Microsoft has had to tell more than 20,000 software testers to change their passwords after a server at a company development site was hacked.

The software giant said that the server handles the details of Microsoft's BetaPlace, where thousands of people participate as beta testers of new software.

Although the hacker would only have had access to certain programs, and not the underlying code, testers' passwords, and some personal information such as email addresses, were at risk.

Microsoft stated that it did not believe that any personal data was compromised, but confirmed that it is taking no chances and is advising its beta testers to change their passwords.

The company is working with law enforcement agencies to find the hacker.