Users of the popular ZoneAlarm firewall have reported dead Internet connections after installing one of the security updates issued by Microsoft Tuesday, according to online message forums.

Early Wednesday, ZoneAlarm told its users to uninstall the Microsoft patch -- which fixed a wide-spread problem with the Web's addressing system -- or make more technical changes to their PCs in order to regain a connection. The company will also release a patch later Wednesday.

Shortly after Microsoft posted its monthly security updates on Tuesday, ZoneAlarm users running Windows XP started posting complaints about broken connections. "After installing the patches, [my son's] computer could not access the Internet," said a user identified as "RFSharpe" on a ZoneAlarm support forum Tuesday. "I noticed that as soon as I disabled ZoneAlarm I was able to access the Internet again."

Traffic on other message forums was also heavy; one thread on dsl.com detailing problems sported more than 120 messages and had been viewed more than 22,000 times in the past 24 hours.

ZoneAlarm, which is part of Check Point Software Technologies, acknowledged the problem Tuesday, and posted several work-arounds Wednesday. The quickest way to regain Internet access, said the company, is to uninstall the security update tagged as KB951748 using Windows' Add or Remove Programs utility. Alternately, users could tweak ZoneAlarm's firewall settings or reduce the security level of the machine.

"The problem caused ZoneAlarm users to lose Internet access," said Laura Yecies, general manager for the ZoneAlarm group. "That's a very serious type of issue."

The trouble was traced to ZoneAlarm hooking into the Windows XP kernel in order to filter out potentially malicious traffic, a company engineer said. "We filter network traffic at the kernel, where malware can't avoid us," said James Grant, a ZoneAlarm team lead. "If you filter traffic in usermode, malware can see what we're doing."

Kernel hooking -- intercepting Windows' system calls and modifying the kernel dispatch table -- is a common practice by security vendors, who defend it on the grounds that it lets them provide stronger protection against malware, including rootkits. The practice is undocumented in Windows XP, although Microsoft has traditionally looked the other way. In Windows Vista, however, it documented API calls in the Windows Filtering Platform, or WFP, that let third-party security companies directly access data traffic.

Microsoft's patch for a serious DNS cache poisoning vulnerability changed components that ZoneAlarm hooks into, said Yecies, with the result that the firewall blocked all connections to the Web.

The problem notwithstanding, she defended kernel hooking. "It's undocumented, but it's in widespread use. Every major security vendor makes use of it," said Yecies.