To prevent user enumeration on a registration page, you need to ensure that your application doesn’t leak any information that reveals whether a particular email or username is already registered.
1. Standardized Responses
Ensure that your server returns the same response for both existing and non-existing users when they attempt to register.
Example:
Instead of saying "Email is already in use", return a generic message like:
If this email is not registered, an account will be created.
2. Rate Limiting and CAPTCHA
Implement rate limiting and CAPTCHA to prevent attackers from automating user enumeration attempts.
Why it works: Slows down brute-force attempts and discourages attackers from enumerating users at scale.
3. Delayed Responses
Introduce a small random delay in your server’s response time to make it harder for attackers to distinguish responses based on timing.
Example: Use a delay of 100-300ms, regardless of whether the user exists.
4. Generic Email Confirmation Workflow
When sending email confirmations for registration, always indicate that a confirmation email might be sent, without specifying whether the account already exists.
Example Email Message:
If this email is associated with an account, you'll receive further instructions shortly.
5. Server-Side Validation
Do not expose details about account existence in the client-side validation. Always validate on the server and ensure the client-side scripts don’t provide hints about existing users.
6. Secure Logging Practices
Avoid logging registration attempts with specific details about user existence, as these logs could be exploited if leaked.
