James Martin/CNET

Hackers have quietly infiltrated more than a dozen mobile carriers around the world, gaining complete control of networks behind the companies' backs. The attackers have been using that access over the last seven years to steal sensitive data, but have so much control they could shut down communications at a moment's notice, according to Cybereason, a security company based in Boston.

On Tuesday, Cybereason said it's been investigating the campaign, dubbed Operation Soft Cell, through which hackers targeted phone providers in Europe, Asia, Africa and the Middle East. The hackers infected multiple mobile carriers since 2012, gaining control and siphoning off hundreds of gigabytes of data on people.

It constitutes a potentially massive breach -- with more fallout still to come -- as companies across different industries struggle with how to protect their customers' data. The hackers also had highly privileged access to do more than steal information.

"They have all the usernames and passwords, and created a bunch of domain privileges for themselves, with more than one user," said Amit Serper, Cybereason's head of security research. "They can do whatever they want. Since they have such access, they could shut down the network tomorrow if they wanted to."

Gigabytes of data theft

Cyberattacks on infrastructure are a national security concern -- hackers have found ways to shut down electrical power grids and access dams. The US Department of Homeland Security has created its own center for dealing with attacks on infrastructure, which it acknowledged as a frequent target for hackers. If an attacker shut down phone networks, it could cause massive disruption.

Serper said he didn't find any US mobile carriers that were affected, but the hacking campaign is ongoing and it's possible that could change.

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A person familiar with plans at one of the major US mobile carriers said the company is aware of the cyberattacks and is taking precautions against a potential breach.

Cybereason

While they were able to disrupt network signals, the hackers were more focused on espionage than disruption, Cybereason found.

After gaining access to mobile carriers' internal servers, the hackers would have access to call data records on hundreds of millions of customers. That would provide information like geolocation data, call logs and text message records.

While the hackers had access to the data of millions of people, they had stolen data from fewer than 100 targeted victims. The attackers likely targeted high-profile victims involved in government and the military, said Mor Levi, Cybereason's vice president of security practices.

That data could update in real time, as long as mobile carriers didn't catch on that they'd been hacked.

"Hacking a company that has mountains of data that is always updating is the holy grail for an intelligence agency," Serper said. "It's not just about gaining that access; it's about maintaining it."

How the attacks happened

Cybereason's researchers found that the attackers gained access to more than a dozen mobile carriers by exploiting old vulnerabilities, like malware hidden in a Microsoft Word file or finding an exposed public server belonging to a given company.

Once they slipped in, the malware then spreads by searching for all the computers on the same network and attempting to gain access by flooding them with login attempts. It continues to spread as long as the credentials work, until the hackers reach the caller data records database.

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Using that access, the hackers also created accounts for themselves with escalated privileges, essentially hiding among the company's actual staff. Even if the companies take measures to close up their vulnerabilities, the hackers could remain in the network for years after the fix.

Because the attack method was this sophisticated and targeted, Cybereason researchers believe the hackers were backed by a nation-state. All digital forensics signs point to China. The malware used, the method of attack and the servers the attacks are on are tied to APT10, China's elite hacking group.

A Chinese foreign ministry spokesperson said that China "firmly opposes" cyberattacks using the nation's infrastructure, and denied involvement with the hacks.

"Second, with the cyberspace being a highly virtual one filled with multiple actors whose behaviors are difficult to trace, one should present abundant evidence when investigating and determining the nature of a cyberspace activity," the Chinese embassy said in an email. "Making groundless accusations are neither professional nor responsible."

But there's no smoking gun tying China's hackers to this campaign. Despite the attackers using Chinese malware and servers, it's possible they're attempting to frame APT10, researchers said.

"Because the tools that we saw were leaked and are publicly available to anyone who's looking to get those tools, it could be anyone who wants to look like APT10," Levi said.

What to do

Cybereason said it's reached out to all the affected mobile carriers, though it's unclear what fixes they may have implemented to stop the intrusion.

Levi recommended that all mobile carriers strictly monitor their internet-facing properties, especially servers. Mobile carriers should also look for accounts that have high-privilege access.

Serper said the investigation is ongoing, and he continues to find more companies hacked by this group by the day. The hackers' servers are still up and running, he noted.

For people being tracked through this data theft, there's almost nothing they can do to protect themselves from espionage, he noted. Victims wouldn't even know that their call data records were being stolen from mobile carriers.

"There is no residue on your phone. They know exactly where you are and who you're talking to, and they didn't install any piece of code on your phone," Serper said.

Originally published June 24.

Update, June 25, 6:56 a.m. PT: Adds that US mobile carriers didn't respond to requests for comment. At 9:02 a.m. PT: Notes that a US mobile carrier is taking precautions against the attacks. At 1:09 p.m. PT: Adds a response from the Chinese embassy.