BSI C5:2020 COS-03 — Monitoring of connections in the Cloud Service Provider's network

What this control requires

Separates trusted/untrusted networks into security zones, monitoring connections per risk-based requirements. Annually reviews protocols and services, mitigating vulnerabilities per OIS-06 to protect cloud customer data from unauthorized access.

In plain English

Separates trusted/untrusted networks into security zones, monitoring connections per risk-based requirements. In practice, this means you can prove you risk cloud customer data-level controls on a defined cadence.

Why it matters

BSI C5 is the default cloud-security baseline German public-sector buyers and many DAX-listed enterprises expect; gaps here directly block procurement in the DACH region and frequently appear as must-have items in EU public-tender documents.

This control is rated high. Auditors expect a named owner, a remediation SLA, and a tooling artefact that evidences continuous operation.

What an auditor looks for

A C5 attesting auditor (under ISAE 3000 / IDW PS 860) tests both the design and — for a Type 2 — the operating effectiveness over the attestation period. For each criterion they look for both the Basic and any in-scope Additional objectives.

Within Communication & Network Security, the assessor will trace your implementation back to the catalogue reference (Criterion COS-03) and look for objective evidence on the latest two assessment cycles.

How to implement

Cloud Posture

CSPM. Run Wiz / Prisma / Defender for Cloud / Security Command Center over every account; auto-ticket findings above your accepted-risk threshold.

Operational checklist

• Risk. Document the trigger, the owner and the cadence.
• Security. Document the trigger, the owner and the cadence.
• Data. Document the trigger, the owner and the cadence.
• Access. Document the trigger, the owner and the cadence.
• Networks. Document the trigger, the owner and the cadence.
• Scope: cloud customer data, management.

Evidence to collect

• Vendor Risk Management. vendor_risk_management evidence

Third-party risk management evidence (assessments, contracts, due diligence) shows supplier oversight. Focus: Monitoring of connections in the Cloud Service Provider's network.

Common pitfalls

• Treating CSPM findings as a backlog instead of a hot queue: findings older than the policy SLA accumulate until the auditor asks why.

How it maps to other frameworks

Computed from shared tags, control component and key-operation subjects — handy when you are reconciling controls across an audit.

• SOC 2 (2017 TSC) CC9.2 — Vendor Risk Management
• CIS Controls v8 3.8 — Document Data Flows
• ISO/IEC 27001:2022 A.5.20 — Addressing information security within supplier agreements
• NIST SP 800-53 Rev. 5 SC-12(6) — Physical Control Of Keys

Related controls in BSI C5:2020

• Criterion SSO-01 — Policies and instructions for controlling and monitoring third parties
• Criterion SSO-03 — Directory of service providers and suppliers
• Criterion SSO-04 — Monitoring of compliance with requirements
• Criterion COS-02 — Security requirements for connections in the Cloud Service Provider's network
• Criterion SSO-02 — Risk assessment of service providers and suppliers
• Criterion SSO-05 — Exit strategy for the receipt of benefits

References

• Bundesamt für Sicherheit in der Informationstechnik (BSI) — BSI C5:2020
• Catalogue identifier: Criterion COS-03

Topics: vendor riskvendor managementsupply chain securitycloud posturecontinuous monitoring