SAN JOSE, Calif. (AP) - Just days before the Super Bowl, a hacker installed malicious code on the official Web site of Dolphin Stadium designed to secretly implant keystroke-logging software on visitors' computers, security researchers said Friday.

A stadium spokesman said the Web site was cleaned and back to normal within hours of the discovery, which was confirmed by outside security experts.

The type of malicious software used in the attack is easy to detect and easy to protect against, the security experts said. It was unclear how many computers may have been affected.

The attack involved a common type of Trojan horse program targeting Windows computers without the latest security patches from Microsoft Corp., according to security firm Websense Inc., which first reported the breach.

The program can give hackers full access to compromised computers through two vulnerabilities in Windows PCs, both of which have already been patched by Microsoft, according to Websense.

The firm said it discovered Friday that a link to a malicious JavaScript file was inserted onto the front page of the stadium Web site.

Once a visitor unknowingly executes the script, it attempts to download and execute the malicious keystroke-logging file onto the victim's computer.

"We literally find tens of thousands of these things every day - they're everywhere from big-name sites like this one to mom-and-pop bakery shops," said Dan Hubbard, vice president of security research at Websense. "It's definitely a good lesson in staying up to date on the patches."

George Torres, a stadium spokesman, said officials were alerted to the breach around noon on Friday and had the site fixed within three hours.

Torres said the FBI is investigating the attack because of the international attention surrounding the Super Bowl, which is expected to draw some 75,000 attendees this Sunday to the stadium in Miami Gardens, Fla., about 15 miles from Miami.

Researchers said most basic security measures would protect against this type of attack.

"It's low-hanging-fruit malware," said David Marcus, security research and communications manager with McAfee Inc. "If you have any up-to-date anti-malware software on your machine, it's going to mitigate it."