The Masteriyo LMS WordPress plugin before 3.4.1 does not prevent user-supplied values held as metadata from being deserialized when they are read back, allowing users with a minimal account to inject arbitrary PHP objects and, by way of a class shipped in a library bundled with the Masteriyo LMS WordPress plugin before 3.4.1, write and execute arbitrary code on the server. A weaker form of the same issue is reachable without an account and yields an arbitrary file write rather than code execution.