What is the best way to prevent user enumeration when updating email addresses

Question

In scenarios where users can change their email addresses, how can I prevent attackers from exploiting this functionality to enumerate existing accounts? What practices or frameworks are recommended to address this?

Answer 1

Ensuring that the system does not leak data regarding whether an email address is already linked with an account can help to prevent user enumeration during email address updating.

1. Generic Responses

• Always return the same response for valid and invalid email addresses during the update process.
• Example: Instead of saying “This email is already registered,” respond with a generic message like “If this email is valid, further instructions will be sent.”

2. Rate-Limiting and CAPTCHAs

• Rate-limit requests to prevent brute-force attacks that try to enumerate existing accounts.
• Use CAPTCHAs for repeated requests to verify human interaction.

3. Email-Based Verification

• Require the user to verify ownership of the new email address by sending a confirmation link. This avoids confirming the existence of the email directly.
• Don’t disclose whether the email is already in use—just proceed with the verification process regardless.

4. Use Case-Insensitive Checks

• Ensure email uniqueness checks are case-insensitive to avoid creating accidental enumeration vulnerabilities.

5. Logging and Monitoring

• Monitor logs for repeated attempts to check different email addresses.
• Use automated tools to detect patterns indicative of enumeration attempts.

6. Framework-Level Configurations

• Leverage secure identity frameworks (like OAuth or OpenID) that handle email validation securely.
• Frameworks often include features to mitigate enumeration risks out of the box.

Implementation Example

1. User Request to Change Email:
   The user enters a new email address. The system does not confirm if the email exists.

2. Send Verification:
   A generic confirmation email is sent with a link. For example:
   “If this email is valid, you’ll receive a confirmation link shortly.”

3. Process Email Confirmation:
   After the user clicks the confirmation link, verify the ownership and update the email if the address is valid.

By following these practices, attackers cannot determine whether a specific email address is already associated with an account, mitigating user enumeration risks.