Adversaries may leverage external remote services as a point of initial access into your network. These services allow users to connect to internal network resources from external locations. Examples are VPNs, Citrix, and other access mechanisms. Remote service gateways often manage connections and credential authentication for these services. 
External remote services allow administration of a control system from outside the system. Often, vendors and internal engineering groups have access to external remote services to control system networks via the corporate network. In some cases, this access is enabled directly from the internet. While remote access enables ease of maintenance when a control system is in a remote area, compromise of remote access solutions is a liability. The adversary may use these services to gain access to and execute attacks against a control system network. Access to valid accounts is often a requirement.
As they look for an entry point into the control system network, adversaries may begin searching for existing point-to-point VPN implementations at trusted third party networks or through remote support employee connections where split tunneling is enabled. 
|C0020||Maroochy Water Breach||
In the Maroochy Water Breach, the adversary gained remote computer access to the system over radio.
In the Ukraine 2015 Incident, Sandworm Team harvested VPN worker credentials and used them to remotely log into control system networks.    
|M0936||Account Use Policies||
Configure features related to account use like login attempt lockouts, specific login times, and password strength requirements as examples. Consider these features as they relate to assets which may impact safety and availability. 
|M0942||Disable or Remove Feature or Program||
Consider removal of remote services which are not regularly in use, or only enabling them when required (e.g., vendor remote access). Ensure all external remote access point (e.g., jump boxes, VPN concentrator) are configured with least functionality, especially the removal of unnecessary services. 
|M0935||Limit Access to Resource Over Network||
Limit access to remote services through centrally managed concentrators such as VPNs and other managed remote access systems.
Use strong multi-factor authentication for remote service accounts to mitigate an adversary's ability to leverage stolen credentials. Be aware of multi-factor authentication interception techniques for some implementations.
Deny direct remote access to internal systems through the use of network proxies, gateways, and firewalls. Consider a jump server or host into the DMZ for greater access control. Leverage this DMZ or corporate resources for vendor access. 
Set and enforce secure password policies for accounts.
|M0918||User Account Management||
Consider utilizing jump boxes for external remote access. Additionally, dynamic account management may be used to easily remove accounts when not in use.
|ID||Data Source||Data Component||Detects|
|DS0015||Application Log||Application Log Content||
When authentication is not required to access an exposed remote service, monitor for follow-on activities such as anomalous external use of the exposed API or application.
|DS0028||Logon Session||Logon Session Metadata||
Monitor authentication logs and analyze for unusual access patterns, windows of activity, and access outside of normal business hours, including use of Valid Accounts.
|DS0029||Network Traffic||Network Traffic Flow||
Monitor for network traffic originating from unknown/unexpected systems.