AccessDenied: Not authorized to perform sts:AssumeRoleWithWebIdentity

Question

I've looked around similar problems, but couldn't resolve my problem. I'm developing an web application where the user will authenticate using AWS Cognito's authentication. The sign up part is ok, but when I try to sign in, I'm getting the "not authorized" exception. I've already tried to attach custom policies to my IAM Role (authorizing sts:AssumeRoleWithWebIdentity), but didn't work.. Here is how the code is written right now:

        var cognitoUser = new AWSCognito.CognitoIdentityServiceProvider.CognitoUser(userData);
cognitoUser.authenticateUser(authenticationDetails, {
    onSuccess: function (result) {
        var sts = new AWS.STS({apiVersion: '2011-06-15'});

        var params = {
            RoleArn: 'arn:aws:iam::981601120657:role/Cognito_AliceAuth_Role', /* required */
            RoleSessionName: 'AliceUserSession', 
            WebIdentityToken: result.getIdToken().getJwtToken(), 
            Policy: '{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": "sts:AssumeRoleWithWebIdentity", "Resource": "*" } ] }'
        };

        sts.assumeRoleWithWebIdentity(params, function (err, data) {
            if (err)
                console.log(err, err.stack); // ** <-- ERROR HERE
            else
                console.log(data);           // successful response
        });

        //document.getElementById('authForm').submit();
    },
    onFailure: function (err) {
        alert(err);
    }

});

As you can see, I specified the policy in the code too, but I still get the "AccessDenied: Not authorized to perform sts:AssumeRoleWithWebIdentity" error. Please help me :/

EDIT:

Inside the "Cognito_AliceAuth_Role" I've created the role policies: AssumeRoleWithWebIdentityPolicy:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sts:AssumeRoleWithWebIdentity",
            "Resource": "*"
        }
    ]
}

and: GetFederationTokenPolicy

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sts:GetFederationToken",
            "Resource": "*"
        }
    ]
}

The trust relationship:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Federated": "cognito-identity.amazonaws.com"
      },
      "Action": "sts:AssumeRoleWithWebIdentity",
      "Condition": {
        "StringEquals": {
          "cognito-identity.amazonaws.com:aud": "us-east-1:e4c1833d-a62b-402a-b995-1b2513b04c02"
        },
        "ForAnyValue:StringLike": {
          "cognito-identity.amazonaws.com:amr": "authenticated"
        }
      }
    }
  ]
}

Answer 1

Seems like you are using the Id token vended by Cognito user pools to call the assumeRoleWithWebIdentity.

You need to federate this token with Cognito identity first and you can use the Open Id connect token vended by Cognito identity to call assumeRoleWithWebIdentity. You can directly call getCredentialsForIdentity as well using Enhanced flow.

See this to learn more about how to federate user pools token with Cognito identity.