How to remediate – Amazon Cognito User Enumeration

1. Introduction

Amazon Cognito User Enumeration is a vulnerability affecting Amazon Cognito, a cloud service for user authentication and management. It allows an unauthenticated attacker to potentially identify valid accounts on a target application by abusing the sign-in and sign-up features. This can lead to account targeting attacks like credential stuffing or brute force attempts. Confidentiality may be impacted through enumeration of usernames.

2. Technical Explanation

Amazon Cognito allows attackers to infer the existence of user accounts during sign-in and sign-up processes under certain conditions. The vulnerability stems from how Cognito handles errors when attempting to sign in or sign up with existing or non-existing usernames. An attacker can systematically test for account presence by observing the responses received. There is no specific CVE currently associated with this issue, but it’s tracked as a security concern within the AWS ecosystem (CWE-200). For example, an attacker could attempt to sign in with a list of common usernames and observe whether an error message indicates the user does not exist or if another response occurs.

• Root cause: Cognito’s default error handling reveals information about account existence during sign-in attempts.
• Exploit mechanism: An attacker iterates through potential usernames, attempting to sign in and analyzing the responses for indicators of valid accounts.
• Scope: Applications using Amazon Cognito User Pools are affected.

3. Detection and Assessment

You can confirm vulnerability by testing account enumeration during sign-in attempts. A thorough method involves automated scripting to test a large number of usernames.

• Quick checks: Verify the advanced security settings for your Cognito User Pool in the AWS Management Console.
• Scanning: No specific scanners are available, but custom scripts can be used to enumerate users.
• Logs and evidence: Review application logs for sign-in attempts and error messages related to user existence.

aws cognito-idp list-users --userPoolId {your_user_pool_id}

4. Solution / Remediation Steps

The following steps mitigate the Amazon Cognito User Enumeration vulnerability by enabling advanced security options and implementing custom registration handling.

4.1 Preparation

• No services need to be stopped for this remediation. A roll back plan involves disabling the `Prevent user existence errors` option if issues occur.

4.2 Implementation

1. Step 1: Enable the “Prevent user existence errors” advanced security option in your Cognito User Pool settings through the AWS Management Console. This prevents sign-in attempts from revealing whether a username exists.
2. Step 2: For sign-up enumeration mitigation, implement an AWS Lambda function to handle the registration process. The Lambda function should validate input and prevent direct exposure of account creation status.

4.3 Config or Code Example

Before

// Default Cognito User Pool settings with user existence errors enabled

After

// Cognito User Pool settings with "Prevent user existence errors" enabled.  Implement a Lambda function for sign-up handling.

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege limits the impact of potential exploitation, while input validation prevents malicious data from being processed. Secure defaults reduce the risk of misconfiguration.

• Practice 1: Implement least privilege principles for IAM roles associated with Cognito User Pools to restrict access and minimize potential damage.
• Practice 2: Use input validation on all user-provided data, including usernames and passwords, to prevent injection attacks and other malicious inputs.

4.5 Automation (Optional)

Automation is not directly applicable for enabling the security option but can be used for Lambda function deployment.

# Example Terraform snippet for deploying a Cognito Lambda trigger
resource "aws_lambda_function" "cognito_signup_trigger" {
  # ... configuration details ...
}

5. Verification / Validation

• Post-fix check: Attempt a sign-in with an invalid username. The response should not indicate whether the user exists or does not exist.
• Re-test: Repeat the detection steps from Section 3 to confirm account enumeration is no longer possible.
• Monitoring: Monitor application logs for unexpected error messages related to user authentication or registration.

aws cognito-idp initiateAuth --userPoolId {your_user_pool_id} --clientId {your_client_id} --authFlowType USER_SRP_AUTH --clientMetadata '{"username":"testuser"}'

6. Preventive Measures and Monitoring

Regular security baselines, pipeline checks, and asset management processes can help prevent this issue. Update your security baseline to include the “Prevent user existence errors” setting for Cognito User Pools. Add checks in CI/CD pipelines to validate Cognito configurations.

• Baselines: Include the “Prevent user existence errors” option in your AWS security baseline.
• Pipelines: Implement automated checks during deployment to ensure this setting is enabled and Lambda functions are correctly configured.
• Asset and patch process: Review Cognito User Pool configurations regularly as part of a standard asset management cycle.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Existing applications might need code changes if they depend on specific error responses from sign-in attempts.
• Roll back: Disable the “Prevent user existence errors” advanced security option in your Cognito User Pool settings through the AWS Management Console.

8. References and Resources

• Vendor advisory or bulletin: https://aws.amazon.com/cognito/
• NVD or CVE entry: No specific CVE currently exists for this issue.
• Product or platform documentation relevant to the fix: https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pool-managing-errors.html