Mark's coworker recently discovered that he can use the Linux su command to run programs as one of his peers. What type of attack is this?

Horizontal privilege escalation occurs when users at a similar level are able to use privileges or accounts belonging to peer users. Here, Mark's coworker is able to act as another similar user. Vertical privilege escalation focuses on greater privileges like administrative rights. Replay attacks re-send traffic to authenticate or otherwise repeat a transaction. Forgery attacks like cross-site request forgery (XSRF) leverage trusted sessions to allow malicious apps to take action due to the trust between a browser and a web application.