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Control Acronym



Access Control

CMMC Level


800-171 Control #


CMMC Description

Verify and control/limit connections to and use of external information systems.

CMMC Clarification

Make sure to control and manage connections between your company network and outside networks, such as the public internet or a network that does not belong to your company. Be aware of applications that can be run by outside systems. Control and limit personal devices like laptops, tablets, and phones from accessing the company networks and information. You can also choose to limit how and when your network is connected to outside systems and/or decide that only certain employees can connect to outside systems from network resources. Cybersecurity Example You help manage IT for your employer. You and your coworkers are working on a big proposal, and all of you will put in extra hours over the weekend to get it done. Part of the proposal includes Federal Contract Information, or FCI. FCI is information that you or your company get from doing work for the Federal government. Because FCI is not shared publicly, you remind your coworkers to use their company laptops, not personal laptops or tablets, when working on the proposal over the weekend.

800-171 Description

Verify and control/limit connections to and use of external systems.

800-171 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 Mobile devices and computing platforms include, for example, smartphones and tablets. 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.

Other Source Discussion


CIS Control References

CIS Controls v7.1 12.1, 12.4

NIST 800-53 Control Ref.

NIST SP 800-53 Rev 4 AC-20, AC-20(1)

CMMC Derived

NIST CSF Control References

NIST CSF v1.1 ID.AM-4, PR.AC-3

NIST 800-171 References

NIST SP 800-171 Rev 1 3.1.20

Applicable FAR Clause

FAR Clause 52.204-21 b.1.iii

NIST CSF Control Reference

CERT RMM Reference


Modification of NIST 800-171B Reference

NIST 800-171B Reference

UK NCSCCyber Reference

AS ACSC Reference


Assessment Sub-Criteria 1

AC.1.003.[a] connections to external systems are identified;

Assessment Sub-Criteria 2

AC.1.003.[b] the use of external systems is identified;

Assessment Sub-Criteria 3

AC.1.003.[c] connections to external systems are verified;

Assessment Sub-Criteria 4

AC.1.003.[d] the use of external systems is verified;

Assessment Sub-Criteria 5

AC.1.003.[e] connections to external systems are controlled/limited; and

Assessment Sub-Criteria 6

AC.1.003.[f] the use of external systems is controlled/limited.

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