How to Implement ISO 27001 Annex A 5.23 Information Security for Use of Cloud Services

The ultimate how to implement guide to ISO 27001 Annex A 5.23 Information Security for Use of Cloud Services.

1. Establish a Custom Cloud Usage Policy

Control Requirement: Define and communicate organisational processes for the acquisition, use, management, and exit of cloud services.

Required Implementation Step: Open your word processor and draft a policy that specifies exactly which cloud service models (IaaS, PaaS, SaaS) are permitted and the mandatory security configurations for each. Manually distribute this to all department heads and obtain a physical or timestamped digital signature of receipt.

Minimum Requirement: A signed policy document that explicitly forbids the use of unapproved “Shadow IT” cloud services.

2. Mandate Specific Security Requirements in Cloud Contracts

Control Requirement: Ensure agreements with cloud service providers address information security risks.

Required Implementation Step: Review your current Cloud Service Agreements (CSAs). Manually negotiate or attach an addendum that specifies your requirements for data residency, encryption key management, and incident notification windows, rather than accepting the provider’s standard “click-through” terms.

Minimum Requirement: Evidence of a contract review or addendum addressing data jurisdiction and breach notification for all critical cloud vendors.

3. Define and Document the Shared Responsibility Matrix

Control Requirement: Clearly define the security roles and responsibilities between the organisation and the cloud provider.

Required Implementation Step: Create a physical table for each cloud service. Explicitly mark who is responsible for OS patching, application security, and identity management; do not assume the vendor “handles everything” just because they are a Tier 1 provider.

Minimum Requirement: A documented Shared Responsibility Model signed off by the Head of IT for every core cloud platform (e.g., AWS, Azure, M365).

4. Implement Hardened Configuration Standards (IaC)

Control Requirement: Ensure cloud services are configured securely according to organisational standards.

Required Implementation Step: Manually review your Terraform or CloudFormation scripts to ensure they enforce “encryption at rest” and “no public S3 buckets” by default. Export these configuration files as PDF evidence to prove that security is “baked in” to the infrastructure code.

Minimum Requirement: A baseline configuration report showing that all active cloud environments meet the organisation’s hardening standard.

5. Enforce Strict Data Residency Controls

Control Requirement: Manage the location of data stored and processed in the cloud to meet legal and regulatory requirements.

Required Implementation Step: Log into your cloud console and manually set region-lock policies. Take a screenshot of the configuration that limits data storage to specific United Kingdom or EEA regions to provide as evidence for the auditor.

Minimum Requirement: Screenshots or policy exports showing active “Region Restriction” tags on all production data volumes.

6. Verify Cloud Provider Independent Audit Evidence

Control Requirement: Regularly monitor and review cloud service provider security performance and audit reports.

Required Implementation Step: Download the latest SOC 2 Type II or ISO 27001 certificate from the provider’s trust portal. Manually verify that the “Scope” of their audit actually covers the specific data centre and service you are using, and document any “User Entity Controls” you must implement.

Minimum Requirement: A local folder containing the most recent 12 months of provider audit reports with internal review notes.

7. Establish a Manual Cloud Exit Strategy

Control Requirement: Define and maintain an exit strategy for the cessation of cloud services.

Required Implementation Step: Write a step-by-step technical manual on how to extract your data from the provider in a usable format. Perform a manual “Dry Run” where you export a database backup and verify its integrity on a local, non-cloud server.

Minimum Requirement: A documented Exit Plan that includes data portability tests and a list of alternative providers.

8. Configure Multi-Factor Authentication (MFA) for All Administrative Access

Control Requirement: Manage access to cloud services through secure authentication and authorisation.

Required Implementation Step: Manually audit every administrative user in your cloud IAM (Identity and Access Management). Ensure MFA is enforced at the root/tenant level and that no “long-lived” access keys are stored in plain text in local config files or scripts.

Minimum Requirement: An IAM report showing 100% MFA adoption for all users with “Write” or “Delete” permissions.

9. Monitor Cloud Activity via Local Log Aggregation

Control Requirement: Monitor cloud service activity to detect and respond to security incidents.

Required Implementation Step: Set up a local syslog server or an independent log vault. Configure your cloud provider to stream all “CloudTrail” or “Activity Logs” to this local destination so that you maintain an immutable audit trail that the provider cannot modify or delete.

Minimum Requirement: Evidence of log streaming to an off-platform location that is reviewed monthly by a security officer.

10. Perform Manual Change Management for Cloud Infrastructure

Control Requirement: Manage changes to cloud service configurations and architecture.

Required Implementation Step: Create a manual Change Request for every modification to the virtual network (VPC/VNet) or security groups. Ensure the change is peer-reviewed by a second engineer and the “Before/After” configuration state is documented in your internal change log.

Minimum Requirement: A change management log entry for every modification to cloud firewall rules or network topology.

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