Discussion:

External systems are systems or components of systems for which organizations typically have no direct supervision and authority over the application of security requirements and controls or the determination of the effectiveness of implemented controls on those systems. External systems include personally owned systems, components, or devices and privately-owned computing and communications devices resident in commercial or public facilities. This requirement also addresses the use of external systems for the processing, storage, or transmission of CUI, including accessing cloud services (e.g., infrastructure as a service, platform as a service, or software as a service) from organizational systems. Organizations establish terms and conditions for the use of external systems in accordance with organizational security policies and procedures. Terms and conditions address as a minimum, the types of applications that can be accessed on organizational systems from external systems. If terms and conditions with the owners of external systems cannot be established, organizations may impose restrictions on organizational personnel using those external systems. This requirement recognizes that there are circumstances where individuals using external systems (e.g., contractors, coalition partners) need to access organizational systems. In those situations, organizations need confidence that the external systems contain the necessary controls so as not to compromise, damage, or otherwise harm organizational systems. Verification that the required controls have been effectively implemented can be achieved by third-party, independent assessments, attestations, or other means, depending on the assurance or confidence level required by organizations. Note that while “external” typically refers to outside of the organization’s direct supervision and authority, that is not always the case. Regarding the protection of CUI across an organization, the organization may have systems that process CUI and others that do not. And among the systems that process CUI there are likely access restrictions for CUI that apply between systems. Therefore, from the perspective of a given system, other systems within the organization may be considered “external" to that system.

Upon assessment, assessors must determine if-

3.1.20[a] connections to external systems are identified.
3.1.20[b] the use of external systems is identified.
3.1.20[c] connections to external systems are verified.
3.1.20[d] the use of external systems is verified.
3.1.20[e] connections to external systems are controlled/limited.
3.1.20[f] the use of external systems is controlled/limited

Assessors are instructed to-

Examine: [SELECT FROM: Access control policy; procedures addressing the use of external systems; terms and conditions for external systems; system security plan; list of applications accessible from external systems; system configuration settings and associated documentation; system connection or processing agreements; account management documents; other relevant documents or records].

Interview: [SELECT FROM: Personnel with responsibilities for defining terms and conditions for use of external systems to access organizational systems; system or network administrators; personnel with information security responsibilities].

Test: [SELECT FROM: Mechanisms implementing terms and conditions on use of external systems].