Users of Check Point Software's ZoneAlarm personal firewall products are reporting that a patch released Tuesday by Microsoft to fix a serious security flaw breaks their connection to the Internet.

"For those of you using ZoneAlarm who installed the Windows updates today, you probably already know that your internet connection has died," wrote user 'Winchester73', in a post Tuesday on the popular broadband forum DSLreports.com.

Numerous other forum users reported experiencing the same problem. Dozens of posters on ZoneAlarm's own user forum also reported Internet outages. "I service and administer approximately 200 privately owned computers and I have received about 20+ telephone calls from users with this problem today," wrote 'Calken'.

A spokesman for Check Point said the company planned on releasing a fix later in the day on Wednesday.

The problem began when Microsoft on Tuesday sent patch number KB951748 to Windows users. The patch is designed to plug a security vulnerability that leaves computers vulnerable to so-called DNS attacks.

The vulnerability is widespread and affects products made by numerous networking and software vendors beyond Microsoft. It was discovered by Dan Kaminsky, of the Seattle-based security firm IOActive.

Through a DNS attack, a hacker can redirect Internet users' page requests to phishing sites or other malicious pages. Various reports suggest that ZoneAlarm interprets the Microsoft patch itself as malicious code, and automatically severs the user's Internet connection in response.

Until a fix is issued, some forum users are working around the glitch by either uninstalling ZoneAlarm or the Microsoft patch. Either move, however, could leave their computers vulnerable to cyber-attacks.