According to a report at the Verge, a full version of Microsoft's official Xbox One software development kit (SDK) was reportedly leaked by hacking group H4LT. The group also posted an announcement and screenshots on their official Twitter account.
The alleged SDK leak could open up the possibility for developers to create homebrew mods for the console in the future. For now, however, developers still must be registered with and approved by Microsoft before officially releasing software using the SDK.
So far, the hacking group says there is no "definite exploit" that could allow developers to bypass this, but it hopes those familiar with Windows 8 can dig into the files to find more information. According to H4LT, the group leaked these files in an effort to allow for "creativity and research" in the community, but this public leak caps off a rough holiday season for Microsoft, in which its Xbox Live service (along with Sony's PlayStation Network) was taken down by a DDoS attack on Christmas Day.
As a personal aside here, I honestly don't know how often this needs to be said, but hackers can and will get to almost anything they want in today's highly digital world. They look at these kinds of things as "fun" challenges, and care little for copyrights, morality and/or ethics in these situations. They value freedom of information and ideas above all things...and sometimes they'll do these things just because they can.