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IAM policy elements: Variables and tags

Use Amazon Identity and Access Management (IAM) policy variables as placeholders when you don't know the exact value of a resource or condition key when you write the policy.

Introduction

In IAM policies, many actions allow you to provide a name for the specific resources that you want to control access to. For example, the following policy allows the user to list, read, and write objects with a prefix David in the Amazon S3 bucket mybucket.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["s3:ListBucket"],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket"],
      "Condition": {"StringLike": {"s3:prefix": ["David/*"]}}
    },
    {
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket/David/*"]
    }
  ]
}

In some cases, you might not know the exact name of the resource when you write the policy. You might want to generalize the policy so it works for many users without having to make a unique copy of the policy for each user. For example, consider writing a policy to allow each user to have access to his or her own objects in an Amazon S3 bucket, as in the previous example. But don't create a separate policy for each user that explicitly specifies the user's name as part of the resource. Instead, create a single group policy that works for any user in that group.

You can do this by using policy variables, a feature that lets you specify placeholders in a policy. When the policy is evaluated, the policy variables are replaced with values that come from the context of the request itself.

The following example shows a policy for an Amazon S3 bucket that uses a policy variable.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["s3:ListBucket"],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket"],
      "Condition": {"StringLike": {"s3:prefix": ["${aws:username}/*"]}}
    },
    {
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket/${aws:username}/*"]
    }
  ]
}

When this policy is evaluated, IAM replaces the variable ${aws:username}with the friendly name of the actual current user. This means that a single policy applied to a group of users can control access to a bucket. It does this by using the user name as part of the resource's name.

The variable is marked using a $ prefix followed by a pair of curly braces ({ }). Inside the ${ } characters, you can include the name of the value from the request that you want to use in the policy. The values you can use are discussed later on this page.

You can use policy variables in a similar way to allow each user to manage his or her own access keys. A policy that allows a user to programmatically change the access key for user David looks like this:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Action": ["iam:*AccessKey*"],
    "Effect": "Allow",
    "Resource": ["arn:aws-cn:iam::account-id:user/David"]
  }]
}

If this policy is attached to user David, that user can change his own access key. As with the policies for accessing user-specific Amazon S3 objects, you would have to create a separate policy for each user that includes the user's name. You would then attach each policy to the individual users.

By using a policy variable, you can create a policy like this:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Action": ["iam:*AccessKey*"],
    "Effect": "Allow",
    "Resource": ["arn:aws-cn:iam::account-id:user/${aws:username}"]
  }]
}

When you use a policy variable for the user name like this, you don't have to have a separate policy for each individual user. Instead, you can attach this new policy to an IAM group that includes everyone who should be allowed to manage their own access keys. When a user makes a request to modify his or her access key, IAM substitutes the user name from the current request for the ${aws:username} variable and evaluates the policy.

Tags as policy variables

In some Amazon services you can attach your own custom attributes to resources that are created by those services. For example, you can apply tags to Amazon S3 buckets or to IAM users. These tags are key-value pairs. You define the tag key name and the value that is associated with that key name. For example, you might create a tag with a department key and a Human Resources value. For more information about tagging IAM entities, see Tagging IAM resources. For information about tagging resources created by other Amazon services, see the documentation for that service. For information about using Tag Editor, see Working with Tag Editor in the Amazon Web Services Management Console User Guide.

You can tag IAM resources to simplify discovering, organizing, and tracking your IAM resources. You can also tag IAM identities to control access to resources or to tagging itself. To learn more about using tags to control access, see Controlling access to and for IAM users and roles using tags.

Where you can use policy variables

You can use policy variables in the Resource element and in string comparisons in the Condition element.

Resource element

You can use a policy variable in the Resource element, but only in the resource portion of the ARN. This portion of the ARN appears after the fifth colon (:). You can't use a variable to replace parts of the ARN before the fifth colon, such as the service or account. For more information about the ARN format, see IAM ARNs.

The following policy might be attached to a group. It gives each of the users in the group full programmatic access to a user-specific object (their own "home directory") in Amazon S3.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["s3:ListBucket"],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket"],
      "Condition": {"StringLike": {"s3:prefix": ["${aws:username}/*"]}}
    },
    {
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Effect": "Allow",
      "Resource": ["arn:aws-cn:s3:::mybucket/${aws:username}/*"]
    }
  ]
}

The following policy might be used for an IAM group. It gives users in that group the ability to create, use, and delete queues that have their names and that are in the us-west-2 Region.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "ListForConsole",
            "Effect": "Allow",
            "Action": "sqs:ListQueues",
            "Resource": "*"
        },
        {
            "Sid": "AllQueueActions",
            "Effect": "Allow",
            "Action": "sqs:*",
            "Resource": "arn:aws:sqs:us-east-2:*:${aws:username}-queue"
        }
    ]
}

To replace part of an ARN with a tag value, surround the prefix and key name with ${}. For example, the following Resource element refers to only a bucket that is named the same as the value in the requesting user's department tag.

    "Resource": ["arn:aws-cn:s3:::bucket/${aws:PrincipalTag/department}"]

Condition element

You can use a policy variable for Condition values in any condition that involves the string operators or the ARN operators. String operators include StringEquals, StringLike, and StringNotLike. ARN operators include ArnEquals and ArnLike. You can't use a policy variable with other operators, such as Numeric, Date, Boolean, Binary, IP Address, or Null operators. For more information about condition operators, see IAM JSON policy elements: Condition operators.

The following Amazon SNS topic policy gives users in Amazon Web Services account 999999999999 the ability to manage (perform all actions for) the topic. However this permission is granted only if the URL matches their Amazon user name.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Principal": {
        "AWS": "999999999999"
      },
      "Effect": "Allow",
      "Action": "sns:*",
      "Condition": {
        "StringLike": {
          "sns:endpoint": "https://example.com/${aws:username}/"
        },
        "StringEquals": {
          "sns:Protocol": "https"
        }
      }
    }
  ]
}

When referencing a tag in a Condition element expression, use the relevant prefix and key name as the condition key. Then use the value that you want to test in the condition value. For example, the following policy example allows full access to IAM users, but only if the tag costCenter is attached to the user. The tag must also have a value of either 12345 or 67890. If the tag has no value, or has any other value, then the request fails.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
          "iam:*user*"
       ],
      "Resource": "*",
      "Condition": {
        "StringLike": {
          "iam:ResourceTag/costCenter": [ "12345", "67890" ]
        }
      }
    }
  ]
}

Request information that you can use for policy variables

You can use the Condition element of a JSON policy to compare keys in the request context with key values that you specify in your policy. When you use a policy variable, Amazon substitutes a value from the request context key in place of the variable in your policy.

Information available in all requests

Policies contain keys whose values you can use as policy variables. (Under some circumstances, the keys do not contain a value—see the information that follows this list.)

Principal key values

The values for aws:username, aws:userid, and aws:PrincipalType depend on what type of principal initiated the request. For example, the request could be made using the credentials of an IAM user, an IAM role, or the Amazon Web Services account root user. The following table shows values for these keys for different types of principals.

For the items in this table, note the following:

Information available in requests for federated users

Federated users are users who are authenticated using a system other than IAM. For example, a company might have an application for use in-house that makes calls to Amazon. It might be impractical to give an IAM identity to every corporate user who uses the application. Instead, the company might use a proxy (middle-tier) application that has a single IAM identity, or the company might use a SAML identity provider (IdP). The proxy application or SAML IdP authenticates individual users using the corporate network. A proxy application can then use its IAM identity to get temporary security credentials for individual users. A SAML IdP can in effect exchange identity information for Amazon temporary security credentials. The temporary credentials can then be used to access Amazon resources.

Similarly, you might create an app for a mobile device in which the app needs to access Amazon resources. In that case, you might use web identity federation, where the app authenticates the user using a well-known identity provider like Login with Amazon, Amazon Cognito, Facebook, or Google. The app can then use the user's authentication information from these providers to get temporary security credentials for accessing Amazon resources.

The recommended way to use web identity federation is by taking advantage of Amazon Cognito and the Amazon mobile SDKs. For more information, see the following:

Service-specific information

Requests can also include service-specific keys and values in its request context. Examples include the following:

For information about service-specific keys that you can use to get values for policy variables, refer to the documentation for the individual services. For example, see the following topics:

Special characters

There are a few special predefined policy variables that have fixed values that enable you to represent characters that otherwise have special meaning. If these special characters are part of the string, you are trying to match and you inserted them literally they would be misinterpreted. For example, inserting an * asterisk in the string would be interpreted as a wildcard, matching any characters, instead of as a literal *. In these cases, you can use the following predefined policy variables:

These predefined policy variables can be used in any string where you can use regular policy variables.

Specifying default values

If Amazon cannot resolve a variable this might cause the entire statement to be invalid. However, when you add a variable to your policy you can specify a default value for the variable. If a value is not specified for the variable Amazon uses the default text that you provided.

To add a default value to a variable, surround the default value with single quotes (' '), and separate the variable text and the default value with a comma and space (, ).

For example, if a principal is tagged with team=yellow, they can access ExampleCorp's Amazon S3 bucket named DOC-EXAMPLE-BUCKET-yellow. A policy with this resource allows team members to access their team bucket, but not those of other teams. For users without team tags, it sets a default value of company-wide for the bucket name. These users can access only the DOC-EXAMPLE-BUCKET-company-wide bucket where they can view broad information, such as instructions for joining a team.

"Resource":"arn:aws:s3:::DOC-EXAMPLE-BUCKET-${aws:PrincipalTag/team, 'company-wide'}"

For more information

For more information about policies, see the following: