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AT&T and Verizon's implementations of LTE are said to be vulnerable to "to several issues" that could result in eavesdropping, data spoofing, and over-billing for potentially millions of phones.

Android devices on these networks are at most risk because the software "does not have appropriate permissions model" for LTE networks.

T-Mobile customers were affected but the issue has since been "resolved," a spokesperson said.

Apple products are not affected.

The advisory, posted by Carnegie Mellon University's public vulnerability database (CERT) on Friday, was based on a paper published by Korean academics and security researchers, which call for a "comprehensive solution that eliminates the root causes at mobile devices, mobile platforms, and the core network."

LTE (also known as 4G) relies on packet switching, a common way of sending data across the internet, rather than the old method of circuit switching. This new method of sending data allows for new kinds of attacks, particularly against the Session Initiation Protocol (SIP), nowadays more commonly used in voice calls and instant messaging.

Researchers have found a method that exploits the way that SIP works, by spoofing phone numbers for calls or text messages. It's also possible for an attacker to obtain free bandwidth for more data-intensive activities, like video calling, without incurring any additional costs. In some cases, an attacker can establish multiple SIP sessions at the same time, which could lead to a denial-of-service attack on the network.

The advisory said each network was vulnerable to "one or more" of the issues. CERT, which published the advisory, said it was currently unaware of a practical fix to the issues.

T-Mobile and Verizon were both identified as being at-risk from the peer-to-peer attacks. The researchers had not conducted full testing on AT&T but said it was likely at risk.

The researchers warned create a peer-to-peer network within that cell carrier's network, allowing an attacker to "directly retrieve data from other phones," the advisory said.

In one example, a malicious Android app could quietly make phone calls without the user's knowledge. That could be used to generate money on premium lines, over-billing, as well conducting targeted eavesdropping.

The researchers said every version of Android was at risk, whereas other attacks were network dependent.

When contacted, a Google spokesperson said the company will fix the issue for Nexus devices as part of its November Monthly Security Update, but did not confirm which Android versions were affected.

It's also not clear when a patch will land for other affected Android devices, as these are carrier dependent.

An AT&T spokesperson did not immediately provide comment. An email to Verizon was not returned. We will update the piece if we hear back.