The news comes as British law firm Davenport Lyons says that it is sending out more notices of action over alleged file-sharing of computer games.
Many home internet users now operate wireless networks which distribute their internet signal around the house. If these are not secured then others can use those signals and the internet service provider cannot tell whether use is by the owner of the network or a third party.
Some people are unaware that their networks should be secure, while others leave theirs open as a matter of policy to let other people have access to their unused broadband capacity.
Frankfurt's Higher Regional Court has now ruled that the owner of a network is not responsible for the actions of third parties on it. A musician had sued a user because their Internet Protocol (IP) address showed up as one which was involved in file-sharing of their work without permission.
The network owner argued that it was someone else who had undertaken that activity, but the court agreed with the musician that open networks are liable to be abused and that it should have been protected, and the network owner should prove that it was someone else who shared the files.
The Higher Regional Court heard the man's appeal and said that he was not liable. The court said that everybody was responsible for their own behaviour, and that the defendant could not be held responsible for a stranger's behaviour.
The ruling reflects the approach that would be expected in a UK court, according to Struan Robertson, a technology lawyer with Pinsent Masons, the law firm behind OUT-LAW.COM.
"The onus is on the party bringing the action to convince the court on a balance of probabilities that the person being sued is responsible for the infringement," he said. "The legal wrong isn't that you left your network open, it's the file-sharing."
But Robertson cautioned against opening wireless networks to anyone in the neighbourhood.
"The trouble is, if you use the Wi-Fi defence, absent any computer evidence to back either party's case, the judge might simply think that you're lying. That's one reason why you're asking for trouble by leaving your Wi-Fi network open to the world.
"With criminal content, though, the stakes are even higher. Nobody wants to be in the position of having to argue that they did not download illegal images or did not hack into a bank's systems," said Robertson.
In the US in 2006 Tammie Marson successfully argued in a file-sharing case that her open wireless network meant that anyone in her vicinity could have been responsible for downloading files using her IP address. Her case was dropped.
A UK law firm has said this week that it will step up its legal activity on behalf of game company Topware Interactive over allegations that its game Dream Pinball 3D has been illegally shared online.
Davenport Lyons said that it would send letters to 100 more people alleging that they have uploaded copyright material for others to download. Earlier this year it sent letters to others it accused of file-sharing involvement asking them to pay £600 or face court action.
Last month action was successfully taken against four of those people in the Central London County Court. Default judgments were registered in court when the four lodged no defence and did not turn up to court. Those users were fined £750 and ordered to pay £2,000 in costs.
Some people accused of file-sharing argue that the activity has taken place on a shared family computer, and that it is not necessarily the broadband bill payer that has downloaded or shared material. Children of bill payers can often be behind the activity.
Both Scots law and English law provide that a parent generally is not liable for the actions of their child, and that a civil judgment is as binding on a child as it is on an adult.
There are, though, some circumstances in which a parent can become responsible for the child's actions. That can happen when a child causes injury to others or where a parent has previously authorised or subsequently ratified the child's unlawful act.