Symantec stresses out that a successful attack doesn’t require the user to run RealPlayer. Simply the fact that you have the software installed will start the attack when a malicious page is being visited. The flaw is known to affect RealPlayer 11 Beta and RealPlayer 10.5; older versions may also be vulnerable.
In an ironic note, don’t worry about having the malware on your computer and not knowing it: a clip named "videotest" from the "My Library" folder will be played.
RealNetworks has issued a patch for this flaw, available here.