Define Auth challenge Lambda trigger

Define auth challenge

Amazon Cognito invokes this trigger to initiate the custom authentication flow.

The request for this Lambda trigger contains session. The session parameter is an array that contains all of the challenges that are presented to the user in the current authentication process. The request also includes the corresponding result. The session array stores challenge details (ChallengeResult) in chronological order. The challenge session[0] represents the first challenge that the user receives.

You can have Amazon Cognito verify user passwords before it issues your custom challenges. Here is an overview of the process:

1. Your app initiates sign-in by calling InitiateAuth or AdminInitiateAuth with the AuthParameters map. Parameters must include CHALLENGE_NAME: SRP_A, and values for SRP_A and USERNAME.

2. Amazon Cognito invokes your define auth challenge Lambda trigger with an initial session that contains challengeName: SRP_A and challengeResult: true.

3. After receiving those inputs, your Lambda function responds with challengeName: PASSWORD_VERIFIER, issueTokens: false, failAuthentication: false.

4. If the password verification succeeds, Amazon Cognito invokes your Lambda function again with a new session containing challengeName: PASSWORD_VERIFIER and challengeResult: true.

5. To initiate your custom challenges, your Lambda function responds with challengeName: CUSTOM_CHALLENGE, issueTokens: false, and failAuthentication: false. If you don't want to start your custom auth flow with password verification, you can initiate sign-in with the AuthParameters map including CHALLENGE_NAME: CUSTOM_CHALLENGE.

6. The challenge loop repeats until all challenges are answered.

Define Auth challenge Lambda trigger parameters

These are the parameters that Amazon Cognito passes to this Lambda function along with the event information in the common parameters.

JSON

{
    "request": {
        "userAttributes": {
            "string": "string",
                . . .
        },
        "session": [
            ChallengeResult,
            . . .
        ],
        "clientMetadata": {
            "string": "string",
            . . .
        },
        "userNotFound": boolean
    },
    "response": {
        "challengeName": "string",
        "issueTokens": boolean,
        "failAuthentication": boolean
    }
}

Define Auth challenge request parameters

When Amazon Cognito invokes your Lambda function, Amazon Cognito provides the following parameters:

userAttributes

One or more name-value pairs that represent user attributes.

userNotFound

A Boolean that Amazon Cognito populates when PreventUserExistenceErrors is set to ENABLED for your user pool client. A value of true means that the user id (user name, email address, etc.) did not match any existing users. When PreventUserExistenceErrors is set to ENABLED, the service doesn't inform the app of nonexistent users. We recommend that your Lambda functions maintain the same user experience and account for latency. This way, the caller can't detect different behavior when the user exists or doesn’t exist.

session

An array of ChallengeResult elements. Each contains the following elements:

challengeName

One of the following challenge types: CUSTOM_CHALLENGE, SRP_A, PASSWORD_VERIFIER, SMS_MFA, DEVICE_SRP_AUTH, DEVICE_PASSWORD_VERIFIER, or ADMIN_NO_SRP_AUTH.

Important

When your function determines if a user has successfully authenticated and you should issue them tokens, always check challengeName in your DefineAuthChallenge Lambda trigger to make sure it matches the expected value.

challengeResult

Set to true if the user successfully completed the challenge, or false otherwise.

challengeMetadata

Your name for the custom challenge. Used only if challengeName is CUSTOM_CHALLENGE.

clientMetadata

One or more key-value pairs that you can provide as custom input to the Lambda function that you specify for the define auth challenge trigger. To pass this data to your Lambda function, you can use the ClientMetadata parameter in the AdminRespondToAuthChallenge and RespondToAuthChallenge API operations. The request that invokes the define auth challenge function doesn't include data passed in the ClientMetadata parameter in AdminInitiateAuth and InitiateAuth API operations.

Define Auth challenge response parameters

In the response, you can return the next stage of the authentication process.

challengeName

A string that contains the name of the next challenge. If you want to present a new challenge to your user, specify the challenge name here.

issueTokens

If you determine that the user has completed the authentication challenges sufficiently, set to true. If the user has not met the challenges sufficiently, set to false.

failAuthentication

If you want to end the current authentication process, set to true. To continue the current authentication process, set to false.

Define Auth challenge example

This example defines a series of challenges for authentication and issues tokens only if the user has completed all of the challenges successfully.

Node.js

const handler = async (event) => {
  if (
    event.request.session.length == 1 &&
    event.request.session[0].challengeName == "SRP_A"
  ) {
    event.response.issueTokens = false;
    event.response.failAuthentication = false;
    event.response.challengeName = "PASSWORD_VERIFIER";
  } else if (
    event.request.session.length == 2 &&
    event.request.session[1].challengeName == "PASSWORD_VERIFIER" &&
    event.request.session[1].challengeResult == true
  ) {
    event.response.issueTokens = false;
    event.response.failAuthentication = false;
    event.response.challengeName = "CUSTOM_CHALLENGE";
  } else if (
    event.request.session.length == 3 &&
    event.request.session[2].challengeName == "CUSTOM_CHALLENGE" &&
    event.request.session[2].challengeResult == true
  ) {
    event.response.issueTokens = false;
    event.response.failAuthentication = false;
    event.response.challengeName = "CUSTOM_CHALLENGE";
  } else if (
    event.request.session.length == 4 &&
    event.request.session[3].challengeName == "CUSTOM_CHALLENGE" &&
    event.request.session[3].challengeResult == true
  ) {
    event.response.issueTokens = true;
    event.response.failAuthentication = false;
  } else {
    event.response.issueTokens = false;
    event.response.failAuthentication = true;
  }

  return event;
};

export { handler }