The User Session Synchronizer plugin for WordPress is vulnerable to Authentication Bypass leading to Account Takeover in all versions up to, and including, 1.4.0. The `synchronize_session()` function, hooked on `init` and therefore executed on every request, performs no nonce, capability, or shared-secret validation against the attacker-supplied `ussync-key`, `ussync-token`, and `ussync-ref` parameters; when `ussync-key` references an unregistered slot, `get_option()` returns `false` for both the secret key and the domain list, causing the AES-256-CBC encryption key to degrade to the fully predictable `md5('')` and the referer allowlist to collapse to an empty-string match, while the AES IV is unconditionally hard-coded as `md5('another-secret')`. This makes it possible for unauthenticated attackers to supply a crafted request encrypting any known or guessable user email address in the `ussync-ref` parameter, causing the handler to call `wp_set_auth_cookie()` for the matched user and granting full authentication as that user — including administrators — with no prior knowledge of site secrets.

The User Session Synchronizer plugin for WordPress is vulnerable to Authentication Bypass leading to Account Takeover in all versions up to, and including, 1.4.0. The `synchronize_session()` function, hooked on `init` and therefore executed on every request, performs no nonce, capability, or shared-secret validation against the attacker-supplied `ussync-key`, `ussync-token`, and `ussync-ref` parameters; when `ussync-key` references an unregistered slot, `get_option()` returns `false` for both the secret key and the domain list, causing the AES-256-CBC encryption key to degrade to the fully predictable `md5('')` and the referer allowlist to collapse to an empty-string match, while the AES IV is unconditionally hard-coded as `md5('another-secret')`. This makes it possible for unauthenticated attackers to supply a crafted request encrypting any known or guessable user email address in the `ussync-ref` parameter, causing the handler to call `wp_set_auth_cookie()` for the matched user and granting full authentication as that user — including administrators — with no prior knowledge of site secrets.