What is a threat actor?

An intruder who gains network access by modifying software or exploiting software vulnerabilities.

A threat actor is an intruder who gains access to a network by modifying software or exploiting software vulnerabilities.

They can gain access through:

• Software vulnerabilities
• Hardware attacks
• Guessing usernames and passwords

Go deeper:

• Computer security — Wikipedia — the "attackers and motives" coverage shows the kinds of actor (criminal, insider, state) behind these intrusion methods.