Cross-account access management in AWS is a critical capability for organizations operating multiple AWS accounts, enabling secure resource sharing and centralized governance across complex organizational structures. This approach allows principals (users, roles, or services) in one AWS account to …Cross-account access management in AWS is a critical capability for organizations operating multiple AWS accounts, enabling secure resource sharing and centralized governance across complex organizational structures. This approach allows principals (users, roles, or services) in one AWS account to access resources in another account while maintaining security boundaries.
The primary mechanisms for implementing cross-account access include:
**IAM Roles for Cross-Account Access**: You create an IAM role in the trusting account (Account B) that specifies which principals from the trusted account (Account A) can assume it. The trust policy defines who can assume the role, while the permissions policy determines what actions are permitted. Users or applications then use AWS STS AssumeRole to obtain temporary credentials.
**Resource-Based Policies**: Certain AWS services support resource-based policies that can grant cross-account permissions. Services like S3, SNS, SQS, and KMS allow you to attach policies specifying principals from other accounts that can access specific resources.
**AWS Organizations and SCPs**: Service Control Policies provide guardrails across all accounts in an organization, ensuring consistent security boundaries. Organizations also enable features like consolidated billing and account management.
**AWS Resource Access Manager (RAM)**: RAM facilitates sharing of resources like VPC subnets, Transit Gateway attachments, and License Manager configurations across accounts within or outside your organization.
**Best Practices**:
- Apply the principle of least privilege when defining permissions
- Use external IDs to prevent confused deputy problems
- Implement MFA requirements for sensitive cross-account role assumptions
- Leverage AWS Organizations for hierarchical account management
- Monitor cross-account activities using CloudTrail
- Use IAM Access Analyzer to identify resources shared externally
Cross-account access management enables organizations to maintain separate accounts for different environments, business units, or workloads while still allowing necessary collaboration and resource sharing in a controlled, auditable manner.
Cross-Account Access Management
Why Cross-Account Access Management is Important
In enterprise environments, organizations typically operate multiple AWS accounts for various reasons: separating development from production, isolating workloads for security compliance, managing different business units, or implementing billing segregation. Cross-account access management enables secure resource sharing and collaboration across these accounts while maintaining proper security boundaries and governance.
What is Cross-Account Access Management?
Cross-account access management refers to the strategies, mechanisms, and AWS services used to allow principals (users, roles, or services) in one AWS account to access resources in another AWS account. This capability is fundamental to implementing multi-account architectures recommended by AWS for enterprise organizations.
Key Components and How They Work
1. IAM Roles for Cross-Account Access The most common method involves creating an IAM role in the target account (Account B) that trusts the source account (Account A). The trust policy specifies which principals from Account A can assume the role. Users or applications in Account A use sts:AssumeRole to obtain temporary credentials for Account B.
2. Resource-Based Policies Certain AWS services support resource-based policies that can grant cross-account access. Services include S3 buckets, SNS topics, SQS queues, KMS keys, Lambda functions, and Secrets Manager. These policies specify the external account or principal that can access the resource.
3. AWS Organizations and Service Control Policies (SCPs) AWS Organizations provides centralized management across accounts. SCPs act as guardrails, defining maximum permissions boundaries. Organizations enable features like consolidated billing and organizational units (OUs) for hierarchical management.
4. AWS Resource Access Manager (RAM) RAM allows sharing of specific AWS resources across accounts within an organization or with external accounts. Supported resources include VPC subnets, Transit Gateway, Route 53 Resolver rules, License Manager configurations, and AWS App Mesh meshes.
5. AWS Single Sign-On (IAM Identity Center) Provides centralized identity management and access to multiple AWS accounts. Users authenticate once and can access permitted accounts through a portal. Permission sets define what users can do in each account.
Common Cross-Account Patterns
Centralized Logging Account: All accounts send logs to a central account using cross-account roles or resource policies on S3 buckets and CloudWatch Logs destinations.
Shared Services Account: Contains resources used by multiple accounts, such as Transit Gateway, DNS, or container registries accessed via RAM or cross-account roles.
Security Tooling Account: Runs security services like GuardDuty, Security Hub, or Config aggregators with delegated administrator permissions.
Deployment Pipelines: CI/CD tools in a central account deploy to multiple target accounts using cross-account roles.
Security Considerations
Always use the External ID condition when allowing third parties to assume roles to prevent confused deputy attacks. Implement least privilege by granting only necessary permissions. Use condition keys like aws:PrincipalOrgID to restrict access to accounts within your organization. Enable CloudTrail across all accounts to audit cross-account activities.
Exam Tips: Answering Questions on Cross-Account Access Management
Tip 1: When a question mentions sharing resources between accounts in the same organization, consider AWS RAM first for supported resource types like VPC subnets or Transit Gateway.
Tip 2: For scenarios involving third-party vendor access, look for answers that include External ID in the trust policy to prevent confused deputy problems.
Tip 3: If the question involves centralized identity management across many accounts, IAM Identity Center (AWS SSO) is typically the best answer.
Tip 4: When S3, SNS, SQS, or KMS cross-account access is mentioned, resource-based policies are often simpler than role assumption, but remember that KMS requires both key policy AND IAM permissions.
Tip 5: For questions about restricting cross-account access to only organization members, look for aws:PrincipalOrgID condition key in policies.
Tip 6: Understand that SCPs set permission boundaries but do not grant permissions themselves. Cross-account access still requires proper IAM policies or resource policies.
Tip 7: In scenarios requiring centralized governance with multiple accounts, AWS Organizations combined with delegated administrators for services like GuardDuty or Security Hub is the recommended approach.
Tip 8: Remember that cross-account role assumption provides temporary credentials, which is more secure than sharing long-term access keys.