IOptimize

I keep getting the following warning:

Visitor IP Address:	146.59.243.31
Firewall Rule:	Exe File Uploads
Firewall Pattern:	\.(dll|rb|py|exe|php[3-6]?|pl|perl|ph[34]|phl|phtml|phtm|sql|ini|jsp|asp|git|svn|tar)$
Request Path:	/wp-content/plugins/ioptimization/IOptimize.php
Parameter Name:	userfile
Parameter Value:	zwxnwyuqwg.php

I know that ioptimization/IOptimize.php is not a plugin, but a malware script. How it has got on my Shield-protected site is beyond me.

I am aware that I should make sure this is removed. However, looking at my site via FTP, there appears to be no such file or directory. I have, of course, looked for hidden files as well.

Why am I seeing this on a daily basis?

Cheers,

Mike