Apple's software is sandboxed to the point of frustration of developers.
CAN it happen. Of course. Software is pliable and breakable. Thats a large part of the reason why Apple has the ability revoke certs for any Application distributed through the App store. If one gets out, and you aren't jailbroken then that should fix the issue pretty quickly.
Jailbroken devices though COULD potentially have issues.
Apple's device security is strictly enforced by Apple's own POLICY to not allow unsigned applications from running. If a signed app is malicious, using this POLICY Apple could kill the app.