I host my website on a shared hosting server, and use cPanel as the control panel. Am I the only one can access the control panel? Who else can access it besides me and (maybe) the web hosting company?

I'm not certain, but I'm fairly sure that you are the only one who can access your cPanel. Your hosting provider would also have access to it, but any other access would come from either security breaches or your choice to share your log in information with others, I would think.

I agree with Manny. The hosting company from which you purchased the shared hosting package is responsible for the security and system updates of the server. They will make sure that you are the only user who can access your cPanel.