IAM roles

While creating an identity pool, you're prompted to update the IAM roles that your users assume. IAM roles work like this: When a user logs in to your app, Amazon Cognito generates temporary AWS credentials for the user. These temporary credentials are associated with a specific IAM role. With the IAM role, you can define a set of permissions to access your AWS resources.

You can specify default IAM roles for authenticated and unauthenticated users. In addition, you can define rules to choose the role for each user based on claims in the user's ID token. For more information, see Role-based access control.

By default, the Amazon Cognito console creates IAM roles that provide access to Amazon Mobile Analytics and to Amazon Cognito Sync. Alternatively, you can choose to use existing IAM roles.

Modify IAM roles to allow or restrict access to other services. To do so, log in to the IAM Console. Then select Roles, and select a role. The policies attached to the selected role are listed in the Permissions tab. You can customize an access policy by selecting the corresponding Manage Policy link. To learn more about using and defining policies, see Overview of IAM Policies.

Note

As a best practice, define policies that follow the principle of granting least privilege. In other words, the policies include only the permissions that users require to perform their tasks. For more information, see Grant Least Privilege in the IAM User Guide.

Remember that unauthenticated identities are assumed by users who do not log in to your app. Typically, the permissions that you assign for unauthenticated identities should be more restrictive than those for authenticated identities.

Set up a trust policy

Amazon Cognito uses IAM roles to generate temporary credentials for your application's users. Access to permissions is controlled by a role's trust relationships. Learn more about Role trust and permissions.

Reuse roles across identity pools

To reuse a role across multiple identity pools, because they share a common permission set, you can include multiple identity pools, like this:

"StringEquals": {
    "cognito-identity.amazonaws.com:aud": [
        "us-east-1:12345678-abcd-abcd-abcd-123456790ab",
        "us-east-1:98765432-dcba-dcba-dcba-123456790ab"
    ]
}

Limit access to specific identities

To create a policy limited to a specific set of app users, check the value of cognito-identity.amazonaws.com:sub:

"StringEquals": {
    "cognito-identity.amazonaws.com:aud": "us-east-1:12345678-abcd-abcd-abcd-123456790ab",
    "cognito-identity.amazonaws.com:sub": [
        "us-east-1:12345678-1234-1234-1234-123456790ab",
        "us-east-1:98765432-1234-1234-1243-123456790ab"
    ]
}

Limit access to specific providers

To create a policy limited to users who have logged in with a specific provider (perhaps your own login provider), check the value of cognito-identity.amazonaws.com:amr:

"ForAnyValue:StringLike": {
    "cognito-identity.amazonaws.com:amr": "login.myprovider.myapp"
}

For example, an app that trusts only Facebook would have the following amr clause:

"ForAnyValue:StringLike": {
    "cognito-identity.amazonaws.com:amr": "graph.facebook.com"
}

Access policies

The permissions that you attach to a role apply to all users who assume that role. To partition your users' access, use policy conditions and variables. For more information, see IAM policy elements: Variables and tags. You can use the sub condition to restrict actions to Amazon Cognito identity IDs in your access policies. Use this option with caution, particularly for unauthenticated identities, which lack a consistent user ID. For more information about the IAM policy variables for web federation with Amazon Cognito, see IAM and AWS STS condition context keys in the AWS Identity and Access Management User Guide.

For additional security protection, Amazon Cognito applies a scope-down policy to credentials that you assign your unauthenticated users in the enhanced flow, using GetCredentialsForIdentity. The scope-down policy adds an Inline session policy to the IAM policies that you apply to your unauthenticated role. Because you must grant access in both the IAM policies for your role and the session policies, the scope-down policy limits users' access to services other than those in the list that follows.

Note

In the basic (classic) flow, you make your own AssumeRoleWithWebIdentity API request, and can apply these restrictions to the request. As a best security practice, don't assign any permissions above this scope-down policy to unauthenticated users.

In a successful request with the enhanced flow, Amazon Cognito makes an AssumeRoleWithWebIdentity API request in the background. Among the parameters in this request, Amazon Cognito includes the following.

1. Your user's identity ID.

2. The ARN of the IAM role that your user wants to assume.

3. A policy parameter that adds an inline session policy.

The inline session policy

The inline session policy restricts your user's effective permissions from including access to any AWS services outside those in the following list. You must also grant permissions to these AWS services in the policies that you apply to the user's IAM role. A user's effective permissions for an assumed-role session are the intersection of the policies assigned to their role, and their session policy. For more information, see Session policies in the AWS Identity and Access Management User Guide.

Services that unauthenticated enhanced flow users can access

Category	Service
Analytics	Amazon Kinesis Data Firehose Amazon Kinesis Data Analytics
Application Integration	Amazon Simple Queue Service
AR & VR	Amazon Sumerian¹
Business Applications	Amazon Mobile Analytics Amazon Simple Email Service
Compute	AWS Lambda
Cryptography & PKI	AWS Key Management Service¹
Database	Amazon DynamoDB Amazon SimpleDB
Front-end Web & Mobile	AWS AppSync Amazon Location Service Amazon Simple Notification Service
Game Development	Amazon GameLift
Internet of Things (IoT)	AWS IoT
Machine Learning	Amazon CodeWhisperer Amazon Comprehend Amazon Lex Amazon Machine Learning Amazon Personalize Amazon Polly Amazon Rekognition Amazon SageMaker¹ Amazon Textract¹ Amazon Transcribe Amazon Translate
Management & Governance	Amazon CloudWatch Amazon CloudWatch Logs
Neworking & Content Delivery	Amazon API Gateway
Security, Identity, & Compliance	Amazon Cognito Identity Amazon Cognito user pools Amazon Cognito Sync
Storage	Amazon Simple Storage Service

¹ For the AWS services in the following table, the inline policy grants a subset of actions. The table displays the available actions in each.

AWS service	Maximum permissions for unauthenticated enhanced flow users
AWS Key Management Service	Encrypt Decrypt ReEncrypt GenerateDataKey
Amazon SageMaker	InvokeEndpoint
Amazon Textract	DetectDocumentText AnalyzeDocument
Amazon Sumerian	View*

To grant access to Services that unauthenticated enhanced flow users can access, activate the basic (classic) authentication flow in your identity pool. If your users see NotAuthorizedException errors from AWS services that are allowed by the policies assigned to the IAM role for unauthenticated users, evaluate whether you can eliminate that service from your use case or switch to the basic flow.

Access policy examples

In this section, you can find example Amazon Cognito access policies that grant only the permissions your identities must complete a specific operation. You can further limit the permissions for a given identity ID by using policy variables where possible. For example, using ${cognito-identity.amazonaws.com:sub}. For more information, see Understanding Amazon Cognito Authentication Part 3: Roles and Policies on the AWS Mobile Blog.

Note

As a security best practice, policies should include only the permissions that users require to perform their tasks. This means that you should try to always scope access to an individual identity for objects whenever possible.

Grant an identity read access to a single object in Amazon S3

The following access policy grants read permissions to an identity to retrieve a single object from a given S3 bucket.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "s3:GetObject"
      ],
      "Effect": "Allow",
      "Resource": ["arn:aws:s3:::mybucket/assets/my_picture.jpg"]
    }
  ]
}

Grant an identity both read and write access to identity specific paths in Amazon S3

The following access policy grants read and write permissions to access a specific prefix "folder" in an S3 bucket by mapping the prefix to the ${cognito-identity.amazonaws.com:sub} variable.

With this policy, an identity such as us-east-1:12345678-1234-1234-1234-123456790ab inserted via ${cognito-identity.amazonaws.com:sub} can get, put, and list objects into arn:aws:s3:::mybucket/us-east-1:12345678-1234-1234-1234-123456790ab. However, the identity would not be granted access to other objects in arn:aws:s3:::mybucket.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": ["s3:ListBucket"],
      "Effect": "Allow",
      "Resource": ["arn:aws:s3:::mybucket"],
      "Condition": {"StringLike": {"s3:prefix": ["${cognito-identity.amazonaws.com:sub}/*"]}}
    },
    {
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Effect": "Allow",
      "Resource": ["arn:aws:s3:::mybucket/${cognito-identity.amazonaws.com:sub}/*"]
    }
  ]
}

Assign identities fine-grained access to Amazon DynamoDB

The following access policy provides fine-grained access control to DynamoDB resources using Amazon Cognito environment variables. These variables grant access to items in DynamoDB by identity ID. For more information, see Using IAM Policy Conditions for Fine-Grained Access Control in the Amazon DynamoDB Developer Guide.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:BatchGetItem",
        "dynamodb:Query",
        "dynamodb:PutItem",
        "dynamodb:UpdateItem",
        "dynamodb:DeleteItem",
        "dynamodb:BatchWriteItem"
      ],
      "Resource": [
        "arn:aws:dynamodb:us-west-2:123456789012:table/MyTable"
      ],
      "Condition": {
        "ForAllValues:StringEquals": {
          "dynamodb:LeadingKeys":  ["${cognito-identity.amazonaws.com:sub}"]
        }
      }
    }
  ]
}

Grant an identity permission to invoke a Lambda function

The following access policy grants an identity permissions to invoke a Lambda function.

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

Grant an identity permission to publish records to Kinesis Data Streams

The following access policy allows an identity to use the PutRecord operation with any of the Kinesis Data Streams. It can be applied to users that need to add data records to all streams in an account. For more information, see Controlling Access to Amazon Kinesis Data Streams Resources Using IAM in the Amazon Kinesis Data Streams Developer Guide.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "kinesis:PutRecord",
            "Resource": [
                "arn:aws:kinesis:us-east-1:111122223333:stream/stream1"
            ]
        }
    ]
}

Grant an identity access to their data in the Amazon Cognito Sync store

The following access policy grants an identity permissions to access only their own data in the Amazon Cognito Sync store.

{
  "Version": "2012-10-17",
  "Statement":[{
      "Effect":"Allow",
      "Action":"cognito-sync:*",
      "Resource":["arn:aws:cognito-sync:us-east-1:123456789012:identitypool/${cognito-identity.amazonaws.com:aud}/identity/${cognito-identity.amazonaws.com:sub}/*"]
      }]
  }