Wordpress site with Viruses

I had an inquiry from a potential customer who has a Wordpress website hosted on BlueHost.  When you go into the site, it alerts the user to a trojan in one of the plugins.  There are many articles on the Internet about different fixes for this everytthing from soup to nuts to taking it off the hosting provider and getting a new one.  I have developed a lot of custom websites but never a WordPress one.so just looking for an answer that would make some sense of ithis.  I would assume it came from the persons computer and was uploaded to the webserver but I dont know that for a fact. Thoughts?
Daren Anderson, MSISPresidentAsked:
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Jason C. LevineNo oneCommented:
>> I would assume it came from the persons computer and was uploaded to the webserver but I dont know that for a fact. Thoughts?

Incorrect.  Broadly speaking, there are a few possibilities:

1) The affected plugin was written in an inherently insecure way (either intentionally or through ignorance) and someone leveraged that into a vector to spread the trojan.

2) The user is running an older version of WordPress or a WordPress theme and a robot scanning for older versions found it and exploited a known vulnerability

3) Bluehost itself is hacked and the attacker is running a script on the server(s) that will modify WordPress to serve up malware.  Because WordPress is so common now, it represents a good target for attackers (much like Windows instead of Mac -- sheer numbers makes it an attractive target) who get shell permissions on a shared server at an ISP.  

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