Get the Whitepaper called The Joomla Hacking Compendium. This document should provide you with security related information about Joomla and the extensions which are available for it. This paper focuses on how to hack Joomla installations and how to protect them.
You can download it from the following link: https://packetstormsecurity.com/files/download/96819/joomla_hacking_compendium.txt
Source: https://packetstormsecurity.com/files/96819/The-Joomla-Hacking-Compendium.html