Security experts are finding an increasing number of websites hosting malicious JavaScript code first detected on Super Bowl-related sites last week.

Sites covering topics ranging from health care to government have been hacked to host the JavaScript, SANS Internet Storm Centre Director Marcus H. Sachs wrote on the SANS blog, listing some of the hacked sites.

"System administrators might want to check their network flow logs for any traffic to these sites and for any traffic to the five sites that hosted the hostile JavaScript," Sachs said.

The attack targets two known vulnerabilities in Microsoft's Windows operating systems, for which patches were issued in April and January. Computers with unpatched software are vulnerable to the attack. If one of the hacked sites is visited, the JavaScript code directs the browser to a second web server, based in China, and tries to install a Trojan Horse downloader and password-stealing program on the victim's computer.

Initially, the exploit appeared isolated to websites related to US football, as hackers tried to capitalise on the surge of traffic to sites concerning the Super Bowl sporting event, which was played on Sunday. The site of the Miami Dolphins team, and another site for its stadium, were hacked, although they were eventually cleaned of malware.

Security company Websense reported the problem on the stadium site on Friday. Websense recommended that users stay away from the affected sites until they had been cleaned up.