User pool authentication flow - Amazon Cognito

User pool authentication flow

To verify the identity of users, modern authentication flows incorporate new challenge types, in addition to passwords. Amazon Cognito authentication typically requires that you implement two API operations in the following order:

  1. InitiateAuth

  2. RespondToAuthChallenge

A user authenticates by answering successive challenges until authentication either fails or Amazon Cognito issues tokens to the user. You can repeat these steps with Amazon Cognito, in a process that includes different challenges, to support any custom authentication flow.

Typically, your app generates a prompt to gather information from your user, and submits that information in an API request to Amazon Cognito. Consider an InitiateAuth flow in a user pool where you have configured your user with multi-factor authentication (MFA).

  1. Your app prompts your user for their user name and password.

  2. You include the user name and password as parameters in InitiateAuth.

  3. Amazon Cognito returns an SMS_MFA challenge and a session identifier.

  4. Your app prompts your user for the MFA code from their phone.

  5. You include that code and the session identifier in the RespondToAuthChallenge request.

Depending on the features of your user pool, you can end up responding to several challenges to InitiateAuth before your app retrieves tokens from Amazon Cognito. Amazon Cognito includes a session string in the response to each request. To combine your API requests into an authentication flow, include the session string from the response to the previous request in each subsequent request. By default, your users have three minutes to complete each challenge before the session string expires. To adjust this period, change your app client Authentication flow session duration. The following procedure describes how to change this setting in your app client configuration.

Amazon Cognito console

To configure app client authentication flow session duration (AWS Management Console)

  1. From the App integration tab in your user pool, select the name of your app client from the App clients and analytics container.

  2. Select Edit in the App client information container.

  3. Change the value of Authentication flow session duration to the validity duration that you want, in minutes, for SMS MFA codes. This also changes the amount of time that any user has to complete any authentication challenge in your app client.

  4. Select Save changes.

Amazon Cognito API

To configure app client authentication flow session duration (Amazon Cognito API)

  1. Prepare an UpdateUserPoolClient request with your existing user pool settings from a DescribeUserPoolClient request. Your UpdateUserPoolClient request must include all existing app client properties.

  2. Change the value of AuthSessionValidity to the validity duration that you want, in minutes, for SMS MFA codes. This also changes the amount of time that any user has to complete any authentication challenge in your app client.

For more information about app clients, see Configuring a user pool app client.

You can use AWS Lambda triggers to customize the way users authenticate. These triggers issue and verify their own challenges as part of the authentication flow.

You can also use the admin authentication flow for secure backend servers. You can use the user migration authentication flow to make user migration possible without the requirement that your users to reset their passwords.


Users can attempt but fail to sign in correctly five times before Amazon Cognito temporarily locks them out. Lockout time starts at one second and increases exponentially, doubling after each subsequent failed attempt, up to about 15 minutes. Amazon Cognito ignores attempts to log in during a temporary lockout period, and these attempts don't initiate a new lockout period. After a user waits 15 minutes, Amazon Cognito resets the temporary lockout. This behavior is subject to change.

Client-side authentication flow

The following process works for user client-side apps that you create with the AWS Mobile SDK for Android, AWS Mobile SDK for iOS, or AWS SDK for JavaScript:

  1. The user enters their user name and password into the app.

  2. The app calls the InitiateAuth operation with the user's user name and Secure Remote Password (SRP) details.

    This API operation returns the authentication parameters.


    The app generates SRP details with the Amazon Cognito SRP features that are built in to AWS SDKs.

  3. The app calls the RespondToAuthChallenge operation. If the call succeeds, Amazon Cognito returns the user's tokens, and the authentication flow is complete.

    If Amazon Cognito requires another challenge, the call to RespondToAuthChallenge returns no tokens. Instead, the call returns a session.

  4. If RespondToAuthChallenge returns a session, the app calls RespondToAuthChallenge again, this time with the session and the challenge response (for example, MFA code).

Server-side authentication flow

If you don't have a user app, but instead you use a Java, Ruby, or Node.js secure backend or server-side app, you can use the authenticated server-side API for Amazon Cognito user pools.

For server-side apps, user pool authentication is similar to authentication for client-side apps, except for the following:

  • The server-side app calls the AdminInitiateAuth API operation (instead of InitiateAuth). This operation requires AWS credentials with permissions that include cognito-idp:AdminInitiateAuth and cognito-idp:AdminRespondToAuthChallenge. The operation returns the required authentication parameters.

  • After the server-side app has the authentication parameters, it calls the AdminRespondToAuthChallenge API operation (instead of RespondToAuthChallenge). The AdminRespondToAuthChallenge API operation only succeeds when you provide AWS credentials.

For more information about signing Amazon Cognito API requests with AWS credentials, see Signature Version 4 signing process in the AWS General Reference.

The AdminInitiateAuth and AdminRespondToAuthChallenge API operations can't accept username-and-password user credentials for admin sign-in, unless you explicitly enable them to do so in one of the following ways:

  • Include ALLOW_ADMIN_USER_PASSWORD_AUTH (formerly known as ADMIN_NO_SRP_AUTH) in the ExplicitAuthFlow parameter when you call CreateUserPoolClient or UpdateUserPoolClient.

  • Add ALLOW_ADMIN_USER_PASSWORD_AUTH to the list of Authentication flows for you app client. Configure app clients on the App integration tab in your user pool, under App clients and analytics. For more information, see Configuring a user pool app client.

Custom authentication flow

Amazon Cognito user pools also make it possible to use custom authentication flows, which can help you create a challenge/response-based authentication model using AWS Lambda triggers.


You can't use advanced security features with custom authentication flows. For more information, see Adding advanced security to a user pool.

The custom authentication flow makes possible customized challenge and response cycles to meet different requirements. The flow starts with a call to the InitiateAuth API operation that indicates the type of authentication to use and provides any initial authentication parameters. Amazon Cognito responds to the InitiateAuth call with one of the following types of information:

  • A challenge for the user, along with a session and parameters.

  • An error if the user fails to authenticate.

  • ID, access, and refresh tokens if the supplied parameters in the InitiateAuth call are sufficient to sign the user in. (Typically the user or app must first answer a challenge, but your custom code must determine this.)

If Amazon Cognito responds to the InitiateAuth call with a challenge, the app gathers more input and calls the RespondToAuthChallenge operation. This call provides the challenge responses and passes it back the session. Amazon Cognito responds to the RespondToAuthChallenge call similarly to the InitiateAuth call. If the user has signed in, Amazon Cognito provides tokens, or if the user isn't signed in, Amazon Cognito provides another challenge, or an error. If Amazon Cognito returns another challenge, the sequence repeats and the app calls RespondToAuthChallenge until the user successfully signs in or an error is returned. For more details about the InitiateAuth and RespondToAuthChallenge API operations, see the API documentation.

Built-in authentication flow and challenges

Amazon Cognito contains built-in AuthFlow and ChallengeName values so that a standard authentication flow can validate a user name and password through the Secure Remote Password (SRP) protocol. The AWS SDKs have built-in support for these flows with Amazon Cognito.

The flow starts by sending USER_SRP_AUTH as the AuthFlow to InitiateAuth. You also send USERNAME and SRP_A values in AuthParameters. If the InitiateAuth call is successful, the response has included PASSWORD_VERIFIER as the ChallengeName and SRP_B in the challenge parameters. The app then calls RespondToAuthChallenge with the PASSWORD_VERIFIER ChallengeName and the necessary parameters in ChallengeResponses. If the call to RespondToAuthChallenge is successful and the user signs in, Amazon Cognito issues tokens. If you activated multi-factor authentication (MFA) for the user, Amazon Cognito returns the ChallengeName of SMS_MFA. The app can provide the necessary code through another call to RespondToAuthChallenge.

Custom authentication flow and challenges

An app can initiate a custom authentication flow by calling InitiateAuth with CUSTOM_AUTH as the Authflow. With a custom authentication flow, three Lambda triggers control challenges and verification of the responses.

  • The DefineAuthChallenge Lambda trigger uses a session array of previous challenges and responses as input. It then generates the next challenge name and Booleans that indicate whether the user is authenticated and can be granted tokens. This Lambda trigger is a state machine that controls the user’s path through the challenges.

  • The CreateAuthChallenge Lambda trigger takes a challenge name as input and generates the challenge and parameters to evaluate the response. When DefineAuthChallenge returns CUSTOM_CHALLENGE as the next challenge, the authentication flow calls CreateAuthChallenge. The CreateAuthChallenge Lambda trigger passes the next type of challenge in the challenge metadata parameter.

  • The VerifyAuthChallengeResponse Lambda function evaluates the response and returns a Boolean to indicate if the response was valid.

A custom authentication flow can also use a combination of built-in challenges, such as SRP password verification and MFA through SMS. It can use custom challenges such as CAPTCHA or secret questions.

Use SRP password verification in custom authentication flow

If you want to include SRP in a custom authentication flow, you must begin with SRP.

  • To initiate SRP password verification in a custom flow, the app calls InitiateAuth with CUSTOM_AUTH as the Authflow. In the AuthParameters map, the request from your app includes SRP_A: (the SRP A value) and CHALLENGE_NAME: SRP_A.

  • The CUSTOM_AUTH flow invokes the DefineAuthChallenge Lambda trigger with an initial session of challengeName: SRP_A and challengeResult: true. Your Lambda function responds with challengeName: PASSWORD_VERIFIER, issueTokens: false, and failAuthentication: false.

  • The app next must call RespondToAuthChallenge with challengeName: PASSWORD_VERIFIER and the other parameters required for SRP in the challengeResponses map.

  • If Amazon Cognito verifies the password, RespondToAuthChallenge invokes the DefineAuthChallenge Lambda trigger with a second session of challengeName: PASSWORD_VERIFIER and challengeResult: true. At that point, the DefineAuthChallenge Lambda trigger responds with challengeName: CUSTOM_CHALLENGE to start the custom challenge.

  • If MFA is enabled for a user, after Amazon Cognito verifies the password, your user is then challenged to set up or sign in with MFA.


The Amazon Cognito hosted sign-in webpage can't activate Custom authentication challenge Lambda triggers.

For more information about the Lambda triggers, including sample code, see Customizing user pool workflows with Lambda triggers.

Admin authentication flow

Best practice for authentication is to use the API operations described in Custom authentication flow with SRP for password verification. The AWS SDKs use that approach, and this approach helps them to use SRP. However, if you want to avoid SRP calculations, an alternative set of admin API operations is available for secure backend servers. For these backend admin implementations, use AdminInitiateAuth in place of InitiateAuth. Also, use AdminRespondToAuthChallenge in place of RespondToAuthChallenge. Because you can submit the password as plaintext, you do not have to do SRP calculations when you use these operations. . Here is an example:

AdminInitiateAuth Request { "AuthFlow":"ADMIN_USER_PASSWORD_AUTH", "AuthParameters":{ "USERNAME":"<username>", "PASSWORD":"<password>" }, "ClientId":"<clientId>", "UserPoolId":"<userPoolId>" }

These admin authentication operations require developer credentials and use the AWS Signature Version 4 (SigV4) signing process. These operations are available in standard AWS SDKs, including Node.js, which is convenient for Lambda functions. To use these operations and have them accept passwords in plaintext, you must activate them for the app in the console. Alternatively, you can pass ADMIN_USER_PASSWORD_AUTH for the ExplicitAuthFlow parameter in calls to CreateUserPoolClient or UpdateUserPoolClient. The InitiateAuth and RespondToAuthChallenge operations do not accept the ADMIN_USER_PASSWORD_AUTH AuthFlow.

In the AdminInitiateAuth response ChallengeParameters, the USER_ID_FOR_SRP attribute, if present, contains the user's actual user name, not an alias (such as email address or phone number). In your call to AdminRespondToAuthChallenge, in the ChallengeResponses, you must pass this user name in the USERNAME parameter.


Because backend admin implementations use the admin authentication flow, the flow doesn't support device tracking. When you have turned on device tracking, admin authentication succeeds, but any call to refresh the access token fails.

User migration authentication flow

A user migration Lambda trigger helps migrate users from a legacy user management system into your user pool. If you choose the USER_PASSWORD_AUTH authentication flow, users don't have to reset their passwords during user migration. This flow sends your users' passwords to the service over an encrypted SSL connection during authentication.

When you have migrated all your users, switch flows to the more secure SRP flow. The SRP flow doesn't send any passwords over the network.

To learn more about Lambda triggers, see Customizing user pool workflows with Lambda triggers.

For more information about migrating users with a Lambda trigger, see Importing users into user pools with a user migration Lambda trigger.