The ShiftController Employee Shift Scheduling plugin is vulnerable to PHP Object Injection via deserialization of untrusted input via the `hc3_session`-cookie in versions up to, and including, 4.9.57. This makes it possible for an authenticated attacker with contributor access-level or above to inject a PHP Object. No POP chain is present in the vulnerable plugin. If a POP chain is present via an additional plugin or theme installed on the target system, it could allow the attacker to delete arbitrary files, retrieve sensitive data, or execute code.