Hackers have managed to find a way around one of the key antipiracy protections built into Windows 7.
The software doesn't actually get confirmed as legitimate, but users are able to keep using the product indefinitely.
Microsoft confirmed on Friday it is aware of the technique, but said that it is working to shore up the activation procedure.
"We're aware of this workaround and are already working to address it," a Microsoft representative said in a statement, which also urged customers to only use genuine software, noting the fake stuff can contain malware and other bad things.
It's the latest in a long history of cat-and-mouse moves between the makers of Windows and those who would rather not have to pay for the privilege.