SA-17(9): Design Diversity

To meet the sa-17(9): design diversity requirement, you must intentionally implement different designs for selected system components (or functions) that provide the same required outcomes, so a single design flaw, vulnerability, or supply-chain compromise is less likely to break everything at once ¹. Operationalize it by scoping where diversity matters, defining “different” in your engineering standard, and retaining proof that the diverse designs were actually deployed and maintained.

Key takeaways:

• Scope and document what needs design diversity (components/functions) and why.
• Define acceptable diversity patterns (e.g., different implementations, stacks, vendors, architectures) and prohibit “fake diversity.”
• Keep evidence that the diverse designs exist in production and are maintained through change management.

SA-17(9) sits in the System and Services Acquisition family and focuses on a specific resilience tactic: design diversity. The intent is practical. If every critical path in your system depends on the same design, implementation, or supplier pattern, one exploit, design defect, or compromised update can create a single point of systemic failure. SA-17(9) pushes you to break that monoculture for selected targets by using different designs that still meet the same requirements or deliver equivalent functionality ¹.

For a Compliance Officer, CCO, or GRC lead, the fast way to operationalize SA-17(9) is to treat it as an engineering requirement with a narrow, defensible scope: identify where diversity reduces mission/business risk, define what counts as “different,” then ensure architecture decisions and procurement choices create real independence. Auditors will not accept intent statements alone. They will look for architecture artifacts, configuration evidence, and change records showing that diversity exists and persists over time.

This page gives requirement-level implementation guidance you can hand to control owners and assessors, with concrete steps, evidence expectations, and common audit traps.

Regulatory text

Requirement (excerpt): “Use different designs for {{ insert: param, sa-17.09_odp }} to satisfy a common set of requirements or to provide equivalent functionality.” ¹

Operator interpretation (what you must do):

1. Select the scope object represented by the organization-defined parameter (ODP). In practice, you define whether the target is a system component, subsystem, service, critical function, or control mechanism.
2. Implement at least two meaningfully different designs that meet the same requirements or provide equivalent functionality for that scoped object.
3. Prove the designs are independent enough that a single design flaw, common vulnerability, or single compromised supply-chain path is less likely to impact all implementations at once.
4. Maintain the diversity through SDLC and change management so “diverse by design” does not decay into “identical over time.”

This is a design/architecture control. Your compliance job is to make the requirement unambiguous, testable, and evidenced.

Plain-English interpretation

SA-17(9) requires you to avoid a single shared design for selected critical items. You do that by building or sourcing the same capability in different ways (different implementation, different stack, different architecture pattern, or different supplier), while still meeting the same functional and security requirements ¹.

“Different” has to be real. Two instances of the same container image in two availability zones is redundancy, not design diversity. Two different WAF products with separate rule engines and update pipelines can be design diversity, if they are configured to meet the same requirements.

Who it applies to

Entity types (common contexts):

• Federal information systems implementing NIST SP 800-53 controls ².
• Contractor systems handling federal data where 800-53 is flowed down contractually or via program requirements ².

Operational scope (where it shows up):

• High-impact or mission-critical services where compromise or outage has major consequences.
• Security enforcement points (identity, key management, network boundary controls, monitoring pipelines).
• Supply-chain sensitive components (build systems, signing, update mechanisms) where a single compromised path could have broad blast radius.

You should expect SA-17(9) to be most relevant where you already worry about systemic/common-mode failure.

What you actually need to do (step-by-step)

1) Assign ownership and define the ODP scope object

• Control owner: usually the Chief Architect, Head of Platform Engineering, or Security Architecture lead; GRC supports.
• Define what “{{ sa-17.09_odp }}” means in your environment. Pick a concrete scope object such as:
  • “critical security services” (e.g., MFA, PAM, key management),
  • “boundary protection mechanisms,”
  • “software supply-chain enforcement mechanisms,”
  • or “mission-critical application components.”

Deliverable: a short SA-17(9) control standard that states the chosen ODP scope, decision criteria, and evidence expectations.

2) Perform a “design monoculture” inventory

Create a list of candidate components/functions and document:

• primary implementation (product/service/library, versioning approach),
• deployment pattern (centralized vs distributed),
• update pipeline (who publishes updates, how they are promoted),
• single points of dependency (one CA, one IdP, one logging pipeline, one CI runner fleet).

Deliverable: an inventory table that highlights where a single design is a common dependency across critical paths.

3) Select diversity targets using explicit criteria

Pick targets where diversity meaningfully reduces risk. Common selection criteria:

• Blast radius: failure impacts many systems or users.
• Exploitability: internet-facing or high-privilege.
• Common-mode risk: identical codebase, identical ruleset engine, identical supply-chain path.
• Recovery dependence: you need the component to recover other components.

Deliverable: a scoped list of SA-17(9) “in-scope diversity targets” with rationale.

4) Define acceptable diversity patterns (make “different” testable)

Write an engineering standard that describes acceptable approaches, for example:

• Different implementations: separate codebases for the same function (custom + COTS, or two independent libraries).
• Different suppliers/products: two different third parties for equivalent capability, with separate update channels.
• Different architectures: centralized vs distributed enforcement; agent-based plus network-based; static plus behavioral.
• Different execution environments: separate control planes; separate identity trust chains; separate build/signing systems.

Also define non-acceptable “fake diversity”, such as:

• same vendor product deployed twice,
• same managed service replicated across regions with shared control plane,
• same library forked without meaningful divergence or independent maintenance.

Deliverable: “SA-17(9) Design Diversity Standard” with examples and explicit “counts/doesn’t count” statements.

5) Implement and document the designs

For each in-scope target:

• Document requirements that both designs must meet (security and functional).
• Build/procure the alternate design.
• Validate equivalence: show both satisfy the common requirement set or provide equivalent functionality ¹.
• Ensure operational independence: separate admin roles where feasible, separate CI/CD promotion controls, separate configuration management.

Deliverable: architecture decision records (ADRs) and diagrams showing the two designs and their independence points.

6) Add control checks to prevent diversity decay

• Add architecture review gates for changes affecting in-scope targets.
• Add procurement review for substitutions that collapse diversity (e.g., consolidating to a single provider).
• Add configuration drift detection where “diverse” instances inadvertently converge (same rulesets, same shared keys, same logging dependencies).

Deliverable: change management records and review checklists that include SA-17(9) prompts.

Required evidence and artifacts to retain

Auditors typically want proof in three categories: decision, implementation, and operation.

Design/decision evidence

• SA-17(9) control standard with the defined ODP scope object ¹.
• In-scope target list with selection rationale.
• ADRs documenting why specific diversity choices were made and what “different” means for that target.

Implementation evidence

• Architecture diagrams showing the two designs and independence boundaries.
• Configuration baselines for each design (sanitized as needed).
• Bills of materials or dependency manifests if “different implementations” is based on different libraries/toolchains.

Operational evidence

• Change tickets showing diversity was preserved during upgrades/migrations.
• Test results demonstrating equivalent functional/security requirements are met for both designs.
• Exception/risk acceptance records for targets where diversity is not feasible and compensating controls are used.

Daydream (as a workflow, not a promise) fits well here as a control mapping and evidence orchestration layer: assign an owner, document the implementation procedure, and schedule recurring evidence pulls aligned to SA-17(9) so you are not assembling this under audit pressure ¹.

Common exam/audit questions and hangups

What assessors ask

• “What is your organization-defined parameter for SA-17(9), and where is it documented?”
• “Show me which components/functions require design diversity and how you selected them.”
• “Explain how these designs are truly different. Where are they independent?”
• “Demonstrate both designs meet the same requirement set or provide equivalent functionality.” ¹
• “How do you prevent the environment from converging over time?”

Hangups

• Treating high availability or multi-region deployment as “design diversity.”
• Lack of written criteria for what counts as different.
• No operational mechanism to maintain diversity after re-platforming or standardization initiatives.

Frequent implementation mistakes (and how to avoid them)

1. Mistake: counting redundancy as diversity.
   Avoid it by requiring at least one independent element: different codebase, different vendor, or different control plane.

2. Mistake: diversity that still shares the same supply-chain choke point.
   Avoid it by mapping update/signing paths. If both designs rely on the same build pipeline, you did not reduce common-mode compromise risk.

3. Mistake: “diverse” products configured identically with a shared admin group and shared secrets.
   Avoid it by separating privileged roles and secrets management paths where feasible.

4. Mistake: no proof of equivalence.
   Avoid it by writing a minimum requirement set per target and keeping test results that show both paths meet it ¹.

5. Mistake: no exceptions process.
   Avoid it by documenting infeasible areas and requiring compensating controls plus time-bound review of the exception.

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for SA-17(9). Treat SA-17(9) primarily as an assessment and authorization readiness requirement under NIST SP 800-53 programs ². The risk implication is operational: monoculture increases the chance that a single vulnerability class, misconfiguration pattern, or compromised supplier update affects all critical paths.

Practical 30/60/90-day execution plan

Numeric day counts are a planning convenience, not a claim about required timing.

First 30 days (Immediate)

• Assign a control owner and publish the SA-17(9) ODP definition and scope.
• Build the monoculture inventory for critical functions and security enforcement points.
• Pick a small set of high-value diversity targets and document rationale.

Days 31–60 (Near-term)

• Publish the “what counts as different” engineering standard and review checklist.
• Implement diversity for at least one high-impact target (or document an exception with compensating controls).
• Define required evidence artifacts and set up a repeatable collection method (tickets, repositories, GRC evidence tasks).

Days 61–90 (Operationalize)

• Expand implementation to remaining in-scope targets based on risk.
• Add architecture review and change management gates that explicitly check for diversity collapse.
• Run an internal control test: can you produce the required artifacts quickly and explain independence convincingly to an assessor?

Frequently Asked Questions

What can “{{ sa-17.09_odp }}” be in practice?

It is the organization-defined scope object for SA-17(9), such as “critical security services” or “mission-critical components.” You must define it explicitly in your control standard so assessors can test against a stable scope ¹.

Does multi-region deployment satisfy the sa-17(9): design diversity requirement?

Usually no. Multi-region improves availability, but it often keeps the same design, code, and control plane. SA-17(9) expects different designs that meet the same requirements or provide equivalent functionality ¹.

Can two different vendors providing the same service count as design diversity?

Yes, if the implementations are meaningfully independent and both satisfy the same requirement set. Document independence factors like separate update mechanisms, separate admin controls, and separate failure modes.

How do we prove “equivalent functionality” without writing a full test suite?

Define a minimum set of functional and security requirements for the target (inputs/outputs, performance constraints, required security behaviors). Retain test results or validation records showing both designs meet that set ¹.

What if design diversity is infeasible for a legacy system?

Document an exception that names the target, explains why diversity is not feasible, and identifies compensating controls. Add a review trigger tied to modernization milestones so the exception does not become permanent by default.

How should GRC work with engineering without turning this into paperwork?

Keep artifacts close to engineering work: ADRs in the architecture repo, change tickets for gating, diagrams in the system doc set. Use your GRC tool (or Daydream) to map SA-17(9) to the owner, procedure, and recurring evidence pulls so audits become retrieval exercises, not archaeology ¹.

Footnotes

1. NIST SP 800-53 Rev. 5 OSCAL JSON

2. NIST SP 800-53 Rev. 5