Fine-tuning the Resources and Conditions sections of policies

You can restrict the scope of a user's permissions by specifying resources and conditions in an Amazon Identity and Access Management (IAM) policy. Each action in a policy supports a combination of resource and condition types that varies depending on the behavior of the action.

Every IAM policy statement grants permission to an action that's performed on a resource. When the action doesn't act on a named resource, or when you grant permission to perform the action on all resources, the value of the resource in the policy is a wildcard (*). For many actions, you can restrict the resources that a user can modify by specifying the Amazon Resource Name (ARN) of a resource, or an ARN pattern that matches multiple resources.

By resource type, the general design of how to restrict the scope of an action is the following:

To restrict permissions by resource, specify the resource by ARN.

For example, the following policy allows a user in Amazon Web Services account 123456789012 to invoke a function named my-function in the US West (Oregon) Amazon Region.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Invoke",
            "Effect": "Allow",
            "Action": [
                "lambda:InvokeFunction"
            ],
            "Resource": "arn:aws-cn:lambda:us-west-2:123456789012:function:my-function"
        }
    ]
}

This is a special case where the action identifier (lambda:InvokeFunction) differs from the API operation (Invoke). For other actions, the action identifier is the operation name prefixed by lambda:.

Understanding the Condition section in policies

Conditions are an optional policy element that applies additional logic to determine if an action is allowed. In addition to common conditions that all actions support, Lambda defines condition types that you can use to restrict the values of additional parameters on some actions.

For example, the lambda:Principal condition lets you restrict the service or account that a user can grant invocation access to on a function's resource-based policy. The following policy lets a user grant permission to Amazon Simple Notification Service (Amazon SNS) topics to invoke a function named test.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "ManageFunctionPolicy",
            "Effect": "Allow",
            "Action": [
                "lambda:AddPermission",
                "lambda:RemovePermission"
            ],
            "Resource": "arn:aws-cn:lambda:us-west-2:123456789012:function:test:*",
            "Condition": {
                "StringEquals": {
                    "lambda:Principal": "sns.amazonaws.com"
                }
            }
        }
    ]
}

The condition requires that the principal is Amazon SNS and not another service or account. The resource pattern requires that the function name is test and includes a version number or alias. For example, test:v1.

For more information on resources and conditions for Lambda and other Amazon services, see Actions, resources, and condition keys for Amazon services in the Service Authorization Reference.

Referencing functions in the Resource section of policies

You reference a Lambda function in a policy statement using an Amazon Resource Name (ARN). The format of a function ARN depends on whether you are referencing the whole function (unqualified) or a function version or alias (qualified).

When making Lambda API calls, users can specify a version or alias by passing a version ARN or alias ARN in the GetFunction FunctionName parameter, or by setting a value in the GetFunction Qualifier parameter. Lambda makes authorization decisions by comparing the resource element in the IAM policy with both the FunctionName and Qualifier passed in API calls. If there is a mismatch, Lambda denies the request.

Whether you are allowing or denying an action on your function, you must use the correct function ARN types in your policy statement to achieve the results that you expect. For example, if your policy references the unqualified ARN, Lambda accepts requests that reference the unqualified ARN but denies requests that reference a qualified ARN.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "lambda:InvokeFunction",
            "Resource": "arn:aws:lambda:us-west-2:123456789012:function:myFunction"
        }
    ]
}

If your policy references a specific qualified ARN, Lambda accepts requests that reference that ARN but denies requests that reference the unqualified ARN or a different qualified ARN, for example, myFunction:2.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "lambda:InvokeFunction",
            "Resource": "arn:aws:lambda:us-west-2:123456789012:function:myFunction:1"
        }
    ]
}

If your policy references any qualified ARN using :*, Lambda accepts any qualified ARN but denies requests that reference the unqualified ARN.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "lambda:InvokeFunction",
            "Resource": "arn:aws:lambda:us-west-2:123456789012:function:myFunction:*"
        }
    ]
}

If your policy references any ARN using *, Lambda accepts any qualified or unqualified ARN.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "lambda:InvokeFunction",
            "Resource": "arn:aws:lambda:us-west-2:123456789012:function:myFunction*"
        }
    ]
}

Supported IAM actions and function behaviors

Actions define what can be permitted through IAM policies. For a list of actions supported in Lambda, see Actions, resources, and condition keys for Amazon Lambda in the Service Authorization Reference. In most cases, when an IAM action permits an Lambda API action, the name of the IAM action is the same as the name of the Lambda API action, with the following exceptions:

In addition to the resources and conditions defined in the Service Authorization Reference, Lambda supports the following resources and conditions for certain actions. Many of these are related to referencing functions in the resource section of policies. Actions that operate on a function can be restricted to a specific function by function, version, or alias ARN, as described in the following table.