Internet-service subscribers can’t hide from police behind their IP addresses, the numbers assigned to devices connecting online.

Now a federal court in Pittsburgh has ruled that people who piggyback on their neighbors’ Wi-Fi networks forfeit privacy too.

The ruling, issued this month, was the first to address the Fourth Amendment rights of such people and the latest to shed light on technologies used by police to locate criminal suspects.

The amendment protects against unreasonable searches by the government, requiring that police get search warrants when suspects have reasonable expectations of privacy. The case also raises questions about people who connect to the Internet through public wireless-access points.

In a 2011 poll conducted by Wakefield Research and the trade association Wi-Fi Alliance, 32% of respondents said they had tried to get on a wireless network that wasn’t theirs. The Wi-Fi Alliance estimates that more than 200 million households use Wi-Fi networks.

Police have to use special software to identify Wi-Fi interlopers because they squat on the same IP addresses as paying subscribers. The central question in the Pittsburgh case, which is headed to a federal appeals court, is whether police can use such software without obtaining a search warrant.

In the case, Pennsylvania state police tried to find a man suspected of downloading child pornography by tracing the downloads to an IP address that was linked to a Comcast Corp. Internet account. Police obtained the subscriber’s address from Comcast and then got a warrant to search the subscriber’s home, according to court documents. But police soon ruled out the Comcast subscriber as the culprit.

The authorities then reckoned that someone nearby had been mooching the subscriber’s wireless Internet service.

The police used a program called “Moocherhunter” and a directional antenna to find other devices connected to the subscriber’s wireless router. The program allows its users to measure the distance between a wireless router and the devices connected to it.

Police said “Moocherhunter” led them to Richard Stanley, who lived across the street from the subscriber. Police then used the information to obtain a warrant to search Mr. Stanley’s home, and based on evidence they found, Mr. Stanley was indicted last November for possessing child pornography.

Mr. Stanley, who pleaded not guilty, sought to suppress the evidence, arguing that police should have obtained a warrant to use “Moocherhunter.”

U.S. District Judge Joy Flowers Conti ruled that Mr. Stanley could expect no more privacy than the subscriber. The legality of using a private wireless network without permission isn’t an issue in the case.

“An internet subscriber does not have a reasonable expectation of privacy in his IP address or the information he provides to his Internet Service Provider, such as Comcast, in order to legally establish an internet connection, and likewise, a person connecting to another person’s wireless router does not have an expectation of privacy in that connection,” Judge Conti wrote in her Nov. 14 ruling.

Marketa Sims, an assistant federal public defender who is representing Mr. Stanley, said that while subscribers forfeit their privacy because IP addresses are publicly available, that doesn’t mean wireless Internet users necessarily do as well.

“When Stanley connected to the other person’s router, he didn’t reveal his location,” said Ms. Sims, who plans to appeal Judge Conti’s ruling to the Philadelphia-based Third U.S. Circuit Court of Appeals. “The question here is whether the government needs a warrant to find your location when you haven’t broadcast it.”

Orin Kerr, a Fourth Amendment expert and law professor at George Washington University, said the issue isn’t clear-cut. “When you’re connecting to the wireless network, you’re broadcasting a signal, even though you might not know it,” he said. The question then, he said, is whether people enjoy privacy in signals they don’t knowingly disclose.

In a 1979 case cited by Judge Conti, the U.S. Supreme Court held that the police didn’t need a search warrant to use a device that recorded the telephone numbers dialed by a suspect. The court held that a telephone user typically understands that when he dials a number, he is asking the telephone company to connect him, so he is voluntarily giving the information regarding the number he is calling to a third party, losing the expectation of privacy.

But Prof. Kerr said that whether wireless Internet users know they are sending a publicly available signal could be trickier. It is a “black-box problem,” he said. “Do you assume the user knows how computers work or not?”

A spokeswoman for the U.S. attorney’s office in Pittsburgh, which is prosecuting the Stanley case, declined to comment on Judge Conti’s ruling.

Source: Wall Street Journal