A few days ago, I was developing an application to store my records (PRs) while practicing sports such as CrossFit and Weightlifting. Everything seemed to be working correctly in what I called my production environment. However, at some point, a significant issue arose: someone managed to register with the same email address and create two almost identical users username !== Username.
This was a major inconvenience, as the person could log in with both users and would not have their information consistently.
The Problem
After a bit of research (I assumed AWS Cognito prevented this behavior automatically 😔), I discovered that a possible solution was to set the email as a sign-in alias. This required making some changes to the Cognito User Pool.
However, there was a problem: I already had registered users, and Cognito does not allow modifying sign-in aliases in an existing User Pool.
The Solution
Create a New User Pool
The solution was to create a new User Pool. Thus, my User Pool V2 was born (yes, a very original name). Once created, I tested and confirmed that:
- It was not possible to register a user with the same email.
- The system had to be
case-insensitive: a user must always be unique, regardless of uppercase or lowercase.
To handle the latter, I discovered Pre-Signup Triggers. These triggers allow you to run a Lambda Function before completing the signup process. In this function, I implemented the following logic:
import boto3
client = boto3.client('cognito-idp')
def lambda_handler(event, context):
user_pool_id = event['userPoolId']
username = event['userName'].lower() # Normalizes the username
email = event['request']['userAttributes']['email'].lower() # Normalizes the email
# Validate username uniqueness
response = client.list_users(
UserPoolId=user_pool_id,
Filter=f'username = "{username}"'
)
if len(response['Users']) > 0:
raise Exception("The username is already registered.")
# Validate email uniqueness
response = client.list_users(
UserPoolId=user_pool_id,
Filter=f'email = "{email}"'
)
if len(response['Users']) > 0:
raise Exception("The email is already registered.")
# Normalize username and email in the event
event['userName'] = username
event['request']['userAttributes']['email'] = email
return event
- Convert data to lowercase (username and email).
- Check if the user already exists.
- Update the event and return it.
With this, we ensure data consistency and prevent duplicate users, regardless of uppercase or lowercase usage.
Deployment with AWS CDK
To deploy this solution, I used AWS CDK with TypeScript. An example configuration might look like this:
import { Construct } from "constructs";
import { addTags } from "../common/utilities/addTag";
import { RetentionDays } from "aws-cdk-lib/aws-logs";
import { StringParameter } from "aws-cdk-lib/aws-ssm";
import { PolicyStatement,Effect } from "aws-cdk-lib/aws-iam";
import { CfnOutput,NestedStack,NestedStackProps} from "aws-cdk-lib";
import { PROJECT_NAME,ENVIRONMENT, COGNITO_CALLBACK_URL } from "../../constants";
import { Code, Function, ILayerVersion, LayerVersion, Runtime } from "aws-cdk-lib/aws-lambda";
import { UserPool,AccountRecovery,VerificationEmailStyle,ClientAttributes,UserPoolClient,OAuthScope } from "aws-cdk-lib/aws-cognito";
export class UserPoolStack extends NestedStack {
private projectEnv: string;
public readonly userPoolV2: UserPool;
public readonly userPoolClientV2: UserPoolClient;
constructor(scope: Construct,id: string,props?: NestedStackProps) {
super(scope,id,props);
const REGION = this.region;
this.projectEnv = `${PROJECT_NAME}-${ENVIRONMENT}`;
const callbackURl = (COGNITO_CALLBACK_URL) ? COGNITO_CALLBACK_URL : 'http://localhost:3000';
this.userPoolV2 = new UserPool(this, `${this.projectEnv}-user-pool-v2`, {
selfSignUpEnabled: ENVIRONMENT === 'prod',
accountRecovery: AccountRecovery.EMAIL_ONLY,
userVerification: {
emailStyle: VerificationEmailStyle.CODE,
},
autoVerify: {
email: true,
},
standardAttributes: {
email: {
required: true,
mutable: true,
},
},
signInAliases: {
username: true,
email: true,
}
});
const clientWritteAttributes = new ClientAttributes()
.withStandardAttributes({
fullname: true,
email: true,
nickname: true,
profilePicture: true,
});
const clientReadAttributes = new ClientAttributes()
.withStandardAttributes({
emailVerified: true,
email: true,
});
this.userPoolClientV2 = new UserPoolClient(this,`${this.projectEnv}-user-pool-client-v2`, {
userPool: this.userPoolV2,
authFlows: {
userPassword: true,
userSrp: true,
},
generateSecret: true,
preventUserExistenceErrors: true,
writeAttributes: clientWritteAttributes,
readAttributes: clientReadAttributes,
oAuth: {
flows: {
authorizationCodeGrant: true,
},
scopes: [OAuthScope.EMAIL,OAuthScope.OPENID,OAuthScope.PROFILE],
callbackUrls: [`${callbackURl}/auth`],
logoutUrls: [`${callbackURl}/logout`],
},
});
this.userPoolV2.addDomain(`${PROJECT_NAME}-domain`,{
cognitoDomain: { domainPrefix: `${this.projectEnv}-v2` }
});
new CfnOutput(this,'CognitoUspLoginUrlV2',{
value: `https://${this.projectEnv}-v2.auth.${REGION}.amazoncognito.com/login?response_type=code&client_id=${this.userPoolClientV2.userPoolClientId}&redirect_uri=${callbackURl}/auth`
});
new CfnOutput(this,'CognitoURLV2',{
value: `https://${this.projectEnv}.auth.${REGION}.amazoncognito.com`
});
addTags(this.userPoolV2,'project',`${PROJECT_NAME}`);
this.createCognitoPreSignUpLambdaFunction();
}
private createCognitoPreSignUpLambdaFunction(): void {
const lambdaFunction = new Function(this, 'cognitoPreSignUp', {
runtime: Runtime.PYTHON_3_11,
functionName: 'cognito-pre-sign-up',
handler: 'src.handler.lambda_handler',
code: Code.fromAsset('./services/cognito_pre_sign_up'),
environment: {
LOG_LEVEL: `${ENVIRONMENT === 'prod' ? 'INFO' : 'DEBUG'}`,
},
logRetention: ENVIRONMENT === 'prod' ? RetentionDays.ONE_YEAR : RetentionDays.ONE_DAY,
});
// least privileges principle
const lambdaPolicy = new PolicyStatement({
actions: ['cognito-idp:ListUsers'],
resources: [this.userPoolV2.userPoolArn],
effect: Effect.ALLOW,
});
lambdaFunction.addToRolePolicy(lambdaPolicy);
const layerArn = StringParameter.valueForStringParameter(this, `${PROJECT_NAME}-${ENVIRONMENT}-lambda-layer-parameter`);
const newLayerVersion: ILayerVersion = LayerVersion.fromLayerVersionArn(this, 'lambda-layer-common', layerArn);
lambdaFunction.addLayers(newLayerVersion);
lambdaFunction.node.addDependency(this.userPoolV2);
addTags(lambdaFunction, 'project', `${PROJECT_NAME}`);
}
}
Migrating Existing Users
Now another question arises: what to do with the users who were already registered in the previous User Pool?
In my case, there were only 5 users, so I chose the simplest solution:
- Manually re-invite each one with their new data.
- Manually update the userIDs in the DynamoDB table.
Conclusion
With this solution, I managed to solve the user duplication issues and ensure consistent data management. I also learned that:
- Setting the email as the sign-in alias is crucial to avoid similar issues.
- Cognito’s Pre-Signup Triggers offer great flexibility to customize registration logic.
- Planning and testing are essential, especially when there are already users in production.
Now the system is more robust and ready to scale without duplication issues.
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