Last week, we heard whispers of the coming arrival of a new jailbreak for Windows Phone devices. All we knew at the time was that it was supposed to be an alternative to paying the $9 required for a Chevron Labs unlock. Today, details of the hack were announced, and while it won’t replace existing unlocks, it does provide an easy way to expand the usefulness of unlocked hardware.

The WindowBreak project has released a tool for conveniently performing an interop unlock on a handset already set up for development thanks to an AppHub or Chevron Labs unlock (Update: the author has clarified that the tool doesn’t require any previous unlock). The interop lock prevents unauthorized apps (including those you sideload on an unlocked phone) written in native code from accessing system drivers, preventing certain functionality required for homebrew apps like WP7 Root Tools.

The exploit takes advantage of directory-handling during the extraction of files from XAP containers. A properly-constructed XAP can be tailored to deliver its files to directories on your phone that should otherwise be out-of-reach. Then it’s just a short trip to the phone’s diagnostic tools to trigger the payload.

WindowBreak works fine on Samsung phones, but Nokia and HTC models have some extra protection in place that’s making things difficult. Work continues with the hope of bringing the tool to all Windows Phone hardware.