Allowing AWS IAM Users to Read from S3 but Disallowing Writes

Table of Contents#

1. Core Concepts
2. Typical Usage Scenarios
3. Common Practices
4. Best Practices
5. Conclusion
6. FAQ
7. References

Article#

Core Concepts#

AWS IAM#

AWS IAM is a web service that helps you securely control access to AWS resources. You can use IAM to manage users, groups, and permissions. IAM policies are JSON documents that define permissions for accessing AWS resources. These policies can be attached to IAM users, groups, or roles.

Amazon S3#

Amazon S3 is an object storage service that offers industry-leading scalability, data availability, security, and performance. S3 stores data as objects within buckets. Each object consists of a file and optional metadata. To access S3 resources, you need appropriate permissions.

Read and Write Permissions#

Read permissions in S3 allow users to view and download objects from a bucket. Write permissions, on the other hand, enable users to upload, modify, or delete objects in a bucket. By default, no one has access to an S3 bucket except the AWS account owner. You need to explicitly grant permissions using IAM policies.

Typical Usage Scenarios#

Data Consumption#

Suppose you have a data analytics team that needs to access historical data stored in an S3 bucket for analysis. The team only needs to read the data and does not require the ability to modify or delete it. By allowing read-only access, you ensure the integrity of the historical data.

Public Content Distribution#

If you are distributing public content such as images, videos, or documents from an S3 bucket, you might want to allow anonymous read access. However, for internal users who need to manage the content, you can create IAM users with read-only access to review the content before it goes live.

Auditing and Monitoring#

Auditors or monitoring teams may need to access S3 logs or other monitoring data for compliance and troubleshooting purposes. They should only be able to read the data and not make any changes to it.

Common Practices#

Creating an IAM Policy#

To allow an IAM user to read from an S3 bucket but disallow writes, you can create an IAM policy with the following JSON code:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::your-bucket-name",
                "arn:aws:s3:::your-bucket-name/*"
            ]
        },
        {
            "Effect": "Deny",
            "Action": [
                "s3:PutObject",
                "s3:DeleteObject"
            ],
            "Resource": [
                "arn:aws:s3:::your-bucket-name/*"
            ]
        }
    ]
}

In this policy, the first statement allows the user to list the bucket contents (s3:ListBucket) and get objects from the bucket (s3:GetObject). The second statement explicitly denies the user the ability to put objects into the bucket (s3:PutObject) or delete objects from the bucket (s3:DeleteObject).

Attaching the Policy to an IAM User#

After creating the policy, you need to attach it to an IAM user. You can do this through the AWS Management Console, AWS CLI, or AWS SDKs. In the AWS Management Console, go to the IAM service, select the user, and click on "Add permissions". Then, choose "Attach existing policies directly" and select the policy you created.

Best Practices#

Least Privilege Principle#

Follow the principle of least privilege when creating IAM policies. Only grant the minimum permissions necessary for the user to perform their tasks. For example, if the user only needs to access a specific prefix within the bucket, limit the policy to that prefix.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::your-bucket-name",
                "arn:aws:s3:::your-bucket-name/your-prefix/*"
            ]
        },
        {
            "Effect": "Deny",
            "Action": [
                "s3:PutObject",
                "s3:DeleteObject"
            ],
            "Resource": [
                "arn:aws:s3:::your-bucket-name/your-prefix/*"
            ]
        }
    ]
}

Regular Policy Reviews#

Periodically review your IAM policies to ensure they still meet the security requirements. As the business needs change, you may need to adjust the permissions accordingly.

Use of Tags#

Use tags to manage and organize your S3 resources. You can also use tags in IAM policies to control access based on resource tags. For example, you can create a policy that allows read access to all objects with a specific tag.

Conclusion#

Allowing AWS IAM users to read from S3 while disallowing writes is a common security requirement in many AWS environments. By understanding the core concepts of IAM and S3, identifying the typical usage scenarios, following common practices, and implementing best practices, you can effectively manage access to your S3 resources and ensure data integrity and security.

FAQ#

Q: Can I use the same policy for multiple IAM users?#

A: Yes, you can attach the same IAM policy to multiple IAM users or groups. This helps in managing permissions more efficiently.

Q: What if an IAM user has other policies that conflict with the read-only policy?#

A: In case of conflicting policies, the most restrictive policy takes precedence. If a user has a policy that allows write access and a read-only policy, the read-only policy will be enforced.

Q: Can I use this approach for cross-account access?#

A: Yes, you can use similar IAM policies for cross - account access. You need to set up cross - account roles and attach the appropriate policies to those roles.

References#

• AWS Identity and Access Management Documentation
• Amazon S3 Documentation
• AWS IAM Policy Elements Reference