Air gap (networking)

Network security measure

This article is about network security. For other uses, see Air gap.

An air gap, air wall, air gapping[1] or disconnected network is a network security measure employed on one or more computers to ensure that a secure computer network is physically isolated from unsecured networks, such as the public Internet or an unsecured local area network.[2] It means a computer or network has no network interface controllers connected to other networks,[3][4] with a physical or conceptual air gap, analogous to the air gap used in plumbing to maintain water quality.

## Use in classified settings[edit]

An air-gapped computer or network is one that has no network interfaces, either wired or wireless, connected to outside networks.[3][4] Many computers, even when they are not plugged into a wired network, have a wireless network interface controller (WiFi) and are connected to nearby wireless networks to access the Internet and update software. This represents a security vulnerability, so air-gapped computers either have their wireless interface controller permanently disabled or physically removed. To move data between the outside world and the air-gapped system, it is necessary to write data to a physical medium such as a thumbdrive, and physically move it between computers. Physical access has to be controlled (man identity and storage media itself). It is easier to control than a direct full network interface, which can be attacked from the exterior insecure system and, if malware infects the secure system, can be used to export secure data. That's why some new hardware technologies are also available like unidirectional data diodes or bidirectional diodes (also called electronic airgaps), that physically separate the network and transportation layers and copy and filter the application data.

In environments where networks or devices are rated to handle different levels of classified information, the two disconnected devices or 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). Access policies are often based on the Bell–LaPadula confidentiality model, where data can be moved low-to-high with minimal security measures, while high-to-low requires much more stringent procedures to ensure protection of the data at a higher level of classification. In some cases (for instance industrial critical systems), the policy is different: data can be moved from high-to-low with minimal security measures, but low-to-high requires a high level of procedures to ensure integrity of the industrial safety system.

The concept represents nearly the maximum protection one network can have from another (save turning the device off). One way to transfer data between the outside world and the air-gapped system is to copy data on a removable storage medium such as a removable disk or USB flash drive and physically carry the storage to the other system. This access still has to be carefully controlled since USB drive may have vulnerabilities (see below). 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.[5] That's why another way to transfer data, used in appropriate situations like critical industries, is to use data diodes and electronic airgaps, that assure a physical cut of the network by a specific hardware.

Sophisticated computer viruses for use in cyberwarfare, such as Stuxnet[6] and Agent.BTZ have been designed to infect air-gapped systems by exploiting security holes related to the handling of removable media. The possibility of using acoustic communication has also been demonstrated by researchers.[7] Researchers have also demonstrated the feasibility of data exfiltration using FM frequency signals.[8][9]

## Examples[edit]

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

* Military/governmental computer networks/systems;[10]
* Financial computer systems, such as stock exchanges;[11]
* Industrial control systems, such as SCADA in Oil & Gas fields;[12]
* National and state lottery game machines or random number generators, which are required to be completely isolated from networks to prevent lottery fraud
* Life-critical systems, such as:
* Controls of nuclear power plants;
* Computers used in aviation,[13] such as FADECs, air traffic control systems, 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 and other devices on the CAN bus in an automobile;
* A digital thermostat for temperature and compressor regulation in home HVAC and refrigeration systems;
* Electronic sprinkler controls for watering of lawns.

Many of these systems have since added features that connect them during limited periods of time to the organisation's internet (for the need of surveillance or updates) or the public internet, and are no longer effectively and permanently air gapped, including thermostats with internet connections and automobiles with Bluetooth, Wi-Fi and cellular phone connectivity.

## Limitations[edit]

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.

Despite a lack of direct connection to other systems, air-gapped networks have been shown to be vulnerable to attack in various circumstances.

Scientists in 2013 demonstrated the viability of air gap malware designed to defeat air gap isolation using acoustic signaling.[citation needed] Shortly after that, network security researcher Dragos Ruiu's BadBIOS received press attention.[14]

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.[8][9]

In 2015, BitWhisper, a covert signaling channel between air-gapped computers using thermal manipulations was introduced. BitWhisper supports bidirectional communication and requires no additional dedicated peripheral hardware.[15][16]

Later in 2015, researchers introduced GSMem, a method for exfiltrating data from air-gapped computers over cellular frequencies. The transmission - generated by a standard internal bus - renders the computer into a small cellular transmitter antenna.[17][18]

ProjectSauron malware discovered in 2016 demonstrates how an infected USB device can be used to remotely leak data off of an air-gapped computer. The malware remained undetected for 5 years and relied on hidden partitions on a USB drive not visible to Windows as a transport channel between the air-gapped computer and a computer connected to the internet, presumably as a way to share files between the two systems.[19]

NFCdrip was the name given to the discovery of stealthy data exfiltration through NFC (Near-field communication) radio abuse and signal detection in 2018. Although NFC enables devices to establish effective communication by bringing them within a few centimeters of each other,[20] researchers showed that it can be abused to transmit information at a much longer range than expected - up to 100 meters.[21]

In general, malware can exploit various hardware combinations to leak sensitive information from air-gapped systems using "air-gap covert channels".[22] These hardware combinations use a number of different media to bridge the air-gap, including: acoustic, light, seismic, magnetic, thermal, and radio-frequency.[23][24][25]

## Software updates[edit]

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From a security perspective, the main drawback of an air gapped network is the inability of software to automatically self update. Users and system administrators must instead download and install updates manually. If a strict update routine is not followed, this results in out-of-date software running on the network, which may contain known security vulnerabilities. If an adversary manages to gain access to the air gapped network (for instance by contacting a disgruntled employee or using social engineering) they may be able to quickly spread within the air gapped network using such vulnerabilities with a possibly higher success rate than on the public Internet.

System administrators may manage software updates in an air gapped network using dedicated solutions such as Windows Server Update Services or network logon scripts. Such mechanisms would allow all computers on the air gapped network to automatically install updates after the system administrator downloads the updates from the Internet once. The problem is not completely eliminated though, especially if users have administrative privileges on their local workstations and are therefore able to install software which is not centrally managed. The presence of IoT devices requiring firmware updates can also complicate matters, since often such updates cannot be centrally managed.

## See also[edit]

* Air gap malware
* Airplane mode
* Firewall (computing)
* Near sound data transfer
* NIPRNet
* SIPRNet
* Sneakernet
* Tempest (codename)
* Van Eck phreaking

## References[edit]

1. ^ "What is air gapping (air gap attack)?". WhatIs.com. Retrieved 2020-12-16.
2. ^ Internet Security Glossary, Version 2. RFC 4949.
3. ^ a b Zetter, Kim (8 December 2014). "Hacker Lexicon: What is an air gap?". Wired. Conde Nast. Retrieved 21 January 2019.
4. ^ a b Bryant, William D. (2015). International Conflict and Cyberspace Superiority: Theory and Practice. Routledge. p. 107\. ISBN 978-1317420385.
5. ^ Lemos, Robert (2001-02-01). "NSA attempting to design crack-proof computer". ZDNet News. CBS Interactive, Inc. Retrieved 2012-10-12. "For example, top-secret data might be kept on a different computer than data classified merely as sensitive material. Sometimes, for a worker to access information, up to six different computers can be on a single desk. That type of security is called, in typical intelligence community jargon, an air gap."
6. ^ "Stuxnet delivered to Iranian nuclear plant on thumb drive". CNET. 12 April 2012.
7. ^ Putz, Florentin; Álvarez, Flor; Classen, Jiska (2020-07-08). "Acoustic integrity codes". Proceedings of the 13th ACM Conference on Security and Privacy in Wireless and Mobile Networks. Linz Austria: ACM. pp. 31–41. arXiv:2005.08572. doi:10.1145/3395351.3399420. ISBN 978-1-4503-8006-5. S2CID 218673467.
8. ^ a b Guri, Mordechai; Kedma, Gabi; Kachlon, Assaf; Elovici, Yuval (November 2014). "AirHopper: Bridging the Air-Gap between Isolated Networks and Mobile Phones using Radio Frequencies". arXiv:1411.0237 [cs.CR].
9. ^ a b Guri, Mordechai; Kedma, Gabi; Kachlon, Assaf; Elovici, Yuval (November 2014). "How to leak sensitive data from an isolated computer (air-gap) to a near by mobile phone - AirHopper". BGU Cyber Security Labs.
10. ^ Rist, Oliver (2006-05-29). "Hack Tales: Air-gap networking for the price of a pair of sneakers". Infoworld. IDG Network. Retrieved 2009-01-16. "In high-security situations, various forms of data often must be kept off production networks, due to possible contamination from nonsecure resources — such as, say, the Internet. So IT admins must build enclosed systems to house that data — stand-alone servers, for example, or small networks of servers that aren't connected to anything but one another. There's nothing but air between these and other networks, hence the term air gap, and transferring data between them is done the old-fashioned way: moving disks back and forth by hand, via 'sneakernet'."
11. ^ "Weber vs SEC" (PDF). insurancenewsnet.com. 2012-11-15. p. 35\. Archived from the original (PDF) on 2013-12-03. Retrieved 2012-12-06. "Stock exchange internal network computer systems are so sensitive that they are "air gapped" and not attached to the internet, in order to protect them from attack, intrusion, or other malicious acts by third party adversaries."
12. ^ "Weber vs SEC". "Industrial internal network computer systems are so sensitive that they are "air gapped" and neither attached to the internet nor insecurely connects to the corporate network, in order to protect them from attack, intrusion, or other malicious acts by third party adversaries."
13. ^ Zetter, Kim (2008-01-04). "FAA: Boeing's New 787 May Be Vulnerable to Hacker Attack". Wired Magazine. CondéNet, Inc. Archived from the original on 23 December 2008. Retrieved 2009-01-16. "(...Boeing...) wouldn't go into detail about how (...it...) is tackling the issue but says it is employing a combination of solutions that involves some physical separation of the networks, known as air gaps, and software firewalls."
14. ^ Leyden, John (5 Dec 2013). "Hear that? It's the sound of BadBIOS wannabe chatting over air gaps". Retrieved 30 December 2014.
15. ^ Guri, Mordechai; Monitz, Matan; Mirski, Yisroel; Elovici, Yuval (April 2015). "BitWhisper: Covert Signaling Channel between Air-Gapped Computers using Thermal Manipulations". arXiv:1503.07919 [cs.CR].
16. ^ Guri, Mordechai; Monitz, Matan; Mirski, Yisroel; Elovici, Yuval (March 2015). "BitWhisper: The Heat is on the Air-Gap". BGU Cyber Security Labs.
17. ^ Guri, Mordechai; Kachlon, Assaf; Hasson, Ofer; Kedma, Gabi; Mirsky, Yisroel; Elovici, Yuval (August 2015). "GSMem: Data Exfiltration from Air-Gapped Computers over GSM Frequencies". 24th USENIX Security Symposium (USENIX Security 15): 849–864. ISBN 9781931971232.
18. ^ Guri, Mordechai; Kachlon, Assaf; Hasson, Ofer; Kedma, Gabi; Mirsky, Yisroel; Monitz, Matan; Elovici, Yuval (July 2015). "GSMem Breaking The Air-Gap". Cyber Security Labs @ Ben Gurion University. Archived from the original on 2021-12-19.
19. ^ Chris Baraniuk (2016-08-09). "'Project Sauron' malware hidden for five years". BBC.
20. ^ Cameron Faulkner. "What is NFC? Everything you need to know". Techradar.com. Retrieved 30 November 2015.
21. ^ "NFCdrip: NFC Data Exfiltration Research". Checkmarx. Retrieved 19 December 2018.
22. ^ Carrara, Brent (September 2016). “Air-Gap Covert Channels.” Ph. D. Thesis. University of Ottawa.
23. ^ Carrara, Brent; Adams, Carlisle (2016). "A Survey and Taxonomy Aimed at the Detection and Measurement of Covert Channels". Proceedings of the 4th ACM Workshop on Information Hiding and Multimedia Security - IH&MMSec '16. pp. 115–126. doi:10.1145/2909827.2930800. ISBN 9781450342902. S2CID 34896818.
24. ^ Carrara, Brent; Adams, Carlisle (2016-06-01). "Out-of-Band Covert Channels—A Survey". ACM Computing Surveys. 49 (2): 1–36. doi:10.1145/2938370. ISSN 0360-0300. S2CID 13902799.
25. ^ Cimpanu, Catalin. "Academics turn RAM into Wi-Fi cards to steal data from air-gapped systems". ZDNet.