Post authentication Lambda trigger

The post authentication trigger doesn't change the authentication flow for a user. Amazon Cognito invokes this Lambda after authentication is complete, before a user has received tokens. Add a post authentication trigger when you want to add custom post-processing of authentication events, for example logging or user profile adjustments that will be reflected on the next sign-in.

A post authentication Lambda that doesn't return the request body to Amazon Cognito can still cause authentication to fail to complete. For more information, see Important considerations.

Authentication flow overview

For more information, see User pool authentication flow.

Post authentication Lambda trigger parameters

The request that Amazon Cognito passes to this Lambda function is a combination of the parameters below and the common parameters that Amazon Cognito adds to all requests.

JSON

{
    "request": {
        "userAttributes": {
             "string": "string",
             . . .
         },
         "newDeviceUsed": boolean,
         "clientMetadata": {
             "string": "string",
             . . .
            }
        },
    "response": {}
}

Post authentication request parameters

newDeviceUsed

This flag indicates if the user has signed in on a new device. Amazon Cognito only sets this flag if the remembered devices value of the user pool is Always or User Opt-In.

userAttributes

One or more name-value pairs representing user attributes.

clientMetadata

One or more key-value pairs that you can provide as custom input to the Lambda function that you specify for the post authentication trigger. To pass this data to your Lambda function, you can use the ClientMetadata parameter in the AdminRespondToAuthChallenge and RespondToAuthChallenge API actions. Amazon Cognito doesn't include data from the ClientMetadata parameter in AdminInitiateAuth and InitiateAuth API operations in the request that it passes to the post authentication function.

Post authentication response parameters

Amazon Cognito doesn't expect any additional return information in the response. Your function can use API operations to query and modify your resources, or record event metadata to an external system.

Authentication tutorials

Immediately after Amazon Cognito signs in a user, it activates the post authentication Lambda function. See these sign-in tutorials for JavaScript, Android, and iOS.

Platform	Tutorial
JavaScript Identity SDK	Sign in users with JavaScript
Android Identity SDK	Sign in users with Android
iOS Identity SDK	Sign in users with iOS

Post authentication example

This post authentication sample Lambda function sends data from a successful sign-in to CloudWatch Logs.

Node.js

const handler = async (event) => {
  // Send post authentication data to Amazon CloudWatch logs
  console.log("Authentication successful");
  console.log("Trigger function =", event.triggerSource);
  console.log("User pool = ", event.userPoolId);
  console.log("App client ID = ", event.callerContext.clientId);
  console.log("User ID = ", event.userName);

  return event;
};

export { handler };

Python

import os
def lambda_handler(event, context):

    # Send post authentication data to Cloudwatch logs
    print ("Authentication successful")
    print ("Trigger function =", event['triggerSource'])
    print ("User pool = ", event['userPoolId'])
    print ("App client ID = ", event['callerContext']['clientId'])
    print ("User ID = ", event['userName'])

    # Return to Amazon Cognito
    return event

Amazon Cognito passes event information to your Lambda function. The function then returns the same event object to Amazon Cognito, with any changes in the response. In the Lambda console, you can set up a test event with data that is relevant to your Lambda trigger. The following is a test event for this code sample:

JSON

{
  "triggerSource": "testTrigger",
  "userPoolId": "testPool",
  "userName": "testName",
  "callerContext": {
      "clientId": "12345"
  },
  "response": {}
}