BSI C5:2020 COS-06 — Segregation of data traffic in jointly used network environments

What this control requires

Segregates customer data traffic in shared networks per documented concepts, ensuring confidentiality and integrity. Prevents cross-tenant data leakage, protecting cloud customer data in multi-tenant environments from unauthorized access.

In plain English

Segregates customer data traffic in shared networks per documented concepts, ensuring confidentiality and integrity. In practice, this means you can prove you segregation - configuration-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-06) 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.

Network Security

AWS. VPC Flow Logs to S3, default-deny security groups, AWS Network Firewall for north-south, and Transit Gateway for hub-and-spoke segmentation.

Azure. NSGs with deny-by-default, Azure Firewall Premium with TLS inspection, Private Endpoints for PaaS, and Microsoft Defender for Cloud's secure score for drift.

GCP. VPC Service Controls perimeters around sensitive APIs, Cloud NGFW Enterprise for L7 inspection, Private Service Connect for managed services.

Configuration Management

Policy-as-code. Codify the control as Service Control Policies (AWS), Azure Policy initiatives, or Organisation Policy (GCP) so the platform enforces what the document promises.

IaC. Gate Terraform / Pulumi / CloudFormation changes with Checkov / tfsec / OPA so misconfigurations never reach production.

Drift. Run a CSPM (Wiz / Prisma / Defender for Cloud / Security Command Center) over every account and route findings to a single risk register.

Operational checklist

• Segregation. Document the trigger, the owner and the cadence.
• Security. Document the trigger, the owner and the cadence.
• Confidentiality. 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.
• Scope: - configuration, cloud customer data, environments, management.

Evidence to collect

• Infra Scan. Continuous infrastructure-posture telemetry — AWS Config, Azure Policy compliance, GCP Organisation Policy, Wiz / Prisma findings.

Infrastructure posture tooling (AWS Config, Azure Policy, Nessus) verifies platform hardening. Focus: Segregation of data traffic in jointly used network environments.

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.
• Default-allow egress rules; East-West traffic with no segmentation; jump hosts on the public internet with password auth.
• Manual cloud-console changes that never make it back to Terraform — drift accumulates until the next outage forces a reconciliation.

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.

• NIST SP 800-171 Rev. 3 3.13.6 — Network Communications - Deny by Default - Allow by Exception
• NIST SP 800-53 Rev. 5 AC-20(4) — Network Accessible Storage Devices -- Prohibited Use
• CIS Controls v8 14.8 — Train Workforce on the Dangers of Connecting to and Transmitting Enterprise Data Over Insecure Networks
• ISO/IEC 27001:2022 A.5.34 — Privacy and protection of personal identifiable information (PII)
• GDPR Article 25 — Data protection by design and by default

Related controls in BSI C5:2020

• Criterion COS-04 — Cross-network access
• Criterion OPS-24 — Separation of Datasets in the Cloud Infrastructure
• Criterion PSS-10 — Software Defined Networking
• Criterion COS-07 — Documentation of the network topology
• Criterion PSS-11 — Images for Virtual Machines and Containers
• Criterion COS-01 — Technical safeguards

References

• Bundesamt für Sicherheit in der Informationstechnik (BSI) — BSI C5:2020
• Catalogue identifier: Criterion COS-06

Topics: network securitycloud infrastructurecloud postureconfiguration managementdata loss prevention