Open source software providers may have less to fear from legal action over patents than proprietary software developers, according to a patent law specialist.

Daniel Ravicher, executive director at the Public Patent Foundation, argues that a number of factors would make it hard for a patent holder to seek an injunction or damages against open source software.

Ravicher produced a report last month for Open Source Risk Management, a risk mitigation consulting firm which claimed that the Linux kernel could infringe 283 patents.

He suggested that a developer of proprietary software may find it hard to defend against a primary injunction because they have control over it.

"The wide spread of free software would render such an injunction meaningless, unless it is attached to every single possessor of a copy of the allegedly infringing code," said Ravicher.

"In essence, the common theme is that, because of the network effects inherent with free [open source] software, no party or project of any significance can be singled out for attack on the basis of patent infringement."

He maintained that there are also difficulties concerning which companies to sue, and the danger of the major software vendors standing together in opposition to any patent claim.

So even if a small non-profit open source development organisation was sued for allegedly breaching a patent, the giants of the software industry would intervene to protect their own products which used that open source code, according to Ravicher.

He added that, because patents cannot cover functionality, it is highly likely that before a patent infringement case is tried and appealed, the free software at issue could be redesigned around the asserted patent.

But Ravicher warned that this still leaves a potential threat from patents.

"The burden placed on parties defending themselves from assertions of patent infringement is indeed substantial in terms of cost and distraction," he explained.

"Significant resources will be required to protect free software from patents, but the battle can and will be won."

Sara Ellacott, a partner in the technology group at UK legal firm NabarroNathanson, told vnunet.com: "There are always exceptions but, in general terms, Ravicher's observations hold true.

"Patent claims are notoriously long-winded and the chances of being able to invalidate them in open source are increasing."