DUBAI, United Arab Emirates—The guard in a control room at Iran’s notorious Evin prison springs to attention as one by one, monitors in front of him suddenly blink off and display something very different from the surveillance footage he had been watching. “Cyberattack,” the monitors flash. Other guards gather around, holding up their mobile phones and filming, or making urgent calls. ”General protest until the freedom of political prisoners” reads another line on the screens. An online account, purportedly by an entity describing itself as a group of hackers, shared footage of the incident, as well as parts of other surveillance video it seized, with The Associated Press. The alleged hackers said the release of the footage was an effort to show the grim conditions at the prison, known for holding political prisoners and those with ties abroad who are often used as bargaining chips in negotiations with the West. …