The TYPO3 community released an new TYPO3 Security Guide! This is the most comprehensive security guide on TYPO3 ever released and its a must read for anyone working with TYPO3. And to quote something from the announcement which caught my attention:
Security is not a state that you can reach and stop worrying about. It is an ongoing process of constant improvement
So its not enough to read this guide and forget it. It must be in your mind all the time.