The Ultimate Multisite – WordPress Multisite SaaS & WaaS Platform plugin for WordPress is vulnerable to Authentication Bypass in all versions up to, and including, 2.15.0 via the checkoutform parameter of the logincustomeraftercheckout function. This is due to the publicly accessible wuajaxnoprivwuvalidateform AJAX handler accepting a freely obtainable checkout nonce, and the checkoutform=wu-finish-checkout parameter causing getvalidationrules() to discard all validation rules while finishcheckoutformfields() returns an empty step list — forcing islaststep() to return true and routing the request directly into full order processing — after which maybecreatecustomer() resolves the attacker-supplied emailaddress to an existing WordPress user ID without any authentication or ownership verification, and logincustomeraftercheckout() calls wpsetauthcookie() for that user ID via a passwordless code path. This makes it possible for unauthenticated attackers to log in as any existing WordPress user — including a Network Super Admin — simply by knowing their email address. Exploitation requires that the targeted user account has no pre-existing Ultimate Multisite customer record; accounts such as a Network Super Admin on a fresh Multisite install, or any administrator or editor added before Ultimate Multisite was configured, satisfy this condition and are therefore exploitable.