Networks are only as secure as the services running on them.
If network services are weak, protected systems quickly become irrelevant.
Annex A 8.21 exists to ensure organisations secure network services themselves, not just the underlying networks, preventing unauthorised access, misuse, disruption, and compromise.
This control is about securing what the network delivers, not just how it is connected.

Annex A 8.21 of ISO 27001:2022 focuses on security of network services.
At a practical level, this means:
The control applies to services, not just infrastructure.
A network service is any service delivered over a network, including:
These services are often:
If network services are insecure:
Annex A 8.21 ensures organisations treat network services as security-critical assets, not background infrastructure.
This control replaces ISO 27001:2013 Annex A 13.1.2 and expands emphasis on service-level security and accountability.
A pragmatic approach to Annex A 8.21 typically includes the following elements.
Organisations should identify:
Security cannot be applied to services that are not known.
Each network service should have defined security requirements, including:
Requirements should reflect:
Access to network services should be:
Controls should define:
Open access is rarely justified.
Annex A 8.21 expects services to:
This may include:
Service access should never rely on trust alone.
Where data is transmitted via network services, organisations should consider:
Protection should reflect:
Encryption protects both confidentiality and integrity.
Network services should:
Service-level restrictions complement network-level controls.
Annex A 8.21 explicitly supports monitoring.
Organisations should monitor:
Monitoring supports:
Unmonitored services are blind spots.
Logs should record relevant service activity, such as:
Logs support:
This aligns closely with Annex A 8.15 (Logging).
Where network services are provided externally, organisations should ensure:
Third-party services remain organisational risk.
Annex A 8.21 highlights the importance of:
Monitoring ensures providers and internal teams meet obligations consistently.
Changes to network services should:
Uncontrolled service changes are a common outage cause.
Annex A 8.21 does not require:
It does require organisations to:
Most network incidents involve services, not cables.
Network services fail when nobody owns them.
Annex A 8.21 is about securing what actually delivers functionality to users and systems.
When network service security is implemented effectively:
Networks connect systems.
Services make them usable.
Annex A 8.21 ensures those services are secure, controlled, and dependable.
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