Wild Apricot Login <= 1.0.18 - Authentication Bypass via SSO Email-Collision Account Binding on Multisite

The Wild Apricot Login plugin for WordPress is vulnerable to Authentication Bypass via SSO Email-Collision Account Binding in all versions up to, and including, 1.0.18. When no WildApricot contact-ID mapping is found for an incoming SSO login, the plugin falls back to a network-global `get_user_by('email', ...)` lookup in `Module.php` to resolve a WordPress user, then `UserSynchronizer::syncWaContact()` binds and `Controller::login()` issues a session cookie via `wp_set_auth_cookie()` for that resolved user — both guarded only by `is_super_admin()` — without ever requiring proof of ownership of the matched WordPress account such as a password, local account-linking confirmation, inbox verification, or administrator approval. This makes it possible for attackers with custom-level WildApricot membership access to authenticate as an existing WordPress subsite administrator on a path-based Multisite network, obtaining a valid network-global authentication cookie for the victim without their credentials. Exploitation requires that the attacker control a WildApricot contact whose email address matches the target WordPress administrator's email — achievable via a public zero-fee membership application on the connected WildApricot organization — and that they complete the SSO flow on any subsite where Wild Apricot Login is active; because WordPress user identities and authentication cookies are network-global, the cookie issued on that subsite is equally valid on any other subsite where the victim user holds an administrator role.