Advanced Custom Fields: Extended < 0.9.2.8 - Unauthenticated RCE via Front-End Post 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 perform any capability check when updating a post through one of its front-end form actions, allowing unauthenticated visitors to overwrite arbitrary posts, including the ones in which the site stores its own field definitions, which in turn leads to arbitrary code execution on the server. Exploitation requires the site to publish a form whose post-update action maps both its target and its content to visitor-submitted values.