Conventional wisdom has it that one of the best ways to protect a sensitive system from external attack is to air-gap it, or to physically and logically separate the system from the Internet and other computers.

The technique is very effective, but not foolproof, as researchers from Israel’s Ben-Gurion University of the Negev (BGU) showed yet again this week with a proof-of-concept exploit for extracting data from air-gapped systems via radio frequency transmissions.

This is at least the third similar exploit that BGU researchers have developed in recent years involving data theft from isolated systems. Last year, they showed how attackers could establish a covert bi-directional communication channel between two air-gapped computers by leveraging the integrated thermal sensors in the systems.

In 2014, they demonstrated a technique for getting an air-gapped computer to transmit sensitive data via radio signals to a nearby smartphone equipped with special software dubbed AirHopper.

The latest proof-of-concept too involves the use of controlled RF signals, but with a twist. In this case, the researchers have developed software dubbed USBee that is designed to generate RF electromagnetic emissions from a system’s USB data bus. In a technical paper, the researchers discuss how the RF signals can be modulated with arbitrary binary data, like passwords and encryption keys, and then sent to a nearby computer using a plugged-in USB drive as the transmitter.

The attack builds on previous research that shows how data can be extracted from an air-gapped computer using USB connectors with embedded RF transmitters in them, the researchers said in their paper. As one example, they pointed to an USB hardware implant dubbed COTTONMOUTH, developed by the National Security Agency for extracting data from disconnected systems.

Unlike the previously described methods however, USBee does not require any hardware modification. An ordinary unmodified USB stick that is plugged into a system running the USBee code, can be used as a transmitter to send data to a nearby computer at a rate of about 80 bytes per second. Tests have shown that a computer equipped with an RF antenna will be able to capture the signals from up to 30 feet away, the researchers said.

Significant as that is, the actual risk of attackers being able to leverage the method to steal data from air-gapped systems remains small. By far the biggest reason is that in order for the attack to work, the air-gapped system has to be infected with the USBee code first, which means having physical access to the system. Systems that are sensitive enough to be air-gapped are also unlikely to permit the use of USB drives on them, especially after the Stuxnet attacks of 2010, which were enabled via infected USB sticks.

“The whole chain-of-attack requires a motivated attacker with high skills,” says Mordechai Guri, head of R&D of the cybersecurity center at BGU and the lead author of the technical paper. “Developing the USBee malware itself is not the hardest part. Infecting the victim with [the] malware and then intercepting the transmitted data is the more challenging part in this attack,” he admits.

There are several well-documented examples of how isolated systems can be infected with malware like USBee, he says. But often, even after that, it is hard to download data from such systems without being noticed. The USBee exploit shows one way of how that can be accomplished without raising any red flags.

Assuming that an attacker is able to infect an air-gapped system with USBee, the data that can be stolen this way is significant. Guri pointed to examples like encryption keys, keyloggings, personal records, small files, usernames, and passwords.

One of the most effective countermeasures for dealing with a threat like this is to define areas around air-gapped systems where RF receivers are prohibited, the researchers said. Partition walls with proper insulation can also help to lower signal reception distance, they said.

Cris Thomas, strategist at Tenable Network Security, said that while there’s a chance that a high-level nation state actor might employ a tactic such as the one described by the BGU researchers, there are few real-world scenarios where such an attack would be useful.

“However, the fact that this technology exists, even in the lab, does mean that people who are in control of air-gapped systems need to make sure that the physical security controlling access to those systems is adequate,” he said.

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