An attacker has managed to gain access to one of iscc video

Full Certification Question

An attacker has managed to gain access to one of the file servers within an organization using a social engineering technique. Now that he has access to the file server, he has planted a backdoor so that he can return regularly and collect data without the organization knowing. He has had this access for several months and has stayed under the radar so that the organization can't detect him. What type of threat is being described here?