What evidence do auditors commonly ask for on AU-4 Audit Log Storage Capacity?

SSP AU-4 statement describing how log storage is sized for the retention period and monitored for growth; Storage capacity report for the log platform with utilisation trend and alert thresholds; List of auditable events and log source inventory; SIEM ingestion configuration showing all in scope systems; Log review procedures with assigned analyst owners; Time synchronisation evidence across logging endpoints.

What is the most common gap auditors find on AU-4?

Reviewers acknowledge alerts but do not document investigation outcomes

FedRAMP High (NIST SP 800-53 Rev 5 baseline) · AU - Audit and Accountability

`AU-4` Audit Log Storage Capacity

Allocate audit log storage capacity to accommodate FedRAMP-defined retention period.

Source: FedRAMP High (NIST SP 800-53 Rev 5 baseline), AU-4. The statement above is our statement of the clause, read from the published text and cited so you can check it against the official text, not the instrument verbatim. Open FedRAMP High (NIST SP 800-53 Rev 5 baseline) in the compliance library.

A FedRAMP baseline is a subset of NIST SP 800-53 Rev 5, enhancements included.

artefacts

held by a system

at each review

moderate

to go live

SIEM / log platform

where the evidence lives

teal = a system already holds it · olive = produced at each review. To go live: easy when half or more of the artefacts are already held by a system, moderate from a quarter to a half, hard below a quarter.

These are the artefacts auditors commonly ask for on this requirement, our guidance rather than text from the standard. Your assessor decides what is sufficient in your context, and asks for samples across the period when a report covers one.

system holds itEvidence a system already holds

Storage capacity report for the log platform with utilisation trend and alert thresholds · SIEM / log platform
SIEM ingestion configuration showing all in scope systems · SIEM / log platform
Audit log integrity controls including write once storage or hashing · SIEM / log platform

periodic reviewEvidence produced at each review

Time synchronisation evidence across logging endpoints · Endpoint management (MDM / EDR)

governing documentDocuments that govern the control

SSP AU-4 statement describing how log storage is sized for the retention period and monitored for growth · Policy repository / GRC workspace
List of auditable events and log source inventory · Policy repository / GRC workspace
Log review procedures with assigned analyst owners · Policy repository / GRC workspace

First move

Start with the 3 of 7 artefacts that already live in a system (SIEM / log platform); keep the periodic reviews but log each one as a dated record with a named reviewer.

Common gaps auditors find

Reviewers acknowledge alerts but do not document investigation outcomes
Clock drift across hosts breaks event correlation
Privileged user activity not isolated for independent review
Critical log sources missing from the SIEM with no detection coverage

Do this for your whole sheet

Paste the rows you run your controls from and get this mapping for every control at once, with the periodic-review ones flagged and a first move per row. No account for the first run.

Build my evidence sheet

AU-3(1) Additional Audit Information · AU-5 Response to Audit Logging Process Failures