DISCUSSION [NIST SP 800-171 R2]
Split tunneling might be desirable by remote users to communicate with local system resources such as printers or file servers. However, split tunneling allows unauthorized external connections, making the system more vulnerable to attack and to exfiltration of organizational information. This requirement is implemented in remote devices (e.g., notebook computers, smart phones, and tablets) through configuration settings to disable split tunneling in those devices, and by preventing configuration settings from being readily configurable by users. This requirement is implemented in the system by the detection of split tunneling (or of configuration settings that allow split tunneling) in the remote device, and by prohibiting the connection if the remote device is using split tunneling.
FURTHER DISCUSSION
Split tunneling for a remote user utilizes two connections: accessing resources on the internal network via a VPN and simultaneously accessing an external network such as a public network or the internet.
Split tunneling presents a potential opportunity where an open unencrypted connection from a public network could allow an adversary to access resources on internal network. As a mitigation strategy, the split tunneling setting should be disabled on all devices so that all traffic, including traffic for external networks or the internet, goes through the VPN.
Example
You are a system administrator responsible for configuring the network to prevent remote users from using split tunneling. You review the configuration of remote user laptops. You discover that remote users are able to access files, email, database and other services through the VPN connection while also being able to print and access resources on their local network. You change the configuration settings for all company computers to disable split tunneling [a]. You test a laptop that has had the new hardening procedures applied and verify that all traffic from the laptop is now routed through the VPN connection.
Potential Considerations
Does the system prevent remote devices that have established connections (e.g., remote laptops) with the system from communicating outside that communications path with resources on uncontrolled/unauthorized networks [a]?
Copyright
Copyright 2020, 2021 Carnegie Mellon University and The Johns Hopkins University Applied Physics Laboratory LLC.
Copyright 2021 Futures, Inc.
This material is based upon work funded and supported by the Department of Defense under Contract No. FA8702-15-D-0002 with Carnegie Mellon University for the operation of the Software Engineering Institute, a federally funded research and development center, and under Contract No. HQ0034-13-D-0003 and Contract No. N00024-13-D-6400 with the Johns Hopkins University Applied Physics Laboratory LLC, a University Affiliated Research Center.
The view, opinions, and/or findings contained in this material are those of the author(s) and should not be construed as an official Government position, policy, or decision, unless designated by other documentation.
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