The Skype flaws have not yet been patched, according to James. EBay, Skype's parent company, did not immediately respond to a request for comment.

The attacks show how tricky it will be to securely integrate the old-school telephone system into the more free-wheeling world of the Internet, James said. "This kind of proves ... how easy VoIP is to screw up," he said. He believes that these kinds of flaws almost certainly affect other VoIP systems as well. "There are people out there who can figure out how to tap your phone lines."