Air gap (networking)

An air gap or air wall^[1] is a network security measure that consists of ensuring that a secure computer network is physically isolated from unsecured networks, such as the public Internet or an unsecured local area network.^[2] It is often taken for computers and networks that must be extraordinarily secure. Frequently the air gap is not completely literal, such as via the use of dedicated cryptographic devices that can tunnel packets over untrusted networks while avoiding packet rate or size variation; even in this case, there is no ability for computers on opposite sides of the air gap to communicate.

Use in classified settings

In environments where networks or devices are rated to handle different levels of classified information, the two disconnected devices/networks are referred to as "low side" and "high side", low being unclassified and high referring to classified, or classified at a higher level. This is also occasionally referred to as red (classified) and black (unclassified). To move data from the high side to the low side, it is necessary to write data to a physical medium, and move it to a device on the latter network. Traditionally based on the Bell-La Padula Confidentiality Model, data can move low-to-high with minimal processes while high-to-low requires much more stringent procedures to ensure protection of the data at a higher level of classification.

The concept represents nearly the maximum protection one network can have from another (save turning the device off). It is not possible for packets or datagrams to "leap" across the air gap from one network to another, but computer viruses such as Stuxnet^[3] and agent.btz have been known to bridge the gap by exploiting security holes related to the handling of removable media. The possibility of using acoustic communication has also been demonstrated by researchers.^[4] Researchers have also demonstrated the feasibility of data exfiltration using FM frequency signals.^[5][6]

The upside to this is that such a network can generally be regarded as a closed system (in terms of information, signals, and emissions security) unable to be accessed from the outside world. The downside is that transferring information (from the outside world) to be analyzed by computers on the secure network is extraordinarily labor-intensive, often involving human security analysis of prospective programs or data to be entered onto air-gapped networks and possibly even human manual re-entry of the data following security analysis.^[7]

Examples

Examples of the types of networks or systems that may be air gapped include:

• Military/governmental computer networks/systems;^[8]
• Financial computer systems, such as stock exchanges;^[9]
• Industrial control systems, such as SCADA in Oil & Gas fields;^[10]
• Life-critical systems, such as:
  • Controls of nuclear power plants;
  • Computers used in aviation,^[11] such as FADECs and avionics;
  • Computerized medical equipment;
• Very simple systems, where there is no need to compromise security in the first place, such as:
  • The engine control unit in an automobile;
  • A digital thermostat for temperature and compressor regulation in home HVAC and refrigeration systems;
  • Electronic sprinkler controls for watering of lawns.

Limitations

Limitations imposed on devices used in these environments may include a ban on wireless connections to or from the secure network, or similar restrictions on EM leakage from the secure network through the use of TEMPEST or a Faraday cage.

Further, scientists in 2013 demonstrated the viability of air gap malware designed to defeat air gap isolation using acoustic signaling.^[12] Shortly after that, network security researcher Dragos Ruiu's BadBIOS received press attention.^[13]

In 2014, researchers introduced "AirHopper", a bifurcated attack pattern showing the feasibility of data exfiltration from an isolated computer to a nearby mobile phone, using FM frequency signals.^[14][15]

See also

References