CVE-2026-75957

Overview

The Ultimate Multisite WordPress plugin is vulnerable to authentication bypass via a public AJAX endpoint. Unauthenticated attackers can log in as any existing user, including Network Super Admins, by supplying an email address. The flaw exists in all versions up to 2.15.0.

Description

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 `checkout_form` parameter of the `login_customer_after_checkout` function. This is due to the publicly accessible `wu_ajax_nopriv_wu_validate_form` AJAX handler accepting a freely obtainable checkout nonce, and the `checkout_form=wu-finish-checkout` parameter causing `get_validation_rules()` to discard all validation rules while `finish_checkout_form_fields()` returns an empty step list — forcing `is_last_step()` to return true and routing the request directly into full order processing — after which `maybe_create_customer()` resolves the attacker-supplied `email_address` to an existing WordPress user ID without any authentication or ownership verification, and `login_customer_after_checkout()` calls `wp_set_auth_cookie()` 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.

Impact

Confidentiality: attackers can access any user account. Integrity: they can modify site content or settings. Availability: potential for site takeover. Network Super Admins, administrators, and editors on multisite installations are at risk.

Remediation

Update Ultimate Multisite to version 2.15.1 or later. If an update is not possible, disable the wu_ajax_nopriv_wu_validate_form AJAX handler or restrict it to authenticated users. Ensure all user accounts have a customer record in Ultimate Multisite before enabling the plugin. Monitor authentication logs for unusual login activity.

Risk context

Severity is critical (CVSS 9.8). No EPSS data is available. Immediate action is recommended to prevent potential takeover of multisite networks.

Affected products

• Ultimate Multisite WordPress plugin
• WordPress Multisite SaaS
• WordPress Multisite WaaS Platform