Its a scan, attempting to find and exploit a zero-day vulnerability (just discovered either today or yesterday) in a system called Roundcube, a mailing system used by some web servers. If you don't have Roundcube, you are fine. Otherwise, update your installation. Now.
Go here for some additional information:
http://www.webhostingtalk.com/showthread.php?p=5491823. There does not seem to be much info available right now, except that roundcube detection just got added to a lot of botnets. It is being theorized that Roundcube might be a platform that might be used to launch attacks against scripts in the /bin/ folder, to gain privileges on target servers.