It is Siri!

Siri, the most prominent feature in iPhone 4S, now has a security hole. Basically, screen lock will not prevent Siri from doing a lot of things: from sending messages to initialize a phone call on your list. Actually, except  launching apps, Siri can do almost everything when screen lock is on.

This is potentially a security hole because any complete stranger can come up to your iPhone,  and give Siri a spoken command. Unauthorized access is always a nightmare for smartphone owners.

To solve this issue? It is actually quite simple.

The cause of this security hole is due to the default setting that allows Siri to work even when screen is locked. To change this setting you simply  go to Setting-General-Passcode Lock on your iPhone 4S. Then, you can change option for Siri as OFF.  Now, Siri will no longer function when the phone screen is locked.