CVE-2026-103883: 6Storage Rentals <= 2.31.0 - Authenticated (Subscriber+) Authentication Bypass to Administrator Account Takeover via 'six_storage_online_api_url' Option Poisoning + Transient-Backed Auth Bypass

The 6Storage Rentals plugin for WordPress is vulnerable to Authentication Bypass leading to Administrator Account Takeover in all versions up to, and including, 2.31.0. The vulnerability exists because the sixstorageportalregistration() AJAX handler — registered on wpajaxsixstorageportalpluginregistration — performs no capability check and no nonce verification before writing the caller-supplied sixstorageonlineapiurl POST parameter to the site-wide option of the same name; the unauthenticated sixstoragecreatewpuser() handler then redeems a transient seeded from that attacker-controlled URL's response and unconditionally calls wpsetcurrentuser() and wpsetauthcookie() with no ownership or credential verification. This makes it possible for authenticated attackers, with Subscriber-level access and above, to authenticate as any existing WordPress user — including any administrator whose email address is known or guessable — resulting in full site compromise. This issue represents an incomplete fix for a prior vulnerability: the 2.29.0 remediation moved the target email out of POST data into a server-side transient, but because any Subscriber can still overwrite the outbound API URL that populates that transient, the full authentication bypass chain remains exploitable in three AJAX requests.