Telephony tower

A team of academics has published research yesterday that describes three attacks against the mobile communication standard LTE (Long-Term Evolution), also known as 4G.

Two of the three attacks are passive, meaning an attacker can watch LTE traffic and determine various details about the target, while the third is an active attack that lets the attacker manipulate data sent to the user's LTE device.

According to researchers, the passive attacks allow an attacker to collect meta-information about the user's traffic (an identity mapping attack), while the second allows the attacker to determine what websites a user might be visiting through his LTE device (a website fingerprinting attack).

aLTEr attack can redirect users to malicious websites

Researchers nicknamed the active attack aLTEr because of its intrusive capabilities, which they used in experiments to redirect users to malicious sites by altering DNS packets (DNS spoofing). Below is a demo of an aLTEr attack recorded by researchers.

aLTEr and other attacks not feasible in the real world

But researchers said regular users have nothing to fear, for now. Carrying out any of the three attacks requires special and expensive equipment, along with custom software, which usually puts this type of attack out of the reach of most crooks. Researchers explain:

To conduct such attacks, the attacker depends on specialized hardware (so called software-defined radios) and a customized implementation of the LTE protocol stack. In addition, a controlled environment helps to be successful within an acceptable amount of time. In particular, the use of a shielding box helps to maintain a stable and noise-free connection to the attack setup. Especially the latter cannot be maintained in a real-world situation and more engineering effort is required for real-world attacks.

"We conducted the attacks in an experimental setup in our lab that depends on special hardware and a controlled environment," researchers said. "These requirements are, at the moment, hard to meet in real LTE networks. However, with some engineering effort, our attacks can also be performed in the wild."

"We think that people of special interest such as politicians or journalists are the most likely targets of an attack," researchers added.

aLTEr attacks require proximity to victims

The equipment needed to pull off such attacks is very similar to so-called "IMSI catchers" or "Stingray" devices, equipment used by law enforcement around the world to trick a target's phone into connecting to a fake telephony tower.

aLTEr attacks are performed similarly, with the attacker needing to trick a victim's LTE device into connecting to it first, and then the attacker's device forwarding traffic to the real telephony tower.

As such, proximity to the victim is paramount, and the attack cannot be performed from across the Internet, requiring the attacker's presence on-site. The difference between an aLTEr attack and a classic IMSI catcher is that IMSI catchers perform passive MitM attacks to determine a target's geo-location, while aLTEr can actually modify what the user is seeing on his device.

Attacks possible because of weak LTE encryption

As for the technical details of the three attacks, the three vulnerabilities exist in one of the two LTE layers called the data layer, the one that transports the user's actual data. The other layer is the control layer and that's the one that controls and keeps the user's 4G connection running.

According to researchers, the vulnerabilities exist because the data layer is not protected, so an attacker can intercept, alter, and then relay the modified packets to the actual cell tower.

They can do this because 4G data packets are not integrity-protected, meaning it's possible to change bits of data, despite the data being encrypted.

The aLTEr attack exploits the fact that LTE user data is encrypted in counter mode (AES-CTR) but not integrity protected, which allows us to modify the message payload: the encryption algorithm is malleable, and an adversary can modify a ciphertext into another ciphertext which later decrypts to a related plaintext.

Flaws also impact upcoming 5G standard

The research team, made up of three researchers from the Ruhr-University in Bochum, Germany and a researcher from New York University, say they have notified relevant institutions such as the GSM Association (GSMA), 3rd Generation Partnership Project (3GPP), and telephone companies about the issues they discovered.

They warned the issue could also affect the upcoming version of the 5G standard in its current form. Experts said the 5G standard includes additional security features (stronger encryption at the data layer) to prevent aLTEr attacks, but these are currently optional.

The research team has published its findings in a research paper entitled "Breaking LTE on Layer Two," which they plan to present at the 2019 IEEE Symposium on Security & Privacy that will be held in May 2019 in San Francisco.

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