Researchers Show How Attackers Can Crack LTE Data Link Layer

A new report by security researchers at Ruhr-University Bochum and New York University Abu Dhabi has shown how the data link layer of the 4G LTE mobile telephony standard can be defeated.

In their paper, entitled "Breaking LTE on Layer Two," the researchers show two passive attacks that can determine many details -- including identity and website fingerprinting -- about the traffic along with an active attack that can change where the traffic ends up.

The snappy name they gave to the active attack is aLTEr. It can carry out DNS spoofing to redirect traffic.

However, the attacks are not something most users will have to concern themselves with right now. While the paper showed the feasibility of the attacks, it required use of an experimental network set up by the researchers that may not translate well into normal use.

(Source: Flickr)

That setup had some limiting factors.

For instance, the researchers note:

"We use a shielding box to prevent our relay from interfering with the commercial network in the licensed spectrum, following ethics guidelines. Further, the shielding box stabilizes the user equipment's (UE) radio connection and prevents non-deterministic behavior of the relay. In other words, the shielding box setup assures that the UE does not connect to any other available cell and the malicious relay does not interfere with itself."

Some might say that the researchers actually stacked the deck to make things work for them.

They also asserted that their results will be real-world possible, even though they did not directly do it: "While we use this to simplify the experimental procedure, the setup is comparable to IMSI catcher [so-called 'Stingray' boxes used by law enforcement -- ed.] attacks when considering the victim's perspective. Such attacks were conducted successfully in real-world environments, i.e., without shielding equipment."

The aLTEr attack "exploits the missing integrity protection of LTE user data to perform a chosen-ciphertext attack," according to the researchers. It depends on a malicious relay acting as a rogue base station towards the user. The implicit assumption here is that the UE will first connect with the rogue station.

See how that UE shielding used in their experimental setup all of a sudden becomes really significant?

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Second, the relay acts as an unauthenticated user in the commercial network, and must be undetected by that network. Researchers think that by forwarding all messages between the user device and the network, the malicious relay will be undetectable.

However, the underlying problem exploited by aLTEr is that 4G data packets are not integrity-protected by the counter mode (AES-CTR) that is used. Instead, an attacker can intercept packets, alter them, and then relay those modified packets. And here the researchers have made a good point: While the 5G standard does include some stronger encryption, operators may not use it because of the overhead that is necessary to do so and can slow the speed of a network.

The researchers state their goals simply: "We urgently demand the implementation of effective countermeasures in the upcoming 5G specification to assure the security and privacy of future mobile communication."

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.