At Clerk, our priority is to provide customers with safe, secure, and easy-to-deploy tools for user management and authentication. When it comes to authentication, each stage of the sign in or sign up flow is designed to minimize friction and get people using your application.

For example, if a user attempts to sign in with an identifier that does not match an existing account on your Clerk application, we inform the user that this identifier doesn't match an existing account. This immediate feedback fits the expectations of ordinary users, who may not remember how or whether they have signed up for your application.

Some of our customers also have a need to protect against user enumeration – when a malicious actor takes advantage of the fact that the error message discloses whether an account exists for a given identifier (like an email or phone number) to create a list of all of the accounts that exist within an application. We already offer all our customers protection against such attacks using a variety of rate limiting techniques.

However, some customers would prefer to remove the ability to determine whether an account exists entirely. Some examples of apps that might fall in this category are financial institutions concerned about targeted phishing attacks, or any website for which an existing account being associated with a given email or phone number is intended to be private to that user, such as perhaps a dating app. To accommodate these needs, we are excited to announce that a set of enhanced protections against user enumeration attacks can now be enabled in the Clerk Dashboard, under the Attack Protection page.

With Enumeration Protection enabled, users attempting to sign in or sign up will no longer receive feedback that reveals if their identifier matches an existing account. Instead, they will be advanced to the next stage of the sign in or sign up flow, but attempts to complete the sign in or sign up will be rejected if the account does not exist, in the same way they would be if the credential in the next step, for example, a password, was incorrect. This makes it such that Clerk's response is the same whether or not a user account already exists, enhancing your application's protection against user enumeration attacks.

User security is our priority, and we are happy to bring these opt-in, enhanced protections against user enumeration attacks to our customers who need them.