see TYPO3 Installation and Upgrade Guide for further details.
When you extract the archive file of new TYPO3 sources into the existing install directory (e.g. the web root of your web server) and update the symbolic links, pointing to the directory of the new version, do not forget to delete the old and possibly insecure TYPO3 core version. Failing doing this creates the risk of leaving the source code of the previous TYPO3 version on the system and as a consequence, the insecure code may still be accessible and a security vulnerability possibly exploitable.
Another option is to store the extracted TYPO3 sources outside of the web root directory (so they are not accessible via web requests) as a general rule and use symbolic links inside the web root to point to the correct and secure TYPO3 version.