How to detect Backdoor?
<p>NB:- I am not sure which type of question i can ask here. Because i saw nobody likes my previous question and then i deleted that. </p>

<p>I am hosting reseller. Some months before i have faced a massive defacement in my reseller hosting. All of hosted site made by wordpress, joomla and so shopping CMS. 
And Recently i have faced again this kinds of problem. Although hackers just only changed the index file . And i recovered it easily. 
But i think he kept a backdoor to come again . 
And now how can i detect backdoor in my hosted sites. I am sure he is just using some sites to attack again. </p>
