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IAM JSON Policy Elements: NotPrincipal

Use the NotPrincipal element to specify an exception to a list of principals. For example, you can deny access to all principals except the one named in the NotPrincipal element. The syntax for specifying NotPrincipal is the same as for specifying IAM JSON Policy Elements: Principal.

Important

Very few scenarios require the use of NotPrincipal, and we recommend that you explore other authorization options before you decide to use NotPrincipal.

NotPrincipal with Allow

We strongly recommend that you do not use NotPrincipal in the same policy statement as "Effect": "Allow". Doing so allows all principals except the one named in the NotPrincipal element.We do not recommend this because the permissions specified in the policy statement will be granted to all principals except for the ones specified. By doing this, you might grant access to anonymous (unauthenticated) users.

NotPrincipal with Deny

When you use NotPrincipal in the same policy statement as "Effect": "Deny", the actions specified in the policy statement are explicitly denied to all principals except for the ones specified. You can use this method to implement a form of whitelisting. When you use NotPrincipal with Deny, you must also specify the account ARN of the not-denied principal. Otherwise, the policy might deny access to the entire account containing the principal. Depending on the service that you include in your policy, AWS might validate the account first and then the user. If an assumed-role user (someone who is using a role) is being evaluated, AWS might validate the account first, then the role, and then the assumed-role user. The assumed-role user is identified by the role session name that is specified when they assumes the role. Therefore, we strongly recommend that you explicitly include the ARN for a user's account, or include both the ARN for a role and the ARN for the account containing that role.

Note

As a best practice, you should include the ARNs for the account in your policy. Some services require the account ARN, although this is not required in all cases. Any existing policies without the required ARN will continue to work, but new policies that include these services must meet this requirement. IAM does not track these services, and therefore recommends that you always include the account ARN.

The following examples show how to use NotPrincipal and "Effect": "Deny" in the same policy statement effectively.

Example 1: An IAM user in the same or a different account

In the following example, all principals except the user named Bob in AWS account 444455556666 are explicitly denied access to a resource. Note that as a best practice, the NotPrincipal element contains the ARN of both the user Bob and the AWS account that Bob belongs to (arn:aws:iam::444455556666:root). If the NotPrincipal element contained only Bob's ARN, the effect of the policy might be to explicitly deny access to the AWS account that contains the user Bob. In some cases, a user cannot have more permissions than its parent account, so if Bob's account is explicitly denied access then Bob might be unable to access the resource.

This example works as intended when it is part of a policy statement in a resource-based policy that is attached to a resource in either the same or a different AWS account (not 444455556666). This example by itself does not grant access to Bob, it only omits Bob from the list of principals that are explicitly denied. To allow Bob access to the resource, another policy statement must explicitly allow access using "Effect": "Allow".

{
    "Version": "2012-10-17",
    "Statement": [{
        "Effect": "Deny",
        "NotPrincipal": {"AWS": [
            "arn:aws:iam::444455556666:user/Bob",
            "arn:aws:iam::444455556666:root"
        ]},
        "Action": "s3:*",
        "Resource": [
            "arn:aws:s3:::BUCKETNAME",
            "arn:aws:s3:::BUCKETNAME/*"
        ]
    }]
}

Example 2: An IAM role in the same or different account

In the following example, all principals except the assumed-role user named cross-account-audit-app in AWS account 444455556666 are explicitly denied access to a resource. As a best practice, the NotPrincipal element contains the ARN of the assumed-role user (cross-account-audit-app), the role (cross-account-read-only-role), and the AWS account that the role belongs to (444455556666). If the NotPrincipal element was missing the ARN of the role, the effect of the policy might be to explicitly deny access to the role. Similarly, if the NotPrincipal element was missing the ARN of the AWS account that the role belongs to, the effect of the policy might be to explicitly deny access to the AWS account and all entities in that account. In some cases, assumed-role users cannot have more permissions than their parent role, and roles cannot have more permissions than their parent AWS account, so when the role or the account is explicitly denied access, the assumed role user might be unable to access the resource.

This example works as intended when it is part of a policy statement in a resource-based policy that is attached to a resource in a different AWS account (not 444455556666). This example by itself does not allow access to the assumed-role user cross-account-audit-app, it only omits cross-account-audit-app from the list of principals that are explicitly denied. To give cross-account-audit-app access to the resource, another policy statement must explicitly allow access using "Effect": "Allow".

{
    "Version": "2012-10-17",
    "Statement": [{
        "Effect": "Deny",
        "NotPrincipal": {"AWS": [
            "arn:aws:sts::444455556666:assumed-role/cross-account-read-only-role/cross-account-audit-app",
            "arn:aws:iam::444455556666:role/cross-account-read-only-role",
            "arn:aws:iam::444455556666:root"
        ]},
        "Action": "s3:*",
        "Resource": [
            "arn:aws:s3:::Bucket_AccountAudit",
            "arn:aws:s3:::Bucket_AccountAudit/*"
        ]
    }]
}