AU-8(2): Secondary Authoritative Time Source

AU-8 is the control family anchor for “time is a security control.” If clocks drift, audit logs lose evidentiary value, correlation across systems breaks, and incident response turns into guesswork. AU-8(2) tightens AU-8 by requiring a secondary authoritative time source, so time synchronization continues even when the primary time source is unavailable or untrusted.

Operationally, this requirement lands across infrastructure, identity, security tooling, and application teams. Your SIEM and centralized logging platform are usually the first place auditors look, but they are not the only systems that matter. Domain controllers, hypervisors, Kubernetes nodes, cloud control planes, VPNs, EDR collectors, databases, and critical business applications all generate audit records. If those records do not share a consistent, reliable time base, you will fail common investigative questions like “what happened first?” and “did the control trigger before the change?”

This page translates AU-8(2) into an implementable build sheet: what to configure, how to prove it works, what to retain as evidence, and what typically causes audit findings.

Regulatory text

Excerpt (as provided): “NIST SP 800-53 control AU-8.2.” ¹

Operator interpretation: AU-8(2) is the enhancement that expects you to establish a secondary authoritative time source to support time synchronization for systems that create or rely on audit records. In practice, an assessor will look for (1) a defined authoritative time hierarchy, (2) independent secondary sources, (3) resilient configuration (failover), and (4) evidence the configuration operates as designed ².

Plain-English interpretation (what the requirement really means)

You must:

1. Pick an authoritative time source (the “truth”) for your environment.
2. Add a second authoritative source that does not share the same single point of failure as the first.
3. Configure systems to sync time to the authoritative sources in a controlled way (typically with authenticated NTP where feasible).
4. Prove it keeps working via monitoring and a repeatable failover test.

The intent is auditability. If you cannot trust timestamps, you cannot trust audit records.

Who it applies to (entity + operational context)

Entity types commonly in scope:

• Federal information systems and supporting infrastructure ².
• Contractor systems handling federal data where AU controls are part of the required baseline ².

Operational scope (what systems auditors expect you to include):

• Central log platforms (SIEM, log pipelines, collectors/forwarders).
• Identity and directory services (IdP, domain controllers).
• Security tooling (EDR management planes, scanners, PAM systems).
• Network/security infrastructure (firewalls, VPN concentrators, WAFs, DNS).
• Compute layers (hypervisors, container nodes, orchestration control plane).
• Critical applications and databases that generate security-relevant logs.
• Cloud-native audit sources (cloud activity logs) and the systems that ingest/correlate them.

A practical scoping rule: if the system produces logs used for detection, response, forensics, or compliance reporting, it belongs in AU-8(2).

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

1) Assign ownership and define the time architecture

• Control owner: usually Infrastructure/SRE with Security oversight (GRC validates evidence).
• Document a time synchronization architecture:
  • Primary authoritative time source (internal stratum-1/2, or approved external authoritative source).
  • Secondary authoritative time source (independent path; not the same device, same site, or same provider dependency).
  • Time distribution model (e.g., internal NTP servers feed endpoints; endpoints do not all hit the internet directly).
• Record approved protocols (NTP, Chrony, Windows Time), and whether authentication is required where supported ².

2) Implement primary + secondary authoritative time sources

Common implementation patterns auditors accept when documented and operated:

• Two internal authoritative time servers in separate fault domains (different hosts, power, racks; ideally different sites).
• Hybrid design: internal NTP servers that each reference different upstream authoritative sources, with independent network egress and DNS resolution paths.
• Cloud environments: regionally redundant time sync service plus a second independent reference path. Document how instances behave during upstream loss and how you detect drift.

Your design goal: if the primary time source fails or becomes unreachable, systems continue to synchronize to the secondary without operator heroics.

3) Configure clients for resilient synchronization

For each major platform, standardize configuration:

• Linux (chrony/ntpd): configure multiple servers, mark preferred/backup behavior, and set drift files. Enforce via configuration management.
• Windows: ensure domain hierarchy is correct and that the PDC emulator (or designated time authority) references multiple reliable upstream sources.
• Network devices / appliances: configure two or more NTP servers; confirm the device actually fails over rather than “sticks” to the first entry.
• Containers/Kubernetes: validate node time sync; containers inherit node time. Focus control on node OS and cluster images.

Also decide and document:

• Maximum acceptable drift for your environment (as an internal standard tied to detection and forensic needs).
• What happens when time cannot be synchronized (alerting, quarantine, or operational runbook steps).

4) Add monitoring, alerting, and logging for time health

Auditors want operational proof, not just config screenshots.

• Monitor sync status (synchronized/unsynchronized, offset, stratum, last sync time).
• Alert on:
  • Client unsynced state.
  • Offset beyond your defined threshold.
  • Loss of reachability to one authoritative source (to confirm redundancy still exists).
• Forward time service logs into the SIEM/log platform so you can show historical operation.

5) Test failover and capture evidence

Run and document a failover test:

• Disable or block access to the primary authoritative time source in a controlled manner.
• Verify clients switch to the secondary source.
• Verify key logging systems continue to stamp events with coherent time and do not “jump” backwards unexpectedly.
• Restore primary and confirm stable operation.

Tie this test to change management so the evidence is credible and repeatable.

6) Operationalize: make it part of build standards and onboarding

• Embed time sync requirements into:
  • Gold images / base AMIs.
  • Infrastructure-as-code modules.
  • Device configuration templates.
  • Third-party onboarding requirements when third parties host systems that send logs into your environment.

A GRC-friendly approach is to maintain a system list with “time sync method + primary/secondary sources + monitoring link” for anything in your audit logging scope.

Required evidence and artifacts to retain

Keep evidence that shows both design and operation:

Design artifacts

• Time synchronization standard (policy or technical standard) describing primary and secondary authoritative sources ².
• Network/dataflow diagram for time distribution (high level is fine).
• Asset scope list: systems in logging/audit scope and their time sync method.

Build/config artifacts

• NTP/chrony/Windows Time configuration samples (templates) showing at least two sources.
• Configuration management proof (IaC repo links, CM tool reports, or signed baselines).
• Firewall/DNS rules supporting reachability to both authoritative sources.

Operational artifacts

• Monitoring dashboards or periodic exports showing sync health over time.
• Alert tickets showing detection and response to time drift/unsync events.
• Failover test record: date, steps, results, screenshots/CLI outputs, and approver.
• Change records for modifications to time sources.

Daydream note (earned mention): many teams pass the technical implementation but fail on repeatable evidence. Daydream helps by mapping AU-8(2) to an owner, a runbook, and a recurring evidence checklist so audit prep becomes a pull, not a scramble.

Common exam/audit questions and hangups

Assessors commonly ask:

• “What are your authoritative time sources, and why are they authoritative?”
• “Show me secondary time source configuration on representative systems.”
• “How do you detect drift and unsynchronized time?”
• “Prove failover works. When did you last test it?”
• “Are your logging systems and directory services synchronized to the same hierarchy?”
• “What happens if time sync fails on a critical host?”

Hangups that trigger findings:

• Secondary source exists “on paper” but is unreachable due to firewall, routing, or DNS.
• Clients list two servers but do not fail over due to misconfiguration.
• No monitoring, so you cannot prove continuous operation.

Frequent implementation mistakes (and how to avoid them)

1. Secondary source shares the same failure domain.
   Avoid: two NTP servers on the same hypervisor cluster or same site power. Put them in separate fault domains and document the rationale.

2. Relying on endpoints to hit external time directly.
   Avoid: uncontrolled egress from all hosts. Prefer internal time distribution with controlled upstream sources.

3. No proof of operation.
   Avoid: “config-only compliance.” Keep sync health telemetry and a failover test record.

4. Ignoring “non-obvious” log producers.
   Avoid: scope only the SIEM. Include identity, network devices, and key apps.

5. Time changes that break log integrity.
   Avoid: ad-hoc manual clock edits. Require a runbook and change control for time source changes.

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for AU-8(2). Practically, the risk shows up during incidents and audits: inconsistent timestamps can invalidate investigative timelines, complicate root-cause analysis, and create gaps in audit record reliability ².

Practical execution plan (30/60/90-day)

No fixed timelines are required by the provided source text, so treat this as a pragmatic rollout sequence you can adapt.

First 30 days (stabilize and scope)

• Name the control owner and approver.
• Inventory systems that generate or process audit records; mark time sync method and gaps.
• Define primary and secondary authoritative time sources and confirm network reachability.
• Draft the time synchronization standard and publish it.

By 60 days (implement and instrument)

• Configure clients across major platforms to use both sources.
• Deploy monitoring for sync state and drift; route alerts into your ticketing system.
• Update gold images, IaC modules, and build checklists.

By 90 days (prove and harden)

• Execute a documented failover test and remediation cycle.
• Close stragglers (network devices, appliances, legacy apps).
• Package evidence for assessment: standard, architecture, configs, monitoring exports, and test record.
• Put evidence collection on a recurring cadence (monthly or per change) so it stays current.

Frequently Asked Questions

Do I need GPS-based stratum-1 clocks to satisfy AU-8(2)?

Not inherently. The requirement is about having a secondary authoritative time source and operating it reliably; your authoritative sources can be internal or external if they are approved and monitored ².

Can the secondary time source be another server in the same data center?

It can, but it is a common audit problem if both sources share the same failure domain. Put the secondary source in a separate fault domain where possible and document the engineering rationale either way.

How do auditors expect me to “prove” the secondary source works?

Provide a failover test record plus monitoring evidence that clients can synchronize to the secondary source. Screenshots/CLI outputs are fine if they are tied to a dated ticket or change record.

Does this apply to cloud-native logs like cloud activity trails?

The cloud provider controls the timestamping for provider-native logs, but AU-8(2) still applies to the systems you operate that generate logs and to the collectors/correlation layers that depend on consistent time ².

What evidence is strongest for AU-8(2) in an assessment?

A short standard that names primary/secondary sources, a representative set of system configs showing both sources, monitoring exports for sync health, and a dated failover test with results.

How should I handle third parties that host systems sending logs into my SIEM?

Add time synchronization requirements to third-party security requirements and onboarding checklists. Require them to document their time sources and show periodic evidence of synchronization for systems that provide audit logs to you.

Footnotes

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

2. NIST SP 800-53 Rev. 5