TYPO3 is an open source PHP based web content management system released under the GNU GPL. TYPO3 is copyright © 1999-2020 by Kasper Skaarhoj.
You can find instructions regarding installation and upgrade at https://docs.typo3.org/installation
Find the official system requirements in TYPO3's documentation.
To ensure a secure installation, you have to make sure that you keep your TYPO3 core and the extensions up to date.
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Subscribe to the announcement mailing list. This will inform you about new releases of the TYPO3 core and security bulletins of core and community extensions. http://lists.typo3.org/cgi-bin/mailman/listinfo/typo3-announce
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Please refer to official TYPO3 Security Guide for further information about security-related topics of TYPO3 CMS and the resources compiled by the Security Team. https://docs.typo3.org/typo3cms/SecurityGuide/ https://typo3.org/teams/security/resources/
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Please consider also hardening your TYPO3 instance further by adjusting permissions on non-changing files/folders.
You can find further information in the README.md.