BSI C5:2020·Criterion COS-04·high·Component: Communication & Network Security

What this control requires

Controls network perimeters with security gateways, authorizing cross-network access based on customer security requirements. Ensures restricted access to protect cloud customer data from unauthorized inter-network communication risks.
— BSI C5:2020, Criterion COS-04

In plain English

Controls network perimeters with security gateways, authorizing cross-network access based on customer security requirements. In practice, this means you can prove you security - 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-04) 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

  • 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.
  • Cross-Network. Document the trigger, the owner and the cadence.
  • Scope: - configuration, cloud customer data, 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: Cross-network access.

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.

Related controls in BSI C5:2020

References

Topics: network securitycloud infrastructurecloud postureconfiguration management