The U.S. Supreme Court today ruled that the government needs to obtain a court-ordered warrant to gather location data on mobile device users. The decision is a major development for privacy rights, but experts say it may have limited bearing on the selling of real-time customer location data by the wireless carriers to third-party companies.

At issue is Carpenter v. United States, which challenged a legal theory the Supreme Court outlined more than 40 years ago known as the “third-party doctrine.” The doctrine holds that people who voluntarily give information to third parties — such as banks, phone companies, email providers or Internet service providers (ISPs) — have “no reasonable expectation of privacy.”

That framework in recent years has been interpreted to allow police and federal investigators to obtain information — such as mobile location data — from third parties without a warrant. But in a 5-4 ruling issued today that flies in the face of the third-party doctrine, the Supreme Court cited “seismic shifts in digital technology” allowing wireless carriers to collect “deeply revealing” information about mobile users that should be protected by the 4th Amendment to the U.S. Constitution, which is intended to shield Americans against unreasonable searches and seizures by the government.

Amy Howe, a reporter for SCOTUSblog.com, writes that the decision means police will generally need to get a warrant to obtain cell-site location information, a record of the cell towers (or other sites) with which a cellphone connected.

The ruling is no doubt a big win for privacy advocates, but many readers have been asking whether this case has any bearing on the sharing or selling of real-time customer location data by the mobile providers to third party companies. Last month, The New York times revealed that a company called Securus Technologies had been selling this highly sensitive real-time location information to local police forces across the United States, thanks to agreements the company had in place with the major mobile providers.

It soon emerged that Securus was getting its location data second-hand through a company called 3Cinteractive, which in turn was reselling data from California-based “location aggregator” LocationSmart. Roughly two weeks after The Times’ scoop, KrebsOnSecurity broke the news that anyone could look up the real time location data for virtually any phone number assigned by the major carriers, using a buggy try-before-you-buy demo page that LocationSmart had made available online for years to showcase its technology.

Since those scandals broke, LocationSmart disabled its promiscuous demo page. More importantly, AT&T, Sprint, T-Mobile and Verizon all have said they are now in the process of terminating agreements with third-parties to share this real-time location data.

Still, there is no law preventing the mobile providers from hashing out new deals to sell this data going forward, and many readers here have expressed concerns that the carriers can and eventually will do exactly that.

So the question is: Does today’s Supreme Court ruling have any bearing whatsoever on mobile providers sharing location data with private companies?

According to SCOTUSblog’s Howe, the answer is probably “no.”

“[Justice] Roberts emphasized that today’s ruling ‘is a narrow one’ that applies only to cell-site location records,” Howe writes. “He took pains to point out that the ruling did not ‘express a view on matters not before us’ – such as obtaining cell-site location records in real time, or getting information about all of the phones that connected to a particular tower at a particular time. He acknowledged that law-enforcement officials might still be able to obtain cell-site location records without a warrant in emergencies, to deal with ‘bomb threats, active shootings, and child abductions.'”

However, today’s decision by the high court may have implications for companies like Securus which have marketed the ability to provide real-time mobile location data to law enforcement officials, according to Jennifer Lynch, a senior staff attorney with the Electronic Frontier Foundation, a nonprofit digital rights advocacy group.

“The court clearly recognizes the ‘deeply revealing nature’ of location data and recognizes we have a privacy interest in this kind of information, even when it’s collected by a third party (the phone companies),” Lynch wrote in an email to KrebsOnSecurity. “I think Carpenter would have implications for the Securus context where the phone companies were sharing location data with non-government third parties that were then, themselves, making that data available to the government.”

Lynch said that in those circumstances, there is a strong argument the government would need to get a warrant to access the data (even if the information didn’t come directly from the phone company).

“However, Carpenter’s impact in other contexts — specifically in contexts where the government is not involved — is much less clear,” she added. “Currently, there aren’t any federal laws that would prevent phone companies from sharing data with non-government third parties, and the Fourth Amendment would not apply in that context.”

And there’s the rub: There is nothing in the current law that prevents mobile companies from sharing real-time location data with other commercial entities. For that reality to change, Congress would need to act. For more on the prospects of that happening and how we wound up here, check out my May 26 story, Why is Your Location Data No Longer Private?

The full Supreme Court opinion in Carpenter v. United States is available here (PDF).