The Shipo plugin for WordPress is vulnerable to Authentication Bypass in versions 1.6 through 1.8.0. This is due to the `authenticate_rest_user()` function parsing the `LOGGED_IN_COOKIE` value and authenticating the named user without verifying the cookie's cryptographic signature. The function's intended `/wp-json/shipo/` route restriction can also be bypassed by placing that substring in the query string of an unrelated REST endpoint. This makes it possible for unauthenticated attackers who know an existing username to impersonate that user, including an administrator, and perform authorized actions through the REST API, including creating another administrator account.