Ultimate Multisite <= 2.16.0 - Unauthenticated Arbitrary Account Takeover
Strategic Overview
- Status
- Patched in 2.17.0
- Affected Version
<= 2.16.0- CVSS
- 7.3High
- Weakness type
- CWE-287 · Improper Authentication
- CVE
CVE-2026-103646
At a glance
CVE-2026-103646 is a high-severity Improper Authentication vulnerability in the Ultimate Multisite WordPress plugin, affecting versions <= 2.16.0. It carries a CVSS score of 7.3 (reachable over the network; low attack complexity). Exploitation requires no authentication. The issue is fixed in version 2.17.0; sites on affected versions should update now. Disclosed October 2026, reported by Erwan LR.
Vulnerability Overview
The Ultimate Multisite plugin for WordPress is vulnerable to Account Takeover in all versions up to, and including, 2.16.0. This is due to missing email normalization before the duplicate-account lookup combined with the absence of a require_new_user guard, allowing a submitted email to be matched to and linked with an existing WordPress user account without authentication. This makes it possible for unauthenticated attackers to log in as any existing WordPress user, including a Network Super Admin, by submitting their known email address through a passwordless checkout form and having the plugin link the unauthenticated checkout session to that existing account. A checkout form must be configured without a password field (auto-generated password mode), and the targeted WordPress account must have no existing customer record in the plugin.
Technical Analysis
The vector marks this flaw as remotely reachable over the network, with low attack complexity — no special timing or configuration is needed, and no privileges on the target site, and no interaction from a victim user.
CWE-287: Improper Authentication
Reaching this weakness in Ultimate Multisite <= 2.16.0 takes no account at all. Improper authentication means the mechanism that proves who a caller is can be satisfied without the secret it was supposed to require.
An attacker authenticates as another user — administrators included — without ever knowing a password, so password policies and login rate limits never come into play. For Ultimate Multisite the fix is 2.17.0: builds <= 2.16.0 are affected, anything from 2.17.0 onward is not.
Remediation
Update to version 2.17.0, or a newer patched version
How does WordSec protect against this?
This one needs no account at all, which puts it outside what login hardening can reach; WordSec's login security narrows the account-level paths around it, and the firewall is what inspects the request itself. None of that substitutes for the fix: Ultimate Multisite 2.17.0 closes this, and updating the plugin is the step that ends it.
- Login Security
- Alerts
External References
Related records
Same weakness class
Other vulnerabilities in Ultimate Multisite – WordPress Multisite SaaS & WaaS Platform
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