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 Using 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
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.
The token presented to AWS STS is generated by an identity pool, which translates a user
pool, social, or OIDC provider token, or a SAML assertion, to its own token. The identity
pool token contains an aud claim that is the identity pool ID.
The following example role trust policy allows the federated service principal
cognito-identity.amazonaws.com to call the AWS STS API
AssumeRoleWithWebIdentity. The request will only succeed if the identity pool
token in the API request has the following claims.
-
An
audclaim of the identity pool IDus-west-2:abcdefg-1234-5678-910a-0e8443553f95. -
An
amrclaim ofauthenticatedthat is added when the user has signed in and isn't a guest user.
{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "Federated": "cognito-identity.amazonaws.com" }, "Action": "sts:AssumeRoleWithWebIdentity", "Condition": { "StringEquals": { "cognito-identity.amazonaws.com:aud": "us-west-2:abcdefg-1234-5678-910a-0e8443553f95" }, "ForAnyValue:StringLike": { "cognito-identity.amazonaws.com:amr": "authenticated" } } } ] }
Trust policies for IAM roles in Basic (Classic) authentication
You must apply at least one condition that limits trust policies for roles that you use with identity pools. When you create or update role trust policies for identity pools, IAM returns an error if you try to save your changes without at least one condition key that limits source identities. AWS STS doesn't permit cross-account AssumeRoleWithWebIdentity operations from identity pools to IAM roles that lack a condition of this type.
This topic includes several conditions that limit source identities for identity pools. For a full list, see Available keys for AWS web identity federation.
In basic, or classic, authentication with an identity pool, you can assume any IAM
role with AWS STS if it has the right trust policy. IAM roles for Amazon Cognito identity pools
trust the service principal cognito-identity.amazonaws.com to assume the
role. This configuration is not sufficient to secure your IAM roles against unintended
access to resources. Roles of this type must apply an additional condition to the role
trust policy. You can't create or modify roles for identity pools without at least one of
the following conditions.
cognito-identity.amazonaws.com:aud-
Restricts the role to operations from one or more identity pools. Amazon Cognito indicates the source identity pool in the
audclaim in the identity pool token. cognito-identity.amazonaws.com:amr-
Restricts the role to either
authenticatedorunauthenticated(guest) users. Amazon Cognito indicates the authentication state in theamrclaim in the identity pool token. cognito-identity.amazonaws.com:sub-
Restricts the role to one or more users by UUID. This UUID is the user's identity ID in the identity pool. This value isn’t the
subvalue from the user’s original identity provider. Amazon Cognito indicates this UUID in thesubclaim in the identity pool token.
Enhanced-flow authentication requires that the IAM role be in the same AWS account as the identity pool, but this isn't the case in basic authentication.
Additional considerations apply to Amazon Cognito identity pools that assume cross-account IAM roles. The trust policies of those roles
must accept the cognito-identity.amazonaws.com service principal and must contain the specific
cognito-identity.amazonaws.com:aud condition. To prevent unintended access
to your AWS resources, the aud condition key restricts the role to users
from the identity pools in the condition value.
The token that an identity pool issues for an identity contains information about the
originating AWS account of the identity pool. When you present an identity pool token in
an AssumeRoleWithWebIdentity API request, AWS STS checks to see if the originating
identity pool is in the same AWS account as the IAM role. If AWS STS determines that the
request is cross-account, it checks to see if the role trust policy has an
aud condition. The assume-role call fails if no such conditions are present
in the role trust policy. If the request is not cross-account, AWS STS doesn’t enforce this
restriction. As a best practice, always apply a condition of this type to the trust
policies of your identity pool roles.
Additional trust policy conditions
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 and an AWS managed 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.
Amazon Cognito also prevents authenticated and unauthenticated users from making API requests to Amazon Cognito identity pools and Amazon Cognito Sync. Other AWS services might place restrictions on service access from web identities.
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.
-
Your user's identity ID.
-
The ARN of the IAM role that your user wants to assume.
-
A
policyparameter that adds an inline session policy. -
A
PolicyArns.member.Nparameter whose value is an AWS managed policy that grants additional permissions in Amazon CloudWatch.
Services that unauthenticated users can access
When you use the enhanced flow, the scope-down policies that Amazon Cognito applies to your user's session prevent them from using any services other than those listed in the following table. For a subset of services, only specific actions are allowed.
| Category | Service |
|---|---|
|
Analytics |
Amazon Data Firehose Amazon Managed Service for Apache Flink |
|
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 Amazon Pinpoint Amazon Location 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 |
| Networking & Content Delivery |
Amazon API Gateway |
| Security, Identity, & Compliance |
Amazon Cognito user pools |
| 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 |
|
| Amazon SageMaker |
|
| Amazon Textract |
|
| Amazon Sumerian |
|
| Amazon Location Service |
|
To grant access to AWS services beyond this list, 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
remove that service from your use case. If you can't, switch to the basic flow.
The inline session policy for guest users
Amazon Cognito first applies an inline policy in the request for IAM credentials. 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.
Amazon Cognito adds the following inline policy to sessions for your users in AWS Regions that are enabled by default. For an overview of the net effect of the inline policy and other session policies, see Services that unauthenticated users can access.
{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": [ "cloudwatch:*", "logs:*", "dynamodb:*", "kinesis:*", "mobileanalytics:*", "s3:*", "ses:*", "sns:*", "sqs:*", "lambda:*", "machinelearning:*", "execute-api:*", "iot:*", "gamelift:*", "scs:*", "cognito-identity:*", "cognito-idp:*", "lex:*", "polly:*", "comprehend:*", "translate:*", "transcribe:*", "rekognition:*", "mobiletargeting:*", "firehose:*", "appsync:*", "personalize:*", "sagemaker:InvokeEndpoint", "cognito-sync:*", "sumerian:View*", "codewhisperer:*", "textract:DetectDocumentText", "textract:AnalyzeDocument", "sdb:*" ], "Resource": [ "*" ] } ] }
For all other Regions, the inline scope-down policy includes everything listed in the
default Regions except for the following Action statements.
"cognito-sync:*", "sumerian:View*", "codewhisperer:*", "textract:DetectDocumentText", "textract:AnalyzeDocument", "sdb:*"
The AWS managed session policy for guests
Amazon Cognito also applies an AWS managed policy as a session policy to the enhanced-flow
sessions of unauthenticated guests. This policy limits the scope of unauthenticated users'
permissions with the policy
AmazonCognitoUnAuthedIdentitiesSessionPolicy.
You must also grant this permission in the policies that you attach to your unauthenticated IAM role. A user's effective permissions for an assumed-role session are the intersection of the IAM policies assigned to their role, and their session policies. For more information, see Session policies in the AWS Identity and Access Management User Guide.
For an overview of the net effect of this AWS managed policy and other session policies, see Services that unauthenticated users can access.
The AmazonCognitoUnAuthedIdentitiesSessionPolicy managed policy has the
following permissions.
{ "Version": "2012-10-17", "Statement": [{ "Effect": "Allow", "Action": [ "rum:PutRumEvents", "polly:*", "comprehend:*", "translate:*", "transcribe:*", "rekognition:*", "mobiletargeting:*", "firehose:*", "personalize:*", "sagemaker:InvokeEndpoint", "geo:GetMap*", "geo:SearchPlaceIndex*", "geo:GetPlace", "geo:CalculateRoute*", "geo:*Geofence", "geo:*Geofences", "geo:*DevicePosition*", "kms:Encrypt", "kms:Decrypt", "kms:ReEncryptTo", "kms:ReEncryptFrom", "kms:GenerateDataKey", "kms:GenerateDataKeyPair", "kms:GenerateDataKeyPairWithoutPlaintext", "kms:GenerateDataKeyWithoutPlaintext" ], "Resource": "*" }] }
Access policy examples
In this section, you can find example Amazon Cognito access policies that grant your users the
minimum permissions necessary to do 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
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:::amzn-s3-demo-bucket/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
${cognito-identity.amazonaws.com:sub} can get, put, and list objects into
arn:aws:s3:::amzn-s3-demo-bucket/us-east-1:12345678-1234-1234-1234-123456790ab.
However, the identity would not be granted access to other objects in
arn:aws:s3:::amzn-s3-demo-bucket.
{ "Version": "2012-10-17", "Statement": [ { "Action": ["s3:ListBucket"], "Effect": "Allow", "Resource": ["arn:aws:s3:::amzn-s3-demo-bucket"], "Condition": {"StringLike": {"s3:prefix": ["${cognito-identity.amazonaws.com:sub}/*"]}} }, { "Action": [ "s3:GetObject", "s3:PutObject" ], "Effect": "Allow", "Resource": ["arn:aws:s3:::amzn-s3-demo-bucket/${cognito-identity.amazonaws.com:sub}/*"] } ] }
Assign identities fine-grained access to Amazon DynamoDB
The following access policy provides fine-grained access control to DynamoDB resources using Amazon Cognito environment variables. These variables grant access to items in DynamoDB by identity ID. For more information, see Using IAM Policy Conditions for Fine-Grained Access Control in the Amazon DynamoDB Developer Guide.
{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": [ "dynamodb:GetItem", "dynamodb:BatchGetItem", "dynamodb:Query", "dynamodb:PutItem", "dynamodb:UpdateItem", "dynamodb:DeleteItem", "dynamodb:BatchWriteItem" ], "Resource": [ "arn:aws:dynamodb:us-west-2:123456789012:table/MyTable" ], "Condition": { "ForAllValues:StringEquals": { "dynamodb:LeadingKeys": ["${cognito-identity.amazonaws.com:sub}"] } } } ] }
Grant an identity permission to invoke a Lambda function
The following access policy grants an identity permission 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}/*"] }] }
Role trust and permissions
The way these roles differ is in their trust relationships. The following is an example trust policy for an unauthenticated role:
{ "Version": "2012-10-17", "Statement": [ { "Sid": "", "Effect": "Allow", "Principal": { "Federated": "cognito-identity.amazonaws.com" }, "Action": "sts:AssumeRoleWithWebIdentity", "Condition": { "StringEquals": { "cognito-identity.amazonaws.com:aud": "us-east-1:12345678-corner-cafe-123456790ab" }, "ForAnyValue:StringLike": { "cognito-identity.amazonaws.com:amr": "unauthenticated" } } } ] }
This policy grants federated users from cognito-identity.amazonaws.com (the
issuer of the OpenID Connect token) permission to assume this role. Additionally, the policy
restricts the aud of the token, in this case the identity pool ID, to match the
identity pool. Finally, the policy specifies that one of the array members of the
multi-value amr claim of the token issued by the Amazon Cognito
GetOpenIdToken API operation has the value
unauthenticated.
When Amazon Cognito creates a token, it sets the amr of the token as either
unauthenticated or authenticated. If amr is
authenticated, the token includes any providers used during authentication.
This means that you can create a role that trusts only users that logged in via Facebook by
changing the amr condition as shown:
"ForAnyValue:StringLike": { "cognito-identity.amazonaws.com:amr": "graph.facebook.com" }
Be careful when changing your trust relationships on your roles, or when trying to use roles across identity pools. If you don't configure your role correctly to trust your identity pool, an exception from STS results, like the following:
AccessDenied -- Not authorized to perform sts:AssumeRoleWithWebIdentity
If you see this message, check that your identity pool and authentication type have an appropriate role.
