If you get asked to download from a website (like Mediafire, but not limited to just that one) or receive over Skype any program to “test it out” or for any reason really, do not do it. Here’s how this social engineering trick is played out – real attempt made on this server:
This is a trojan program written in Java that will steal your Minecraft user and password and email them over to the person that wrote the trojan.
From that point on, that person – let’s call it Mr. Corley – has full control over your Minecraft account and can log on in to any server posing as yourself.
If you’ve downloaded or received over Skype or any file sharing website such programs in the last weeks, you’re advised to immediately change your Minecraft password.
There have been several reports of this going on and already several people lost their in-game items.
Pass this warning on to your friends too. Play safe people, think before you run unknown programs from strangers!
Stay safe, have fun.