Impersonate user in AWS Cognito

Question

I have a file server that uses Cognito so users can access by authenticating themselves with basic authentication or the OAuth2.0 authorization code flow.

I'd like external apps to be able to authenticate themselves using the client credentials flow, and then be able to impersonate a user. Is there a way to do this with Cognito?

Answer 1

I was able to do this by creating custom lambdas for the Cognito triggers: Define Auth Challenge, Create Auth Challenge & Verify Auth Challenge.

My requirement was that I wanted my backend to use a secret to then get access & refresh tokens for any Cognito user.

Define Auth Challenge Lambda

exports.handler = async event => {
  if (
    event.request.session &&
    event.request.session.length >= 3 &&
    event.request.session.slice(-1)[0].challengeResult === false
  ) {
    // The user provided a wrong answer 3 times; fail auth
    event.response.issueTokens = false;
    event.response.failAuthentication = true;
  } else if (
    event.request.session &&
    event.request.session.length &&
    event.request.session.slice(-1)[0].challengeResult === true
  ) {
    // The user provided the right answer; succeed auth
    event.response.issueTokens = true;
    event.response.failAuthentication = false;
  } else {
    // The user did not provide a correct answer yet; present challenge
    event.response.issueTokens = false;
    event.response.failAuthentication = false;
    event.response.challengeName = 'CUSTOM_CHALLENGE';
  }
  return event;
};

Create Auth Challenge Lambda

exports.handler = async event => {
  if (event.request.challengeName == 'CUSTOM_CHALLENGE') {
    // The value set for publicChallengeParameters is arbitrary for our
    // purposes, but something must be set
    event.response.publicChallengeParameters = { foo: 'bar' };
  }
  return event;
};

Verify Auth Challenge Lambda

exports.handler = async event => {
  if (event.request.challengeName == 'CUSTOM_CHALLENGE') {
    // The value set for publicChallengeParameters is arbitrary for our
    // purposes, but something must be set
    event.response.publicChallengeParameters = { foo: 'bar' };
  }
  return event;
};

I was then able to use some JS, using amazon-cognito-identity-js, to provide the secret and get the tokens:

var authenticationData = {
  Username : 'username'
};
var authenticationDetails = new AmazonCognitoIdentity.AuthenticationDetails(authenticationData);
var poolData = {
  UserPoolId : '...', // Your user pool id here
  ClientId : '...' // Your client id here
};
var userPool = new AmazonCognitoIdentity.CognitoUserPool(poolData);
var userData = {
  Username : 'username',
  Pool : userPool
};
var cognitoUser = new AmazonCognitoIdentity.CognitoUser(userData);

cognitoUser.setAuthenticationFlowType('CUSTOM_AUTH');

cognitoUser.initiateAuth(authenticationDetails, {
  onSuccess: function(result) {
    // User authentication was successful
  },
  onFailure: function(err) {
    // User authentication was not successful
  },
  customChallenge: function(challengeParameters) {
    // User authentication depends on challenge response
    var challengeResponses = 'secret'
    cognitoUser.sendCustomChallengeAnswer(challengeResponses, this);
  }
});