Joomla or Drupal.

I am sure there are others; but these 2 came to me immediately.
With open source you want a very active developer community as they will find security issues and fix them quickly. I have used both and both have had security issues; however every issue was corrected quickly.
One thing that made the system more secure is to move the database server off the web server and on to an interior (private ip number) server.

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"The future is here. It's just not evenly distributed.