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Comply Defense
Category: Authorization & Accreditation

Authorization Boundary

Also known as: Security Authorization Boundary
Simply put

An authorization boundary defines exactly which parts of an information system are covered when an official approves that system to operate. It sets the scope of what is being protected and evaluated, and it excludes any related systems that carry their own separate approvals. Drawing this boundary correctly is generally a critical first step because it determines what falls under a given authorization and what does not.

Formal definition

Per OMB Circular A-130 and the NIST CSRC glossary, an authorization boundary comprises all components of an information system to be authorized for operation by an authorizing official, and excludes separately authorized systems to which the system is connected. In practice, the boundary establishes the scope of protection, generally encompassing the people, processes, and technologies within the system, against which risk assessment, security control implementation, assessment, and the authorization decision are conducted. Practitioners should note that boundary definition is scoping, not authorization itself, and that connected but separately authorized systems (for example, external services or a distinct FedRAMP-authorized offering) fall outside the boundary and must be addressed through interconnection or inheritance arrangements rather than assumed to be covered. Agency- and program-specific tailoring may affect how boundaries are drawn, so the current authoritative guidance applicable to the system's environment should be verified.

Why it matters

The authorization boundary is foundational to any authorization decision because it defines the scope of what an authorizing official is actually approving. If the boundary is drawn too narrowly, components that should be protected and assessed may be left out of the risk assessment and control implementation, creating gaps that an authorization does not address. If it is drawn too broadly, an organization may assume coverage over connected systems that in fact carry their own separate authorizations, leading to false confidence about what has been evaluated.

Per OMB Circular A-130 and the NIST CSRC glossary, the boundary comprises all components of a system to be authorized and explicitly excludes separately authorized systems to which the system is connected. This exclusion is a frequent source of error: connected but separately authorized systems, such as external services or a distinct FedRAMP-authorized offering, fall outside the boundary and must be handled through interconnection or inheritance arrangements rather than assumed to be covered. Treating a connected service as automatically within scope, or treating boundary definition as if it were the authorization itself, are mistakes that undermine the integrity of the resulting authorization.

Because boundary definition establishes the scope against which risk assessment, control implementation, assessment, and the authorization decision are conducted, getting it right is generally treated as a critical first step. It shapes everything downstream, and errors here propagate into the entire authorization package. Practitioners should verify the current authoritative guidance applicable to their environment, since agency- and program-specific tailoring may affect how boundaries are drawn.

Who it's relevant to

Authorizing Officials
The authorization boundary defines precisely what an authorizing official is approving for operation. A clear boundary is what allows an AO to understand the scope of risk they are accepting and to avoid inadvertently assuming responsibility for connected systems that carry their own separate authorizations.
Information System Security Managers and System Owners
These practitioners are generally responsible for drawing the boundary correctly and ensuring that risk assessment, control implementation, and assessment activities align with the defined scope. They must document which components are included, which are excluded, and how connected but separately authorized systems are handled through interconnection or inheritance.
Assessors and Auditors
Assessors rely on the authorization boundary to determine what falls within their evaluation. An accurately defined boundary establishes the scope against which controls are assessed and helps confirm that separately authorized systems are addressed appropriately rather than assumed to be covered.
Cloud Service Providers and Government Contractors
Providers of external or cloud-based services must understand where their offering sits relative to a system's boundary, since a distinct authorization, such as a separate FedRAMP-authorized offering, falls outside another system's boundary. Contractors should confirm how their services are treated through interconnection or inheritance arrangements and verify the guidance applicable to the relevant environment.

Inside Authorization Boundary

Information System Components
The hardware, software, firmware, and information resources that make up the system being authorized, including servers, workstations, network devices, and applications that fall under the responsibility of the authorizing official.
System Interconnections and External Interfaces
The connections between the system and other systems or services outside the boundary, which generally must be documented, governed by interconnection agreements where applicable, and accounted for in the risk determination.
Data Flows and Information Types
The movement of information into, out of, and within the boundary, along with the categorization of information types (for example, CUI or other data) that helps drive the applicable control baseline and impact level.
Boundary Documentation
Diagrams and narrative descriptions, typically captured in the System Security Plan (SSP), that define what is inside versus outside the boundary and establish the scope of assessment and continuous monitoring.
Inherited and Shared Responsibilities
Controls or services provided by external entities, such as a cloud service provider, that the system relies upon; the boundary generally clarifies which controls are inherited and which remain the system owner's responsibility.

Common questions

Answers to the questions practitioners most commonly ask about Authorization Boundary.

Does an authorization boundary stay fixed once an ATO is granted?
No. Treating the authorization boundary as static, like treating an ATO as permanent, is a common mistake. The boundary reflects the system as authorized, but systems change over time as components are added, removed, or reconfigured. Because an ATO is time-bound and subject to continuous monitoring, changes to the system may alter the authorization boundary and can trigger reassessment or reauthorization. The boundary should be maintained as a current representation of what is actually authorized, not a one-time snapshot. Confirm change-management and boundary-update expectations against your authorizing official's requirements and the applicable RMF or FISMA guidance.
Is defining the authorization boundary the same as securing the system?
No. Defining an authorization boundary establishes what is included in the scope of an authorization; it does not by itself make the system secure. Compliance and security are distinct: the boundary determines which components are subject to control selection, assessment, and monitoring, but the effectiveness of the controls within that boundary is a separate question. A precisely drawn boundary supports sound scoping and accountability, but readers should not equate boundary definition or authorization with an assured security posture.
How do we decide which components to include inside the authorization boundary?
Generally, the authorization boundary includes the components under the direct management control of the organization that are necessary for the system to operate and that fall within the scope of the authorization. Decisions typically consider factors such as management responsibility, data flows, and how components support the system's mission or function. Interconnections and external services may be handled through inheritance or interconnection agreements rather than by pulling every connected element inside the boundary. Because scoping involves judgment and agency-specific interpretation, coordinate the proposed boundary with your authorizing official and verify expectations against the applicable governing guidance and any agency tailoring.
How should shared or inherited services be represented relative to the authorization boundary?
Shared and inherited services are often represented as being outside the system's own authorization boundary while their relationship to the system is documented, for example through control inheritance or interconnection arrangements. This lets a system rely on controls provided by another authorized environment without absorbing those components into its own boundary. The specific documentation and inheritance mechanisms depend on the applicable framework and the authorizing official's expectations, so confirm the required approach against current authoritative sources rather than assuming a single universal method.
Does a FedRAMP-authorized cloud service define our authorization boundary for us?
No. A FedRAMP authorization applies to the cloud service provider's offering as authorized, and it does not automatically define or satisfy the boundary of a system you build on top of it, nor does it automatically satisfy DoD requirements. Your organization is generally responsible for defining its own authorization boundary, which may leverage the underlying service through inheritance while still accounting for the components and configurations you manage. Confirm how the underlying authorization maps to your responsibilities against the applicable guidance and any agency-specific or DoD requirements.
How does the authorization boundary relate to assessment and monitoring activities?
The authorization boundary generally scopes what assessment and continuous monitoring cover. Components inside the boundary are subject to the selected controls and to ongoing monitoring, while relationships to components outside the boundary are typically managed through inheritance or interconnection documentation. Keep in mind that assessment is distinct from authorization: assessing controls within the boundary informs the authorizing official's decision but is not itself the authorization. Verify the specific scoping, assessment, and monitoring expectations against the applicable framework and your authorizing official's direction.

Common misconceptions

The authorization boundary is a purely physical or network perimeter defined by firewalls and subnets.
The authorization boundary is a management and accountability construct, not simply a network perimeter. It reflects what the authorizing official accepts risk for and may include components, services, and data flows that extend beyond a single physical or network segment.
Once the boundary is drawn and an ATO is granted, the boundary is fixed and requires no further attention.
An ATO is time-bound and subject to continuous monitoring. Changes to components, interconnections, or data flows can alter the boundary and generally require reassessment; the boundary should be maintained as a living element of the system's documentation.
Using an authorized external service, such as a FedRAMP-authorized cloud offering, removes that service from any boundary consideration.
Relying on an authorized external service does not automatically remove it from scope. The system owner generally must document the interconnection and inherited controls, and a FedRAMP authorization does not by itself satisfy DoD requirements; responsibilities that remain with the system owner stay within the boundary.

Best practices

Document the boundary explicitly in the System Security Plan using both diagrams and narrative, clearly distinguishing components inside the boundary from external systems and services.
Identify and record all interconnections and external interfaces, noting which controls are inherited from external providers and which remain the system owner's responsibility.
Align the boundary with the information types and categorization of the system so the applicable control baseline and impact level are consistent with what the boundary actually encompasses.
Treat the boundary as a living artifact by reviewing it whenever components, data flows, or interconnections change, and reassess affected controls as part of continuous monitoring.
Coordinate the boundary definition with the authorizing official early, since the boundary determines the scope of what the AO is accepting risk for and what will be assessed.
Verify boundary scope and inherited responsibilities against current official guidance and any agency-specific tailoring rather than assuming a single interpretation applies across all environments.