Grav Login Plugin before 3.9.1 Email Enumeration via Registration

The Grav Login plugin (getgrav/grav-plugin-login) before 3.9.1 is vulnerable to email address enumeration. The register() method in classes/Login.php throws a distinct exception (EMAIL_NOT_AVAILABLE) when a submitted email address already belongs to an existing account, while allowing registration to proceed otherwise. Because the registration endpoint has no rate limiting, an attacker can enumerate which email addresses have accounts on the site, one guess per request.