Resource-based policies are a fundamental security mechanism in AWS that allow you to control access to specific AWS resources by attaching policies to the resources themselves rather than to IAM users, groups, or roles. These policies define who can access the resource and what actions they can pe…Resource-based policies are a fundamental security mechanism in AWS that allow you to control access to specific AWS resources by attaching policies to the resources themselves rather than to IAM users, groups, or roles. These policies define who can access the resource and what actions they can perform.
Unlike identity-based policies that are attached to IAM principals, resource-based policies are embedded within the resource configuration. Common AWS services supporting resource-based policies include S3 buckets, SNS topics, SQS queues, Lambda functions, KMS keys, and ECR repositories.
The policy structure follows standard JSON format with key elements including Version, Statement, Effect (Allow or Deny), Principal (who can access), Action (what operations are permitted), Resource (the ARN of the resource), and optional Conditions for fine-grained control.
A critical feature of resource-based policies is the Principal element, which specifies the AWS accounts, IAM users, roles, federated users, or AWS services that are granted or denied access. This enables cross-account access scenarios where you can grant permissions to principals in other AWS accounts.
For SysOps Administrators, understanding resource-based policies is essential for implementing least-privilege access, enabling secure cross-account resource sharing, and meeting compliance requirements. When both identity-based and resource-based policies apply, AWS evaluates them together - an explicit deny in either policy results in access denial.
Best practices include regularly reviewing and auditing resource-based policies using AWS Config and IAM Access Analyzer, implementing specific conditions to restrict access by IP address, VPC, or time, and avoiding overly permissive wildcards in Principal statements.
Resource-based policies also support Service Control Policies (SCPs) in AWS Organizations, providing an additional layer of access control at the organizational level. Proper configuration of these policies is crucial for maintaining a strong security posture and ensuring compliance with organizational and regulatory requirements.
Resource-based Policies in AWS
What are Resource-based Policies?
Resource-based policies are JSON policy documents that you attach to a resource such as an S3 bucket, SQS queue, SNS topic, or Lambda function. Unlike identity-based policies that are attached to IAM users, groups, or roles, resource-based policies are attached to the resource itself and specify who has access to that resource and what actions they can perform.
Why are Resource-based Policies Important?
Resource-based policies are crucial for several reasons:
• Cross-account access: They enable you to grant access to principals from other AWS accounts, which is essential for multi-account architectures. • Fine-grained control: They allow resource owners to control exactly who can access their resources. • No role assumption required: Users from other accounts can access resources through resource-based policies using their existing permissions. • Defense in depth: They provide an additional layer of security alongside identity-based policies.
How Resource-based Policies Work
Resource-based policies contain a Principal element that specifies who is allowed or denied access. The policy evaluation follows these rules:
1. When a principal makes a request, AWS evaluates all applicable policies. 2. For same-account access, if either the identity-based policy OR the resource-based policy allows the action, access is granted. 3. For cross-account access, BOTH the identity-based policy (in the requester's account) AND the resource-based policy (on the resource) must allow the action.
• Principal: The AWS account, IAM user, role, or service that is allowed or denied access • Action: The specific API actions that are allowed or denied • Resource: The specific resource ARN the policy applies to • Condition: Optional conditions under which the policy is in effect
Exam Tips: Answering Questions on Resource-based Policies
Tip 1: Remember that cross-account access requires permissions on both sides - the identity policy in the source account AND the resource-based policy on the target resource must both allow the action.
Tip 2: Know which services support resource-based policies. S3 bucket policies, Lambda function policies, and SQS queue policies are frequently tested.
Tip 3: Understand that resource-based policies are the ONLY way to grant cross-account access to some resources like S3 objects when you want the original account to retain ownership.
Tip 4: Be aware that KMS key policies are unique - the key policy must explicitly grant permissions even for the root account, unlike other AWS resources.
Tip 5: When troubleshooting access issues, check both identity-based and resource-based policies. An explicit deny in either policy will result in access being denied.
Tip 6: For Lambda functions invoked by other AWS services (like S3 or API Gateway), resource-based policies are used to grant those services permission to invoke the function.
Tip 7: The Principal element is mandatory in resource-based policies but not used in identity-based policies - this distinction often appears in exam questions.
Tip 8: Remember that resource-based policies support conditions like aws:SourceArn and aws:SourceAccount to prevent confused deputy attacks when granting access to AWS services.