In AWS Identity and Access Management (IAM), Resource Policies are used to control access to specific AWS resources, such as S3 buckets, SNS topics, or Lambda functions. Unlike identity policies that grant permissions to IAM users or roles, resource policies are directly attached to the resource, defining who can access the resource and under what conditions. This approach is essential for managing access to resources across AWS accounts or external entities.
This comprehensive guide will walk you through implementing resource policies in IAM.
1. Prerequisites
1. IAM Permissions: Ensure that you have the necessary permissions to create and modify IAM policies and AWS resources.
2. AWS CLI or Console: Either the AWS Management Console or AWS CLI is required to interact with IAM.
2. Understanding Resource Policies
A resource policy is a JSON document that defines the permissions on a specific resource. It controls which entities (users, roles, accounts, etc.) can access the resource and what actions they are allowed to perform. For example, a resource policy can grant public access to an S3 bucket or allow only specific IAM users from another AWS account to access an SNS topic.
3. Creating a Resource Policy for S3 Bucket
Let’s walk through the process of creating a resource policy for an S3 bucket that allows cross-account access.
1. Access S3 Bucket in the Console:
Navigate to the S3 service in the AWS Management Console.
Select the bucket for which you want to create the resource policy.
2. Edit Bucket Policy:
Under the Permissions tab, select Bucket Policy.
Insert the following policy to allow a specific IAM user from a different AWS account to access the bucket. The policy grants the user permissions to read objects from the bucket.
Example policy:
{
“Version”: “2012-10-17”,
“Statement”: [
{
“Effect”: “Allow”,
“Principal”: {
“AWS”: “arn:aws:iam::123456789012:user/Alice”
},
“Action”: “s3:GetObject”,
“Resource”: “arn:aws:s3:::my-bucket/*”
}
]
}
This policy allows the IAM user Alice in account 123456789012 to perform s3:GetObject action on objects within the my-bucket S3 bucket.
3. Save the Policy:
Click Save changes to apply the resource policy to the S3 bucket.
4. Create a Resource Policy for SNS Topic
Now, let’s create a resource policy for an SNS topic to allow an external AWS account to publish messages.
1. Access SNS Topic:
Navigate to the SNS service in the AWS Management Console.
Select the SNS topic to which you want to apply the policy.
2. Add Policy to Topic:
In the Access Control Policy section, add a policy like the following:
{
“Version”: “2012-10-17”,
“Statement”: [
{
“Effect”: “Allow”,
“Principal”: “*”,
“Action”: “SNS:Publish”,
“Resource”: “arn:aws:sns:us-east-1:123456789012:MyTopic”,
“Condition”: {
“StringEquals”: {
“AWS:SourceAccount”: “987654321098”
},
“StringLike”: {
“AWS:SourceArn”: “arn:aws:iam::987654321098:role/*”
}
}
}
]
}
3. Save the Policy:
Click Save changes to apply the policy.
5. Testing Resource Policies
Once the resource policy is applied, it’s essential to test whether the access is working as expected.
1. For S3 Bucket:
From the external account, try accessing the S3 bucket using the AWS CLI:
aws s3 cp s3://my-bucket/my-file.txt .
Ensure that the IAM user has the correct permissions to access the resource.
2. For SNS Topic:
From the specified external AWS account, try to publish a message to the SNS topic:
aws sns publish –topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic –message “Test Message”
6. Best Practices for Resource Policies
Least Privilege: Always follow the principle of least privilege when defining permissions.
Cross-Account Access: Use resource policies to allow specific actions from other AWS accounts while minimizing exposure.
Granular Conditions: Leverage conditions such as aws:SourceIp or aws:Referer to restrict access based on additional criteria.
Conclusion
Resource policies in IAM provide a granular, flexible way to control access to AWS resources. By using policies like the ones demonstrated in S3 and SNS, you ensure secure and controlled access while maintaining the principle of least privilege. Whether for cross-account access or fine-tuned access controls, mastering resource policies is fundamental to securing your AWS environment.
The article above is rendered by integrating outputs of 1 HUMAN AGENT & 3 AI AGENTS, an amalgamation of HGI and AI to serve technology education globally.