Clock synchronization

What the control asks

The clocks of the organization's information processing systems are to be kept in step with approved time sources. Purpose: allow security events and other recorded data to be correlated and analysed, and support incident investigations. Guidance: document and implement external and internal requirements for how time is represented, how reliably it is synchronized and how accurate it must be, which may come from law, regulation, contracts, standards or internal monitoring. Define one standard reference time for the organization and apply it to all systems, including building management, entry and exit and other systems that may help investigations. Use a reference clock for logging systems tied to a national atomic clock radio broadcast or GPS, giving a consistent, trusted source for accurate timestamps, and keep networked systems aligned with it through protocols such as NTP or PTP. Two external time sources can be used together to improve reliability, with any difference between them managed. Where several cloud services, or cloud and on-premises services, are combined, synchronization is harder, so monitor each service's clock and record the difference to limit the risk of discrepancies. Other information: correct clocks keep event logs accurate, which matters for investigations and for evidence in legal or disciplinary cases; wrong timestamps hamper investigations and weaken evidence.

Purpose in the standard’s terms, in our words: Enables correlation and analysis of security events and recorded data and supports incident investigations.

Attribute values in one line: Detective; Integrity; Protect, Detect; Information security event management; Protection, Defence.

The five attribute values

These are the published values for this control, from ISO/IEC 27002:2022 clause 4.2 and Annex A Table A.1. Where your own value differs, record the difference and the reason in the same row rather than replacing the published value.

Evidence an auditor asks for

The attribute is a claim about how the control behaves. The evidence is what proves it operates. Keep both, and keep them together.

• Documented time requirements (representation, accuracy, synchronization) and the defined organizational reference time
• NTP or PTP configuration showing systems synchronized to the approved reference clocks, including building management and physical access systems
• Evidence of a trusted reference source such as GPS or national time service, and use of two external sources where adopted
• Monitoring of clock drift across systems, with alerts on deviation
• Records of time offsets for cloud services where direct synchronization is not possible

Where audits commonly find gaps

• Physical access and CCTV systems run on unsynchronized clocks, breaking correlation with IT logs
• Systems synchronize to arbitrary internet time servers not approved by the organization
• Time zones are mixed across logs without a standard representation
• Clock drift is not monitored, so failures of the time service go unnoticed

What ISO/IEC 27028 changes for this control

It carries a single control type, detective, which sets what the evidence has to prove. Its earlier numbering (12.4.4) is kept in the register so an older audit file can still be traced. It serves more than one security domain, which is why a domain count taken by row understates the view.

The guidance expands how the five values are used; it does not change the control or add a requirement. A register that carries the values and the evidence can answer the attribute question for this control without a new document. A register that does not can add the columns at the next SoA version, with an owner and a review date.

Controls that share its values

• 8.15 Logging
• 8.7 Protection against malware
• 7.4 Physical security monitoring

Read the five attributes All 93 controls