Microsoft has said it is working to close a breach of its technology that protects music digital files from copyright infringement.
Microsoft's DRM technology is thought to have been breached
A program called Fairuse4wm has been posted on the net and is said to be capable of bypassing Microsoft's Digital Rights Management (DRM) system.
It could spell problems for internet music shops, potentially enabling users to download unlimited files.
Microsoft said it hoped to deploy an update to fix the problem soon.
DRM is used to control people's access to digital data.
It is commonly employed on music downloading sites to restrict the use of music purchased and downloaded online to ensure copyright is not infringed.
Many internet stores offer files for use on a Windows Media platform wrapped in Microsoft's DRM technology.
But a message on the doom9.org internet forum posted links to an application called Fair Use for Windows Media, now mirrored on other sites, which appears to circumvent this system.
The program apparently strips Windows Media audio (wma) files of Microsoft's DRM 10 or 11 protection, removing any usage restrictions.
A post on the forum said: "This program is only designed and intended to enable fair-use rights to purchased media."
Marcus Matthias, senior product manager, Windows Client Division, said: "Microsoft is aware that a tool recently surfaced that circumvents Windows Media Digital Rights Management technology.
"Fortunately, the Windows Media DRM system has built-in renewability, we have an update to address the circumvention, and are working with our partners to deploy this solution."
He added: "¿Microsoft has long stated that no DRM system is impervious to circumvention. End users are not at risk due to this circumvention."
The breach could create problems for file download sites.
Some internet music sites, including Napster, offer monthly subscription services where users can download an unlimited amount of music. This is controlled by DRM technology, so if a payment is missed, the user can no longer listen to any of their songs.
But by removing DRM protection from the files, users could potentially download and keep many files.
Microsoft was not available for immediate comment.
Paul Myers, CEO of music and film download site Wippit, said his site sees many more downloads of mp3 files, which are not copy protected, than DRM protected files.
He said: "We think this is another step towards the avocation of legal unrestricted content being the norm rather than exception."
He added that now the technology had been breached once, Microsoft could see more attempts from hackers to break its defences.
Mark Mulligan, an analyst from Jupiter Research, said: "I'm not sure this means a massive amount - this sort of stuff goes on all the time. Every time a piece of technology is developed you always have the hackers trying to break it.
"I think Microsoft will be putting tools into place to close the hole - they are the big gorilla of the digital media space - whenever they develop a piece of technology people try to hack it and crack it.
"They have good systems in place to dynamically update their system to make sure holes are filled."
He added that those determined to download music illegally could find much easier ways of doing so through peer to peer file sharing sites.