Microsoft confirmed Thursday that portions of the source code for its Windows 2000 and Windows NT operating systems have been leaked on the Internet.

Portions of Windows 2000, archived in a 203-Mbyte file that decompresses to 600 Mbytes, or one CD-ROM's worth of data, were being distributed on peer-to-peer file-sharing networks and in chat rooms as early as Wednesday. The file contains approximately 15% of the program's full source code, including text files, executable programs, scripts, and notes from programmers.

While the leaked code isn't enough to reproduce or run the operating system, it poses a security concern for users of the operating system, since virus writers and malicious hackers will be able to examine the code for vulnerabilities they can exploit. The company says it's working with law-enforcement authorities to determine the source of the leak.