Advanced Custom Fields: Extended 0.9.2.7 - Unauthenticated Administrator Account Takeover via Front-End User Update Action

The Advanced Custom Fields: Extended WordPress plugin before 0.9.2.8, Advanced Custom Fields: Extended PRO WordPress plugin before 0.9.2.8 does not resolve the target of the update-user action in its front-end Forms module consistently between the stage that authorises the request and the stage that carries it out, which allows unauthenticated visitors to bypass the guard protecting administrator accounts, take over the site administrator's account and be issued a valid session for it. Exploitation requires the site to publish two separate front-end form configurations whose targets are mapped to visitor-submitted values.