Answering questions after a speech to government officials in Singapore, Ballmer noted that entities using Linux could be opening themselves up to intellectual-property litigation.
"There was a report out this summer by an open-source group that highlighted that Linux violates over 228 patents," Ballmer said at the company's Government Leaders Forum event.
"Someday, for all countries that are entering (the World Trade Organization), somebody will come and look for money to pay for the patent rights for that intellectual property," Ballmer said. "So the licensing costs are less clear than people think today."
Actually, theby the start-up Open Source Risk Management found that Linux may violate 283 patents, but Ballmer's point was clear.
The Redmond, Wash.-based software maker has been trying hard lately to make a clear distinction between the intellectual-property risks of Linux as contrasted with protections offered with Windows. The company, which already offered indemnity against patent and other claims for volume licensing customers, last weekto all customers, regardless of how they buy Microsoft software.
Microsoft isn't the only one to use indemnification as a marketing tactic.developers and users even when that operating becomes open-source software in coming weeks. Hewlett-Packard has offered some limited protection to its Linux customers, and Novell, the No. 2 Linux seller, has to defend against patent-infringement attacks against customers using its open-source software.
Ballmer made the case that Microsoft also stands behind customers when it comes to security and other issues, echoing an argument he made at Microsoft'slast week.
"It is more secure because we stand behind it, because we fix it, because you actually know who builds it," he said. "Nobody ever knows who builds a piece of open-source software, where it comes from, who did it."
The Open Source Development Lab rebuffed Ballmer's comments.
"At OSDL, we have a lot of confidence in the robustness of Linux around IP, patents and copyright," OSDL CEO Stuart Cohen said in an e-mail statement. "Some of the world's largest vendors share our view and are willing to stand behind Linux to protect their customers, as are we."
Cohen pointed to the offers of legal protection from, and , as well as to OSDL's $10 million legal defense fund for Linux customers.
"Over the past 18 months, a handful of companies and individuals who are threatened by Linux's success have tried to argue that Linux may infringe others' software patents," Cohen said. "We find it interesting that none of those companies or individuals have said which patents Linux may offend."
CNET News.com's Stephen Shankland contributed to this report.