A two-stage attack could allow spies to sneak secrets out of the most sensitive buildings, even when the targeted computer system is not connected to any network, researchers from Ben-Gurion University of the Negev in Israel stated in an academic paper describing the refinement of an existing attack.

The technique, called AirHopper, assumes that an attacker has already compromised the targeted system and desires to occasionally sneak out sensitive or classified data. Known as exfiltration, such occasional communication is difficult to maintain, because government technologists frequently separate the most sensitive systems from the public Internet for security. Known as an air gap, such a defensive measure makes it much more difficult for attackers to compromise systems or communicate with infected systems.

Yet, by using a program to create a radio signal using a computer’s video card—a technique known for more than a decade—and a smartphone capable of receiving FM signals, an attacker could collect data from air-gapped devices, a group of four researchers wrote in a paper presented last week at the IEEE 9th International Conference on Malicious and Unwanted Software (MALCON).

“Such technique can be used potentially by people and organizations with malicious intentions and we want to start a discussion on how to mitigate this newly presented risk,” Dudu Mimran, chief technology officer for the cyber security labs at Ben-Gurion University, said in a statement.

For the most part, the attack is a refinement of existing techniques. Intelligence agencies have long known—since at least 1985—that electromagnetic signals could be intercepted from computer monitors to reconstitute the information being displayed. Open-source projects have turned monitors into radio-frequency transmitters. And, from the information leaked by former contractor Edward Snowden, the National Security Agency appears to use radio-frequency devices implanted in various computer-system components to transmit information and exfiltrate data.

AirHopper uses off-the-shelf components, however, to achieve the same result. By using a smartphone with an FM receiver, the exfiltration technique can grab data from nearby systems and send it to a waiting attacker once the smartphone is again connected to a public network.

“This is the first time that a mobile phone is considered in an attack model as the intended receiver of maliciously crafted radio signals emitted from the screen of the isolated computer,” the group said in its statement on the research.

The technique works at a distance of 1 to 7 meters, but can only send data at very slow rates—less than 60 bytes per second, according to the researchers.