Wired on Friday: Something is changing at Microsoft. It's considerably stepped up its internal patenting process. Already this year, Microsoft has placed over three thousand patent applications with the US patent office. That compares to the less than 4,000 patents held by the company in total before 2004.

It has also recently been advertising licences for independent developers online, allowing individual developers to use Microsoft-patented technology with very few restrictions. It's been increasing the number of cross-licensing agreements with other major companies (some two hundred proposals are in the pipeline).

Then, a fortnight ago, it gave a blanket assurance to many of its customers that they need not worry about being sued for any breaches of intellectual property that Microsoft might have committed with their current code. All of these actions represent a very careful shift in direction for Microsoft. And one that may spell disaster for Microsoft's major competitor - the open source movement.

Microsoft has traditionally shown surprisingly little interest in building a patentable stock of intellectual property. Microsoft had good reason to dislike the expensive and slow world of intellectual property licensing and enforcement. As a fast-moving and reactive company, it had little incentive to pursue the kind of blue-sky research that yields long-term, general purpose patents.

Microsoft's patent strategy until now was largely defensive. Its primary interest in patents was in avoiding being sued over them. Now, with the build-up in Microsoft's patent armoury, the emphasis is on Microsoft not suing you. This year, Microsoft offered two interesting contracts with groups that wouldn't normally be dealing with Microsoft at all: coders implementing internet protocols. Such developers aren't usually bothered by Microsoft at all: it's a bit like car manufacturers having to sign contracts with road builders.

The publicly offered contracts certainly sounded attractive to such a group. They said little other than if you signed this contract, you wouldn't have to worry about being sued by Microsoft's patent lawyers for any Microsoft-owned intellectual property you invadvertently breached while working on your internet design.

Two things were notable about this. Firstly, the tables are now turned. Microsoft is now the one making the promise not to sue partners.

Secondly, this licence, while mostly rather innocuous (and, arguably, required by Microsoft's recent anti-trust settlement), contains conditions which make it a poisoned apple for those who write open source software. Microsoft's contract makes it a condition that you must get others to sign the same contract before they can work on your software.

Ironically, this odd feature of Microsoft's contract has similiarities with one of the favourite open source licenses, the GNU General Public License (GPL). The GPL, which guards much open source software, including Linux, the free operating system, says that you can change or adapt it as much as you want, but if you redistribute the program, you must make its source available on as generous a set of terms.

Microsoft has publicly attacked the GPL, asserting that it stops profit-making companies like Microsoft from taking advantage of new developments, and risks "polluting" proprietary software that uses GPL software with its "free source" requirements. But Microsoft's IP licences pollute the other way: preventing GPL software from taking advantage of Microsoft's patent amnesty.

So here is the current state of play. Microsoft has moved from being a company which mildly dislikes software patents, and spent its time making deals to stop others from pursuing court cases against them using them, to a company that is actively snatching up patents itself. And it has crafted a "protection" contract which is remarkably generous in allowing others to use its patented ideas, but with the tragic - really, quite tragic - catch that open source developers cannot sign it. And Microsoft has made it clear that if you don't sign that contract, you do put yourself at risk of being sued by them.

And now, the final piece in the jigsaw: that strange indemnification that Microsoft has now given to its customers. We guarantee you protection, says Microsoft now. But it's a very dangerous place out there. Certainly, other people could get hurt. Especially other people using open source software who have no-one to protect them.

All in all, its a sign that a company who even a few years ago professed a disdain for litigation and legal solutions is now becoming a master of the art.