The File Manager WordPress plugin before 8.0.5, FileOrganizer WordPress plugin before 1.2.1, File Manager Pro WordPress plugin before 2.1.3 do not correctly validate the origin of window messages received by the file browser they load on their admin screens, accepting any origin that is a leading string prefix of the site's own address, which allows an unauthenticated attacker to run arbitrary JavaScript in the session of a logged-in administrator who visits a page under their control.
The defect is in the file-manager library all three bundle, and every version below 2.1.70 carries it. Updating the bundled library closes it.