Hackers have attacked Harvard’s computer systems a multitude of times over recent weeks, resulting in a printer server downtime for a day last week and several hours of reduced Internet access on campus Tuesday, according to Director of Harvard Arts and Sciences Computer Services Franklin M. Steen.

Steen said the recent attacks had been traced to Brazilian hackers, which meant it could be more difficult to pursue legal remedies because of the need to deal with foreign authorities.

He said most of the attacks were “denial of service attacks,” in which a hacker searches for a vulnerable computer on a network and floods the system with activity. This influx can slow down and even temporarily shut down a computer network.

“If the attacks are intensive, which they have been, it slows down the network to the point where things don’t work,” Steen said.

He said it’s often difficult to prevent such attacks because setting up firewalls—which theoretically could prevent some or most hacking attempts—also restrict access for those operating their computers within the network.

“As a University, we’re not generally setting up firewalls,” he said. “Firewalls, as much as protecting, can also slow down things and they can keep things out that you probably want. Universities generally have an open environment.”

Steen said the attacks made it necessary to reboot the core router—the mechanism through which multiple computers are connected to one another on a network—on Tuesday, which caused e-mail to go down for several hours that afternoon and kept some Harvard-based computers from accessing the Internet altogether.

—LAURA L. KRUG