What are the main things I need to pay attention to them
Process of building a strong security system?

Can you provide some more background, security for what exactly?

content management system for a site that need high protection to the admin zone

  • Use the latest versions of your tools (Apache/PHP/MySQL).
  • Use strong encryption where needed.
  • Run on SSL, perhaps a sub-domain, or separate domain.
  • Make sure your code is free from injection attacks.
  • Be paranoid about user input.
  • Log everything.

And browse this site. It is a lot of information but it is not a bad idea to become familiar with most of it.

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