With Amazon S3 bucket policies, you can secure access to objects in your buckets, so that only users with the appropriate permissions can access them. You can even prevent authenticated users without the appropriate permissions from accessing your Amazon S3 resources.
This section presents examples of typical use cases for bucket policies. These sample policies use DOC-EXAMPLE-BUCKET as the resource value. To test these policies, replace the user input placeholders with your own information (such as your bucket name).
To grant or deny permissions to a set of objects, you can use wildcard characters (*) in Amazon Resource Names (ARNs) and other values. For example, you can control access to groups of objects that begin with a common prefix or end with a given extension, such as .html.
For more information about AWS Identity and Access Management (IAM) policy language, see Policies and Permissions in Amazon S3.
Note
When testing permissions by using the Amazon S3 console, you must grant additional permissions that the console requires—s3:ListAllMyBuckets, s3:GetBucketLocation, and s3:ListBucket. For an example walkthrough that grants permissions to users and tests those permissions by using the console, see Controlling access to a bucket with user policies.
Here are some key components and concepts related to bucket policies in AWS S3:
JSON Format: Bucket policies are written in JSON (JavaScript Object Notation) format. This allows for a flexible and expressive way to define permissions.
Resources: The primary resource in a bucket policy is the S3 bucket itself, identified by its Amazon Resource Name (ARN), which looks like arn:aws:s3:::your-bucket-name.
Actions: Actions define the permitted operations on the specified resources. Examples of S3 actions include s3:GetObject, s3:PutObject, s3:ListBucket, etc.
Effect: The Effect field in a policy statement can have two values: "Allow" or "Deny." If set to "Allow," the specified actions are permitted. If set to "Deny," the actions are explicitly prohibited.
Principal: The Principal field specifies the AWS identity or account that the policy applies to. It can be an AWS account ID, * (indicating all identities), or an AWS service principal.
Condition: The Condition field allows you to specify additional criteria that must be met for the policy to take effect. For example, you might restrict access based on the requester's IP address or require the use of a secure connection (HTTPS).
Bucket Policy Examples:
Allow Public Read Access to All Objects in a Bucket:
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/*"
}
]
}
Allow Access to Specific IP Range:
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/*",
"Condition": {
"IpAddress": {"aws:SourceIp": "192.168.1.0/24"}
}
}
]
}
Deny Access to a Specific User:
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Deny",
"Principal": {"AWS": "arn:aws:iam::123456789012:user/username"},
"Action": "s3:*",
"Resource": ["arn:aws:s3:::your-bucket-name/*"]
}
]
}
Remember to carefully craft bucket policies, as misconfigurations can lead to unintended exposure of data or restricted access for legitimate users. Always test and review policies thoroughly before applying them to production environments.
Here's a checklist along with examples and explanations:
1. Use the Least Privilege Principle:
Explanation: Assign only the permissions necessary for the intended use. Avoid granting overly broad permissions.
2. Explicitly Allow Actions:
Explanation: Explicitly specify allowed actions rather than relying on implicit permissions.
{
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/*"
}
3. Restrict Access to Specific Resources:
Explanation: Be specific about the resources to which the policy applies.
{
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/private/*"
}
4. Use Conditions for Additional Control:
Explanation: Apply conditions to control access based on various factors like IP address or request type.
{
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/*",
"Condition": {
"IpAddress": {"aws:SourceIp": "192.168.1.0/24"}
}
}
5. Avoid Using "*" as the Principal:
Explanation: Limit the use of wildcard principals to specific use cases and avoid using * for all identities.
{
"Effect": "Allow",
"Principal": {"AWS": "arn:aws:iam::123456789012:root"},
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::your-bucket-name/*"
}
6. Regularly Review and Audit Policies:
Explanation: Regularly review and audit bucket policies to ensure they align with the current requirements.
7. Enable Logging for S3 Bucket:
Explanation: Enable S3 bucket logging to capture detailed information about access requests.
8. Use IAM Roles Instead of IAM Users in Policies:
Explanation: Prefer using IAM roles and assume them in your applications rather than embedding IAM user credentials in policies.
9. Deny by Default:
Explanation: If possible, use "Deny" policies to explicitly deny actions and follow a principle of least privilege.
{
"Effect": "Deny",
"Principal": "*",
"Action": "s3:PutObject",
"Resource": "arn:aws:s3:::your-bucket-name/*"
}
10. Regularly Rotate Access Keys and Credentials:
Explanation: Rotate access keys and credentials regularly to enhance security.
11. Test Policies in a Staging Environment:
Explanation: Before applying policies to production, thoroughly test them in a staging environment to identify any unintended consequences.
12. Document Policies and Permissions:
Explanation: Document the purpose of each policy, its associated permissions, and any conditions applied for clarity and future reference.
Adhering to these best practices will help you create secure and effective S3 bucket policies in AWS. Always consider the specific needs and security requirements of your applications and users.
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