S3 bucket policies

What is an S3 bucket policy and how does it differ from IAM policies?

An S3 bucket policy is a set of permissions attached directly to an Amazon S3 bucket that governs access to that bucket and the objects within it. Unlike IAM policies, which are attached to AWS user accounts or groups to manage permissions across AWS services, S3 bucket policies are attached only to S3 buckets. Bucket policies are resource-based policies that allow you to grant access permissions to resources that the bucket owner might not own, such as allowing cross-account access to the S3 bucket.

Can you describe the structure of an S3 bucket policy?

An S3 bucket policy is a JSON document that defines who can access the bucket (Principal), what actions they can perform (Action), on which resources (Resource), and under what conditions (Condition). A bucket policy includes a series of statements, each of which includes those elements to specify the exact permissions.

How do S3 bucket policies interact with IAM policies when both are in place?

When S3 bucket policies and IAM policies are both in place, AWS evaluates both to determine whether a request should be allowed or denied. If either the IAM policy or the bucket policy explicitly denies access, the request will be denied. If the request is allowed by one policy but not explicitly denied by the other, then the request will be allowed. AWS uses the principle of least privilege, so if permissions are not explicitly granted, they are by default denied.

What are policy conditions and how can they be used in an S3 bucket policy?

Policy conditions in an S3 bucket policy allow the bucket owner to specify constraints or conditions under which the policy statement applies. Conditions can be based on various factors, such as IP address, date/time, whether MFA (Multi-Factor Authentication) is used, SSL/TLS usage, etc. By using conditions, you can enforce fine-grained access control to your S3 resources, for example, allowing access to an S3 bucket only from a certain IP range during specific times.

When would you use cross-account access in S3 bucket policies, and what’s the benefit?

Cross-account access in S3 bucket policies is used when you want to grant access to your S3 buckets to users from another AWS account. This can be beneficial when working with partners, subcontractors, or separate departments within an organization that use different AWS accounts. It allows for secure sharing of resources without the need to create IAM users in your own account for external entities.

How can you ensure that objects uploaded to an S3 bucket are encrypted by default?

To ensure that objects uploaded to an S3 bucket are encrypted by default, you can use a bucket policy to deny PUT requests that don’t include the x-amz-server-side-encryption header. Alternatively, you can also enable default encryption in the bucket settings, which will apply server-side encryption to all new objects without the need to specify it during upload.

What are some best practices for securing sensitive data in S3 buckets?

Best practices for securing sensitive data in S3 buckets include enabling bucket versioning to protect against accidental deletions, using S3’s default encryption feature to encrypt data at rest, implementing least privilege access by crafting precise bucket policies and IAM policies, enabling logging and monitoring with AWS CloudTrail and Amazon S3 access logs, employing MFA (Multi-Factor Authentication) deletion protection, and following the AWS shared responsibility model by keeping the data encrypted in transit (using SSL/TLS).

How do bucket policies help with regulatory compliance when using S3 for data storage?

Bucket policies help with regulatory compliance by allowing you to enforce specific access controls and conditions to meet various compliance requirements. For example, you can restrict access to data in S3 buckets to specific IP addresses or insist on secure data transfer protocols, thus addressing compliance concerns for data residency and data protection.

Can you explain the difference between ‘s3:*’ and specific actions like ‘s3:GetObject’ in a bucket policy?

The ‘s3:*’ is a wildcard permission in a bucket policy that represents all possible actions within the S3 namespace. When used, it grants the specified principal permission to perform any action on the specified resources. In contrast, specifying an action like ‘s3:GetObject’ grants permission for that specific action only. This is far more restrictive and aligned with the principle of least privilege, allowing for finer control over access permissions.

How would you prevent accidental deletion of objects in an S3 bucket through a policy?

To prevent accidental deletion of objects in an S3 bucket through a policy, you can use a bucket policy to deny the s3:DeleteObject permission. Additionally, you can enable MFA (Multi-Factor Authentication) delete on the bucket, which requires additional authentication to delete versioned objects, adding another layer of protection against accidental deletions.

What is the effect of making an S3 bucket public, and how would you control this using a bucket policy?

Making an S3 bucket public gives anyone on the internet permission to view and possibly manipulate the data in the bucket, which might lead to data breaches. To control access via a bucket policy, you can enforce specific rules to block public access altogether or conditionally allow it. AWS also provides the option to block all public access at the bucket or account level as a safeguard against public exposure.

Is it possible to restrict access to an S3 bucket based on the HTTP referer header? If so, how would you do that in a bucket policy?

Yes, it’s possible to restrict access based on the HTTP referer header by using a condition in the bucket policy to allow or deny access based on the value of the “aws:Referer” key. This can be an additional safeguard to ensure that requests are coming from trusted domains. However, this isn’t foolproof since the referer header can be spoofed, so it should not be the sole method of securing your S3 buckets.