Executive Summary

Service Control Policies (SCPs) are powerful tools for managing permissions in AWS Organizations, but they can be complex to implement effectively. This article explores the key aspects of SCPs, common pitfalls, and best practices for their use. We'll cover the relationship between SCPs and IAM policies, methods for viewing resultant permissions, and strategies for optimizing SCP management.

What are Service Control Policies (SCPs)?

Service Control Policies are a type of organization policy used within AWS Organizations to manage permissions across accounts. They provide central control over the maximum available permissions for all accounts in your organization, ensuring that accounts do not have more permissions than intended.

Key Characteristics of SCPs:

• SCPs do not grant permissions; they only define what actions are allowed or denied at the account level.
• SCPs take precedence over IAM policies when denying actions.
• SCPs can be applied at the root, organizational unit (OU), or individual account level.

The Relationship Between SCPs and IAM Policies

SCP and IAM Policy Relationship

SCPs and IAM policies work together to define the permissions of your AWS accounts and principals:

• SCPs set the outer boundaries of permissions.
• IAM policies grant specific permissions within those boundaries.
• The effective permissions are the intersection of what's allowed by both SCPs and IAM policies.

Common Pitfalls of Misunderstanding SCPs

If you are an AWS architect like me, I know that you have run into this issue before. Here are some of the common problems we face:

• Unexpected Denials: Users might face denied actions despite having the necessary IAM permissions due to SCPs overriding IAM policies.
• Overly Restrictive Access: Applying restrictive SCPs without thorough consideration can block access to essential AWS services.
• Operational Inefficiencies: Troubleshooting permission issues can become complex, delaying service rollouts and hindering productivity.
• Compliance and Security Risks: Misaligned SCPs might lead to either insufficient restriction or broader access than intended, violating security best practices.
• Ineffective Policy Management: Poorly designed SCPs and IAM policies can lead to conflicts and overlapping permissions, complicating policy management.
• Scalability Challenges: Managing SCPs and ensuring alignment with IAM policies becomes increasingly complex as your organization grows.
• Reduced Flexibility: SCPs restrict the ability to delegate permissions effectively within the organization.

Viewing Resultant SCP and IAM Policies

Understanding the combined effect of SCPs and IAM policies is crucial. Here are three methods to assess the resultant permissions:

1. AWS Policy Simulator

   The AWS Policy Simulator allows you to simulate the effective permissions of IAM policies, resource-based policies, and SCPs.

   • Navigate to the IAM Console: Open the IAM console in AWS Management Console.
   • Open Policy Simulator: Select Policy Simulator from the navigation pane.
   • Select an Entity: Choose the IAM user, group, or role you want to simulate.
   • Add Policies: Include the relevant IAM policies and SCPs.
   • Simulate Actions: Enter the actions you want to simulate and run the simulation to see the allowed or denied actions.

2. AWS IAM Access Analyzer

   AWS IAM Access Analyzer helps you analyze access permissions of IAM policies, resource policies, and SCPs.

   • Navigate to the IAM Console: Open the IAM console in AWS Management Console.
   • Open Access Analyzer: Select Access Analyzer from the navigation pane.
   • Create an Analyzer: If not already set up, create an analyzer.
   • Analyze Policies: Access Analyzer will scan your policies and provide insights into permissions.

3. Manual Review and Documentation

   Performing a manual review helps in understanding the net permissions by combining SCPs and IAM policies:

   • Review SCPs: Examine the SCPs applied at the root, OUs, and individual accounts.
   • Review IAM Policies: Check the IAM policies attached to the user, groups, and roles.
   • Combine Policies: Document the permissions from both SCPs and IAM policies to understand the effective permissions.

Example Scenario:

SCP:
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Action": "s3:DeleteBucket",
      "Resource": "*"
    }
  ]
}

IAM Policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:*",
      "Resource": "*"
    }
  ]
}

Resultant Permissions:

• s3:DeleteBucket is denied due to the SCP.
• All other S3 actions are allowed as per the IAM policy.

Best Practices for Managing SCPs

• Start with Least Privilege: Implement SCPs with a least privilege approach, allowing only necessary permissions and explicitly denying risky actions.
• Test Policies in Development: Before applying SCPs in production, test them in a development environment to ensure they do not inadvertently block essential services.
• Use Explicit Denies Sparingly: While explicit denies are powerful, use them sparingly and only when necessary to block high-risk actions.
• Regularly Review and Update SCPs: As your organization evolves, regularly review and update SCPs to ensure they align with current security and compliance requirements.
• Document Policies and Changes: Maintain clear documentation of SCPs and any changes made, providing a reference for troubleshooting and audits.
• Implement Gradual Rollout: Roll out changes to SCPs gradually to monitor and adjust for any unintended consequences.
• Use Version Control for SCPs: Track changes to SCPs using version control to maintain an audit trail and enable rollback if needed.
• Conduct Regular Audits: Regularly audit your SCPs to ensure they are effective and up-to-date.

SCP Design Patterns

Here are some common SCP design patterns to consider:

• Deny High-Risk Services
• Allow List for Approved Services
• Enforce Multi-Factor Authentication
• Restrict Region Usage
• Prevent Resource Sharing Outside the Organization

Troubleshooting SCP Issues

Common issues and their solutions:

• Unexpected Permission Denials
  • Solution: Review SCP hierarchy and use AWS Policy Simulator.
• SCP Not Taking Effect
  • Solution: Check SCP attachment and ensure it's enabled.
• Conflicts Between Multiple SCPs
  • Solution: Understand SCP evaluation logic and simplify policies.

Industry-Specific SCP Considerations

• Healthcare
  • Enforce HIPAA compliance controls.
  • Restrict access to PHI data stores.
• Financial Services
  • Implement controls for PCI DSS compliance.
  • Enforce encryption for data at rest and in transit.
• Government
  • Restrict usage to GovCloud regions.
  • Enforce strict data residency requirements.

Conclusion

Service Control Policies are a vital tool for managing permissions in AWS Organizations. By understanding their complexities and following best practices, you can ensure a secure, efficient, and compliant AWS environment. Navigating these intricacies with care will enable you to leverage the full power of AWS while minimizing potential risks and operational disruptions.

Additional Resources

• AWS Organizations Documentation
• AWS SCP Examples
• AWS Policy Simulator