Policy examples for ACLs - Amazon Simple Storage Service
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Policy examples for ACLs

You can use condition keys in bucket policies to control access to Amazon S3.

Granting s3:PutObject permission with a condition requiring the bucket owner to get full control

The PUT Object operation allows access control list (ACL)–specific headers that you can use to grant ACL-based permissions. Using these keys, the bucket owner can set a condition to require specific access permissions when the user uploads an object.

Suppose that Account A owns a bucket, and the account administrator wants to grant Dave, a user in Account B, permissions to upload objects. By default, objects that Dave uploads are owned by Account B, and Account A has no permissions on these objects. Because the bucket owner is paying the bills, it wants full permissions on the objects that Dave uploads. The Account A administrator can do this by granting the s3:PutObject permission to Dave, with a condition that the request include ACL-specific headers that either grant full permission explicitly or use a canned ACL. For more information, see PUT Object.

Require the x-amz-full-control header

You can require the x-amz-full-control header in the request with full control permission to the bucket owner. The following bucket policy grants the s3:PutObject permission to user Dave with a condition using the s3:x-amz-grant-full-control condition key, which requires the request to include the x-amz-full-control header.

{ "Version": "2012-10-17", "Statement": [ { "Sid": "statement1", "Effect": "Allow", "Principal": { "AWS": "arn:aws-cn:iam::AccountB-ID:user/Dave" }, "Action": "s3:PutObject", "Resource": "arn:aws-cn:s3:::awsexamplebucket1/*", "Condition": { "StringEquals": { "s3:x-amz-grant-full-control": "id=AccountA-CanonicalUserID" } } } ] }

This example is about cross-account permission. However, if Dave (who is getting the permission) belongs to the Amazon Web Services account that owns the bucket, this conditional permission is not necessary. This is because the parent account to which Dave belongs owns objects that the user uploads.

Add explicit deny

The preceding bucket policy grants conditional permission to user Dave in Account B. While this policy is in effect, it is possible for Dave to get the same permission without any condition via some other policy. For example, Dave can belong to a group, and you grant the group s3:PutObject permission without any condition. To avoid such permission loopholes, you can write a stricter access policy by adding explicit deny. In this example, you explicitly deny the user Dave upload permission if he does not include the necessary headers in the request granting full permissions to the bucket owner. Explicit deny always supersedes any other permission granted. The following is the revised access policy example with explicit deny added.

{ "Version": "2012-10-17", "Statement": [ { "Sid": "statement1", "Effect": "Allow", "Principal": { "AWS": "arn:aws-cn:iam::AccountB-ID:user/AccountBadmin" }, "Action": "s3:PutObject", "Resource": "arn:aws-cn:s3:::awsexamplebucket1/*", "Condition": { "StringEquals": { "s3:x-amz-grant-full-control": "id=AccountA-CanonicalUserID" } } }, { "Sid": "statement2", "Effect": "Deny", "Principal": { "AWS": "arn:aws-cn:iam::AccountB-ID:user/AccountBadmin" }, "Action": "s3:PutObject", "Resource": "arn:aws-cn:s3:::awsexamplebucket1/*", "Condition": { "StringNotEquals": { "s3:x-amz-grant-full-control": "id=AccountA-CanonicalUserID" } } } ] }
Test the policy with the Amazon CLI

If you have two Amazon Web Services accounts, you can test the policy using the Amazon Command Line Interface (Amazon CLI). You attach the policy and use Dave's credentials to test the permission using the following Amazon CLI put-object command. You provide Dave's credentials by adding the --profile parameter. You grant full control permission to the bucket owner by adding the --grant-full-control parameter. For more information about setting up and using the Amazon CLI, see Developing with Amazon S3 using the Amazon CLI.

aws s3api put-object --bucket examplebucket --key HappyFace.jpg --body c:\HappyFace.jpg --grant-full-control id="AccountA-CanonicalUserID" --profile AccountBUserProfile

Require the x-amz-acl header

You can require the x-amz-acl header with a canned ACL granting full control permission to the bucket owner. To require the x-amz-acl header in the request, you can replace the key-value pair in the Condition block and specify the s3:x-amz-acl condition key, as shown in the following example.

"Condition": { "StringEquals": { "s3:x-amz-acl": "bucket-owner-full-control" } }

To test the permission using the Amazon CLI, you specify the --acl parameter. The Amazon CLI then adds the x-amz-acl header when it sends the request.

aws s3api put-object --bucket examplebucket --key HappyFace.jpg --body c:\HappyFace.jpg --acl "bucket-owner-full-control" --profile AccountBadmin

Granting s3:PutObject permission with a condition on the x-amz-acl header

The following bucket policy grants the s3:PutObject permission for two Amazon Web Services accounts if the request includes the x-amz-acl header making the object publicly readable. The Condition block uses the StringEquals condition, and it is provided a key-value pair, "s3:x-amz-acl":["public-read"], for evaluation. In the key-value pair, the s3:x-amz-acl is an Amazon S3–specific key, as indicated by the prefix s3:.

{ "Version":"2012-10-17", "Statement": [ { "Sid":"AddCannedAcl", "Effect":"Allow", "Principal": { "AWS": [ "arn:aws-cn:iam::Account1-ID:root", "arn:aws-cn:iam::Account2-ID:root" ] }, "Action":"s3:PutObject", "Resource": ["arn:aws-cn:s3:::awsexamplebucket1/*"], "Condition": { "StringEquals": { "s3:x-amz-acl":["public-read"] } } } ] }

Not all conditions make sense for all actions. For example, it makes sense to include an s3:LocationConstraint condition on a policy that grants the s3:CreateBucket Amazon S3 permission. However, it does not make sense to include this condition on a policy that grants the s3:GetObject permission. Amazon S3 can test for semantic errors of this type that involve Amazon S3–specific conditions. However, if you are creating a policy for an IAM user or role and you include a semantically invalid Amazon S3 condition, no error is reported because IAM cannot validate Amazon S3 conditions.