Managing multiple AWS accounts while maintaining secure access to shared resources can feel like navigating a complex maze. AWS cross-account access allows you to safely share resources between different AWS accounts without compromising security or creating unnecessary duplicates.
This guide is designed for AWS administrators, DevOps engineers, and security professionals who need to implement cross-account IAM roles and establish secure resource sharing across their organization’s AWS infrastructure.
We’ll walk through the core concepts behind AWS cross-account access and show you exactly how cross-account policies work in practice. You’ll learn how to set up cross-account IAM roles that follow AWS security best practices, and we’ll cover implementing resource-based policies for direct access control. We’ll also explore cross-account VPC peering options and network security considerations, plus essential monitoring techniques to track cross-account access control activities across your AWS environment.
Understanding AWS Cross-Account Access Fundamentals
A. Define cross-account resource sharing and its business value
AWS cross-account access lets organizations share resources between separate AWS accounts while keeping strong security boundaries. This approach helps companies manage different departments, subsidiaries, or environments without duplicating resources or creating complex permission structures within single accounts. The business value comes from reduced costs through shared resources, improved security through account isolation, better compliance management, and streamlined operations across organizational units.
B. Identify key AWS services that enable secure cross-account access
Several AWS services power secure cross-account resource sharing. IAM roles serve as the foundation, allowing trusted accounts to assume permissions temporarily. S3 bucket policies and resource-based policies grant direct access to specific resources without role switching. AWS Organizations provides centralized account management and policy enforcement across multiple accounts. VPC peering and Transit Gateway enable secure network connectivity between account VPCs. AWS Resource Access Manager (RAM) simplifies sharing of resources like subnets, Route 53 resolver rules, and License Manager configurations.
C. Compare cross-account access methods and their use cases
| Method | Best For | Security Level | Setup Complexity |
|---|---|---|---|
| Cross-account IAM roles | Programmatic access, temporary permissions | High | Medium |
| Resource-based policies | Direct resource access, S3 buckets, SNS topics | High | Low |
| AWS Organizations | Large-scale account management, consolidated billing | Very High | High |
| VPC Peering | Network-level resource sharing, private connectivity | High | Medium |
| AWS RAM | Infrastructure sharing, centralized resource management | High | Low |
Cross-account IAM roles work best when you need temporary, auditable access with fine-grained permissions. Resource-based policies shine for direct access to specific resources like S3 buckets or DynamoDB tables. Organizations excels at enterprise-level governance across many accounts, while VPC peering handles network connectivity needs. AWS RAM simplifies sharing infrastructure components like subnets and DNS resolvers across organizational units.
Setting Up Cross-Account IAM Roles for Secure Access
Create assumable IAM roles with proper trust policies
Building cross-account IAM roles starts with crafting precise trust policies that define which AWS accounts can assume your roles. The trust policy acts as the gatekeeper, specifying the principal accounts and conditions required for role assumption. Create roles with trust relationships that explicitly list trusted account IDs and include condition blocks for additional security layers like MFA requirements or IP restrictions. Your trust policy should use the “sts:AssumeRole” action and specify the trusted account’s root ARN or specific IAM users. Always include “sts:ExternalId” conditions when working with third-party services to prevent confused deputy attacks.
Configure least-privilege permissions for cross-account scenarios
AWS cross-account access demands strict adherence to least-privilege principles to maintain security boundaries. Design permission policies that grant only the minimum access required for specific cross-account operations. Start by identifying the exact AWS services and resources your cross-account users need to access, then create targeted policies using resource ARNs and condition statements. Avoid wildcard permissions and instead use specific resource patterns. Implement time-based conditions, IP restrictions, and request context conditions to further limit access scope. Regular policy reviews ensure permissions remain aligned with actual business requirements while preventing privilege creep.
Implement role switching mechanisms for users and applications
Role switching provides seamless access management for both human users and automated applications across AWS accounts. For users, configure AWS CLI profiles with role assumption capabilities using the “role_arn” and “source_profile” parameters. Applications should use AWS SDKs with STS AssumeRole API calls to programmatically switch between roles. Implement session management with appropriate session duration limits and rotation policies. Create standardized switching procedures that include proper error handling and fallback mechanisms. Document the switching process clearly and provide code examples for common programming languages to ensure consistent implementation across your organization.
Establish secure role naming conventions and documentation
Consistent role naming conventions streamline AWS cross-account access management and improve security visibility. Develop a naming structure that includes account purpose, environment, and functional identifiers like “CrossAccount-Production-S3ReadOnly-Role”. Create comprehensive documentation that maps each cross-account role to its intended use case, trusted accounts, and permission boundaries. Maintain an inventory of all cross-account roles with their creation dates, last usage timestamps, and review schedules. Implement tags that categorize roles by department, project, and compliance requirements. Regular audits of role names and documentation help identify unused roles and ensure naming standards remain current with organizational changes.
Implementing Resource-Based Policies for Direct Access Control
Configure S3 bucket policies for cross-account file sharing
Cross-account S3 access requires carefully crafted bucket policies that grant specific permissions to external AWS accounts while maintaining security boundaries. The bucket policy acts as a gatekeeper, allowing you to define which accounts can access your S3 resources and what actions they can perform. When configuring these policies, you’ll specify the principal using the target account ID and define permissions like s3:GetObject or s3:PutObject. A typical policy includes conditions that restrict access based on IP ranges, request time, or SSL requirements. For example, you can allow Account B to read objects from your bucket while requiring all requests to use HTTPS encryption. The beauty of resource-based policies lies in their simplicity – external accounts can access your resources directly without assuming roles, streamlining the authentication process. Remember to test policies in a development environment first, as overly restrictive policies can break legitimate access patterns while overly permissive ones create security vulnerabilities.
Set up Lambda function resource policies for cross-account invocation
Lambda resource policies enable external AWS accounts to invoke your functions directly, creating powerful cross-account automation workflows. Unlike IAM roles, these policies attach directly to the Lambda function, specifying which accounts or services can trigger execution. The policy structure includes the principal (source account or service), the action (typically lambda:InvokeFunction), and optional conditions for enhanced security. AWS provides built-in support for common triggers like S3 events, SNS notifications, and API Gateway requests from external accounts. When setting up cross-account Lambda invocation, you can use the AWS CLI command add-permission or define policies through the console. The source account needs corresponding IAM permissions to invoke the function, creating a two-way security check. For event-driven architectures, you might configure an S3 bucket in Account A to trigger a Lambda function in Account B when files are uploaded. Consider implementing least privilege access by restricting invocation to specific source accounts and adding conditions based on request attributes like source IP or time windows.
Apply KMS key policies for encrypted cross-account data access
AWS KMS key policies control cross-account access to encrypted data, ensuring that only authorized accounts can decrypt sensitive information stored in S3, RDS, or other encrypted services. The key policy document defines which accounts can use the key for encryption and decryption operations, with granular permissions for different cryptographic operations. Cross-account KMS access typically involves granting kms:Decrypt and kms:DescribeKey permissions to external accounts while maintaining strict control over key management operations. When sharing encrypted S3 objects, both the S3 bucket policy and KMS key policy must align to allow seamless access. The external account also needs corresponding IAM permissions to use the KMS key, creating a multi-layered security model. Best practices include using separate keys for different data classifications and implementing key rotation policies. For automated cross-account workflows, grant permissions to specific IAM roles rather than entire accounts to maintain principle of least privilege. KMS key policies support conditions based on encryption context, allowing you to enforce additional security requirements like specific application tags or data classifications during decryption operations.
Managing Cross-Account VPC Connectivity and Network Security
Establish VPC peering connections for secure network access
VPC peering creates a direct network connection between two Virtual Private Clouds, allowing resources in different AWS accounts to communicate privately without traversing the public internet. When setting up cross-account VPC peering, the peer VPC owner must accept the peering request and update route tables to direct traffic through the peering connection. Security groups and network ACLs can filter traffic between peered VPCs, ensuring only authorized communication occurs. This approach works well for simple, one-to-one connections but becomes complex with multiple accounts requiring full mesh connectivity.
Configure AWS Transit Gateway for multi-account network architecture
AWS Transit Gateway acts as a central hub for connecting multiple VPCs across different accounts, eliminating the need for complex peering relationships. Through resource sharing using AWS Resource Access Manager (RAM), you can share Transit Gateway attachments with other accounts in your organization. Each account can attach their VPCs to the shared gateway and control routing through Transit Gateway route tables. This architecture scales efficiently as new accounts join, requiring only a single attachment rather than multiple peering connections. Network segmentation becomes easier through separate route tables for different business units or environments.
Implement security groups and NACLs for cross-account traffic control
Cross-account traffic control requires careful configuration of both security groups and Network Access Control Lists (NACLs) to maintain security boundaries. Security groups can reference other security groups across accounts using their group IDs, enabling precise application-level access control. For example, a web server security group in Account A can allow traffic from a load balancer security group in Account B. NACLs provide subnet-level filtering and work in conjunction with security groups for defense in depth. Best practice involves creating separate security groups for cross-account access patterns and documenting all cross-account references to maintain visibility.
Set up VPC endpoints for private service connectivity
VPC endpoints enable private connectivity to AWS services and third-party applications without internet gateways or NAT devices. Interface endpoints create private network interfaces within your VPC, while gateway endpoints provide direct routes to S3 and DynamoDB. Cross-account scenarios benefit from interface endpoints when accessing services like EC2, Lambda, or custom applications running in another account’s VPC. Resource-based policies on the endpoint can restrict which accounts and principals can access the service through the endpoint. This approach keeps traffic within the AWS backbone network, improving security and reducing data transfer costs compared to internet-based access.
Monitoring and Auditing Cross-Account Access Activities
Enable CloudTrail logging for cross-account API calls
Setting up CloudTrail across all AWS accounts creates a comprehensive audit trail for cross-account activities. Configure CloudTrail to capture AssumeRole events, resource access attempts, and API calls between accounts. Enable data events for S3 buckets and Lambda functions to track cross-account resource usage. Store logs in a centralized S3 bucket with proper encryption and access controls to maintain security while ensuring all accounts can contribute logs.
Set up CloudWatch alarms for suspicious cross-account activities
CloudWatch alarms help detect unusual cross-account access patterns before they become security incidents. Create alarms for failed AssumeRole attempts, unexpected source IP addresses, and access during off-hours. Monitor metrics like the number of cross-account API calls per hour and failed authentication attempts. Set thresholds based on your organization’s normal usage patterns and configure SNS notifications to alert security teams immediately when anomalies occur.
Use AWS Config for compliance monitoring across accounts
AWS Config provides continuous compliance monitoring across your multi-account environment. Deploy Config rules that check for proper cross-account IAM role configurations, resource-based policy compliance, and security group settings. Use the multi-account multi-region data aggregation feature to centralize compliance data. Create custom rules that validate cross-account access control policies align with your security standards and automatically flag non-compliant resources for remediation.
Implement automated alerts for unauthorized access attempts
Automated alerting systems catch unauthorized cross-account access attempts in real-time. Use Amazon GuardDuty to detect suspicious cross-account activities and unusual API patterns. Set up EventBridge rules to trigger alerts when specific cross-account events occur, such as role creation or policy modifications. Configure automated responses like temporarily disabling compromised roles or blocking suspicious IP addresses. Integrate alerts with your incident response platform for streamlined security workflows.
Create audit reports for cross-account resource usage
Regular audit reports provide visibility into cross-account resource sharing patterns and help identify optimization opportunities. Use AWS Cost and Usage Reports to track cross-account resource consumption and associated costs. Generate monthly reports showing which accounts access resources, frequency of access, and resource utilization patterns. Create dashboards using QuickSight or similar tools to visualize cross-account access trends and present findings to stakeholders for better governance decisions.
Cross-account access in AWS opens up powerful possibilities for organizations managing multiple accounts while maintaining strong security boundaries. The combination of IAM roles, resource-based policies, and proper VPC connectivity creates a robust framework that lets you share resources safely without compromising your security posture. Getting the setup right from the start saves countless headaches down the road and builds the foundation for scalable, secure cloud operations.
The real magic happens when you pair these technical implementations with solid monitoring and auditing practices. CloudTrail logs and AWS Config give you the visibility you need to track who’s accessing what and when. Start with the principle of least privilege, test your configurations thoroughly, and regularly review your cross-account relationships. Your future self will thank you for taking the time to build these secure bridges between your AWS accounts properly.