Simple Membership <= 4.8.3 - Missing Authorization to Unauthenticated Account Takeover and Sensitive Information Disclosure via 'email' Parameter on Activation Endpoints

The Simple Membership plugin for WordPress is vulnerable to unauthorized modification of data and sensitive information disclosure in versions up to, and including, 4.8.3 via the resend-activation and email-activation endpoints. The endpoints are dispatched from SwpmInitTimeTasks::check_and_do_email_activation() on frontend init with no authentication, nonce, capability, or ownership check, and the recipient address used by SwpmRegistration::send_reg_email() is taken from an attacker-controlled $_POST['email'] parameter (overriding the member's registered address). This makes it possible for unauthenticated attackers to redirect an arbitrary pending member's activation email — and the follow-up 'registration complete' email containing the member's username and plaintext password — to an attacker-chosen address, and to then activate that member's account without their consent.