SC-45(2): Secondary Authoritative Time Source

SC-45(2) requires you to designate a secondary authoritative time source in a different geographic region from your primary source and make it operationally usable for time synchronization failover. To implement it quickly, document primary/secondary sources, enforce NTP configuration across systems, test failover, and retain evidence that the secondary source is geographically separate and actually used when needed ¹.

Key takeaways:

• Pick a secondary authoritative time source in a different geographic region than the primary ¹.
• Make failover real: configuration, monitoring, and periodic testing matter as much as naming the source.
• Keep assessor-ready evidence: architecture, configs, logs, test results, and ownership mapped to the control.

Accurate, consistent time is a security dependency. It drives log correlation, incident response timelines, authentication checks, certificate validation, and forensics. SC-45(2) focuses on a simple but frequently brittle point of failure: if your primary authoritative time source becomes unavailable or unreliable, your environment can drift. Drift breaks investigations and can create real operational faults.

This requirement is narrow by design. You are not being asked to redesign your whole time architecture. You are being asked to identify a secondary authoritative time source that is geographically separate from the primary authoritative time source ¹. “Identify” is the minimum verb in the text, but auditors and system assessors commonly expect you to show that the secondary source is configured, reachable, and ready for use in the way your environment actually synchronizes time.

For a CCO or GRC lead, the fastest path is to treat SC-45(2) as a resilience control with measurable operational checks: documented sources, standardized configuration, monitoring, and a repeatable failover test. If you run third-party-hosted infrastructure, you’ll also need clear responsibility boundaries so your cloud provider or managed service cannot become an unexamined single point of failure.

Regulatory text

Requirement excerpt: “Identify a secondary authoritative time source that is in a different geographic region than the primary authoritative time source; and” ¹.

Operator translation (what you must do):

1. Name a primary authoritative time source for your environment (the “gold” reference you trust).
2. Name a secondary authoritative time source.
3. Verify the secondary source is in a different geographic region than the primary (not the same metro area, data center campus, or cloud region).
4. Make the secondary source usable for synchronization during outage, routing failure, or compromise of the primary source.

NIST’s control text here is short; your implementation has to make it defensible in assessment. A common assessor reaction to weak implementations is: “You identified a secondary source on paper, but systems can’t fail over to it, or it’s effectively the same region.”

Plain-English interpretation of the requirement

SC-45(2) is a continuity and integrity requirement for time synchronization. You need an alternate trusted time reference that won’t fail for the same regional reason as your primary source. “Different geographic region” is the key design constraint. It reduces correlated risk such as regional outages, routing events, natural disasters, or localized service disruptions.

Practical interpretation for most environments:

• If Primary = on-prem stratum 1/2 in one data center, Secondary should be hosted in a different data center region (or an independent time service) with independent upstream references.
• If Primary = cloud provider time service in Region A, Secondary should not be the same provider endpoint in Region A. Prefer Region B (or a separate provider) to avoid a single-region blast radius.
• If Primary = internal NTP pool, Secondary must be geographically distinct and authoritative, not just another internal host in the same site.

Who it applies to (entity and operational context)

SC-45(2) applies wherever NIST SP 800-53 controls are in scope, including:

• Federal information systems ².
• Contractor systems handling federal data (for example, systems supporting federal contracts or programs that inherit NIST requirements through SSPs and contract clauses) ¹.

Operational contexts where SC-45(2) becomes high-friction in audits:

• Hybrid environments (on-prem plus multiple clouds) with inconsistent NTP settings.
• Container platforms where node time drifts and propagates into workloads.
• OT/ICS or segmented networks where outbound NTP is restricted and time sources are scarce.
• High-security environments where inbound NTP is blocked and time must be distributed internally.

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

1) Assign ownership and define the system boundary

• Name a control owner (often Infra/SRE, Network Engineering, or Security Engineering).
• Define which systems are in scope: domain controllers, Linux/Windows servers, hypervisors, network devices, security tools (SIEM, EDR managers), key applications, and logging collectors.
• Document any inherited responsibility from a third party (cloud provider NTP, managed network, managed SOC). Put it in writing.

Deliverable: Control narrative with owner, scope, and dependencies.

2) Select primary and secondary authoritative time sources

Choose sources that meet three tests:

• Authoritative: trusted, stable, and appropriate for your environment’s assurance needs.
• Geographically separate: secondary is in a different geographic region than primary ¹.
• Operationally reachable: systems can reach them under expected failure scenarios.

Decision matrix (use this in your design review):

Decision point	Acceptable	Audit risk / usually rejected
Geographic separation	Different data center region or separate provider region	Same city/metro; same cloud region; same campus
Independence	Different upstream reference or different service boundary	Same upstream path and single failure mode
Reachability	Documented routes/firewalls allow NTP under normal and failover	“In theory reachable,” but blocked by ACLs

Deliverable: Time source register (primary, secondary, region, owner, endpoints, rationale).

3) Standardize time client configuration across platforms

You want consistent behavior: systems query a local tier (if you use one), which queries the authoritative sources.

• Windows: define time source hierarchy (AD domain time hierarchy or explicit NTP peers) and ensure the secondary peer is configured.
• Linux: configure chrony/ntpd with primary and secondary servers; set ordering and constraints appropriate to policy.
• Network/security appliances: configure NTP servers and verify they support multiple servers and failover.
• Cloud workloads: confirm the method (provider time sync service vs NTP) and whether it supports a secondary in another region.

Deliverables: Configuration baselines, golden images, and approved configuration standards.

4) Implement monitoring and alerting for time health

Auditors will ask how you know this control works outside of a one-time configuration screenshot.

• Monitor reachability of primary and secondary time servers.
• Monitor offset/drift on representative systems.
• Alert on loss of sync and excessive offset relative to your operational tolerances (set tolerances based on system requirements; document them).

Deliverables: Monitoring dashboards, alert rules, and incident runbooks.

5) Test failover and document results

Run a controlled test:

• Simulate primary time source unavailability (block NTP to primary or stop service in a lab).
• Verify clients switch to secondary.
• Record timestamps, affected hosts, and evidence (logs, screenshots, command output).

Deliverables: Test plan, test record, and remediation tickets if gaps found.

6) Document the implementation in the SSP/control narrative

Map SC-45(2) to:

• Owner
• Primary/secondary sources and regions
• How systems are configured to use them
• How monitoring and tests validate operation
• Evidence list and review cadence

If you use Daydream for GRC workflow, this is a good place to store the control narrative, assign the owner, and set recurring evidence tasks so SC-45(2) doesn’t become a once-a-year scramble.

Required evidence and artifacts to retain

Keep evidence that answers: “What did you choose, why is it geographically separate, and does it work?”

Minimum evidence set:

• Time Source Register showing primary and secondary, with geographic region noted ¹.
• Network/security approvals: firewall rules/ACLs that permit NTP to both sources.
• Configuration evidence (by platform):
  • Windows GPO/export or w32time configuration output
  • chrony.conf/ntp.conf snippets or configuration management records
  • Appliance NTP settings exports
• Monitoring evidence: alert definitions and a sample of health status.
• Failover test record: date, steps, results, and issues/tickets.
• Change management artifacts: change tickets for rollout and updates.
• Third-party documentation if time service is provided externally: responsibility matrix and service description.

Common exam/audit questions and hangups

Expect these questions in a FedRAMP/NIST-style assessment:

1. “Show me the primary and secondary authoritative time sources and their regions.” Have the register ready ¹.
2. “Prove systems can actually use the secondary.” Provide client configs plus a failover test record.
3. “Is the secondary truly geographically separate?” Be prepared to explain your definition of “region” and why it avoids correlated outages.
4. “What happens if the primary time source is compromised?” Your narrative should describe switching behavior and detection/response.
5. “How do you ensure new assets inherit the configuration?” Show your baseline, IaC, images, or endpoint management standards.

Hangup to anticipate: teams confuse “secondary server” with “another internal time distributor” in the same site. That usually fails the geographic-region intent.

Frequent implementation mistakes and how to avoid them

• Mistake: Primary and secondary are in the same cloud region.
  Fix: choose a different cloud region for the secondary, or a separate provider, and document the region boundary ¹.

• Mistake: Secondary exists on paper only.
  Fix: implement it in client configs and prove it with a failover test record.

• Mistake: No evidence of “authoritative.”
  Fix: document why you trust the source (provider docs, internal stratum architecture, governance decision record). Keep the reasoning crisp.

• Mistake: Time sync is blocked by firewall in one segment.
  Fix: include segmented networks in the scope review; use internal time relays that can reach both authoritative sources.

• Mistake: Monitoring checks only the server, not the clients.
  Fix: sample endpoints across tiers and environments and track drift/offset.

Risk implications (why assessors care)

Bad time creates cascading failures:

• Logs become hard to correlate across hosts and tools.
• Investigations and legal hold timelines become disputed.
• Authentication and token validation can fail if system clocks drift. SC-45(2) reduces the odds that a single regional event breaks time integrity across your environment by requiring geographic separation between primary and secondary sources ¹.

Practical 30/60/90-day execution plan

First 30 days (establish design and ownership)

• Assign control owner and in-scope system inventory.
• Document current time sync architecture and gaps.
• Select primary and secondary authoritative sources and confirm geographic separation ¹.
• Create the Time Source Register and draft the control narrative.

By 60 days (deploy configuration and monitoring)

• Roll out standardized NTP configuration across major platforms.
• Implement network rules for both sources across segments.
• Stand up monitoring/alerting for reachability and drift on representative systems.
• Create runbooks for time-sync incidents and failover actions.

By 90 days (prove operation and lock evidence)

• Execute at least one failover test in a controlled environment and document results.
• Remediate uncovered gaps (segments blocked, devices without secondary configured).
• Finalize SSP/control narrative and evidence checklist.
• Set recurring evidence tasks (configuration attestation, monitoring review, periodic failover retest). Daydream can track ownership and recurring evidence collection so you always have current artifacts ready for assessment.

Frequently Asked Questions

What counts as a “different geographic region” for SC-45(2)?

The control requires the secondary authoritative time source to be in a different geographic region than the primary ¹. Define “region” in your environment (cloud region, data center region) and document why it avoids shared regional outages.

Can my secondary time source be a third-party service?

Yes, if you treat it as authoritative for your environment and document governance and responsibility boundaries. Keep evidence of the provider endpoint/region and confirm clients can fail over to it.

If I already have multiple internal NTP servers, do I still need SC-45(2)?

Multiple internal servers in the same site don’t satisfy the geographic separation intent by themselves ¹. You may still use internal relays, but ensure they can reach a secondary authoritative source in a different region.

How do I show auditors that failover works?

Provide client configuration showing both primary and secondary sources plus a documented test where the primary was unavailable and clients synchronized to the secondary. Pair it with monitoring evidence that would detect future failure.

Does SC-45(2) require a specific NTP stratum level?

The excerpt provided focuses on identifying a geographically separate secondary authoritative time source ¹. Choose a design appropriate to your risk and architecture, then document why it is authoritative and resilient.

What’s the minimum evidence set a CCO should insist on?

A time source register with regions, standardized configuration evidence, monitoring/alerting proof, and a failover test record. If any of those are missing, you’ll struggle to prove the control operates beyond policy text.

Footnotes

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

2. NIST SP 800-53 Rev. 5; Source: NIST SP 800-53 Rev. 5 OSCAL JSON