Is it possible to modify AWS Cognito user attributes in the Lambda triggers

Question

Having a look at the AWS documentation,

https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-identity-pools-working-with-aws-lambda-triggers.html#cogni

Answer 1

For anyone else looking into insight on this question, here is an example below

The lambda function #1 below takes into two custom attributes ida and ethaddress. The lambda is invoked during the PreSignUpHook for Cognito user pools

#2 (Before event changed logs) the original values for these attributes is ida=1 and ethaddress=ABCD

#3 (After event changed logs) reflects the changed values of these attributes: ida=2 and ethaddress=EFGH

However the values which are saved to cognito are the original ones: ida=1 and ethaddress=ABCD. Therefore updateing the userAttributes during the presignuphook does NOT work as suggested in some of the answers.

On a side note when the predefined attributes in the response object are modified they are updated as expected:

"response": {
    "autoConfirmUser": true,
    "autoVerifyEmail": false,
    "autoVerifyPhone": false
}

1. LAMBDA:

'use strict';
global.fetch = require('node-fetch')

module.exports.preSignUp = async (event, context, callback) => {
// Set the user pool autoConfirmUser flag after validating the email domain

let data = await fetch("http://***.***.***/api/members/create",
{
    headers: {
      'Accept': 'application/json',
      'Content-Type': 'application/json'
    },
    method: "POST",
})
.then(res => res.json())
.then(res => res);

event.response.autoConfirmUser = true;
console.log('before event:', JSON.stringify(event)); 
event.request.userAttributes['custom:ethaddress'] = String(data.address); 
event.request.userAttributes['custom:ida'] = "2";  
console.log('Received event:', JSON.stringify(event));  
console.log('Address:', data.address);


 // Return to Amazon Cognito
callback(null, event);
 };

2.

BEFORE EVENT CHANGE LOG:

2019-01-20T01:02:24.639Z    edce636e-75ea-492b-b6a0-dd4f22dc9038    before event:
{
    "version": "1",
    "region": "us-east-1",
    "userPoolId": "us-east-1-*****",
    "userName": "*******@gmail.com",
    "callerContext": {
        "awsSdkVersion": "aws-sdk-unknown-unknown",
        "clientId": "******************"
    },
    "triggerSource": "PreSignUp_SignUp",
    "request": {
        "userAttributes": {
            "custom:ida": "1",
            "custom:ethaddress": "ABCD",
            "email": "*******@gmail.com"
        },
        "validationData": {}
    },
    "response": {
        "autoConfirmUser": true,
        "autoVerifyEmail": false,
        "autoVerifyPhone": false
    }
}

3 .

AFTER EVENT CHANGE LOG:

Received event:
{
    "version": "1",
    "region": "us-east-1",
    "userPoolId": "us-east-1_0BaE6eaTY",
    "userName": "*******@gmail.com",
    "callerContext": {
        "awsSdkVersion": "aws-sdk-unknown-unknown",
        "clientId": "*****************"
    },
    "triggerSource": "PreSignUp_SignUp",
    "request": {
        "userAttributes": {
            "custom:ida": "2",
            "custom:ethaddress": "EFGH",
            "email": "*******@gmail.com"
        },
        "validationData": {}
    },
    "response": {
        "autoConfirmUser": true,
        "autoVerifyEmail": false,
        "autoVerifyPhone": false
    }
}

---

UPDATE:

It seems like there is no way to do this as a part of the PRESIGNUP process However it is possible to do this as a POSTCONFIRMATION trigger in cognito example provided below.

Some things to watch out for.

1. the custom attribute has added in cognito and is mutable.
2. in App client --> show details --> "Set attribute read and write permission" Ensure there is below read and write perms on the custom attribute.
3. Ensure that the lambda function has a ROLE which allows it to execute: adminUpdateUserAttributes E.g. Attach AmazonCognitoPowerUser Policy to the LambaRole.

module.exports.postConfirmation = async (event, context,callback) => {
        const cognitoIdServiceProvider = new CognitoIdentityServiceProvider({
          region: 'us-east-1'
        });

        var params =  {
            UserAttributes: [
              {
                  Name: 'custom:sillyName',
                  Value: 'customSillyName'
              }
            ],
            UserPoolId: event.userPoolId,
            Username: event.userName
        }

        cognitoIdServiceProvider.adminUpdateUserAttributes(params, function(err, data) {
          if (err) console.log(err, err.stack); // an error occurred
          else     console.log(data);           // successful response
        }); 

        callback(null,event);

};

Note if you attempt to user cognitoIdServiceProvider.adminUpdateUserAttributes in the preSignUp trigger hook you;ll get an exception saying the user does not exit yet