No other security vendor has reported problems with the Microsoft DNS patch, however. "That's a good question," Yecies said when asked why only ZoneAlarm users were affected. "We all use [kernel hooking] in different ways."

"We've each done things slightly differently," echoed Grant. "We're all reinventing the wheel."

The problem appeared during the part of the traffic filtering where the ZoneAlarm firewall does error checking on the data it intercepts, he added. "We paid the price for being careful."

ZoneAlarm engineers worked through the night in cooperation with Microsoft programmers, said Yecies, who added that a patch would be posted on ZoneAlarm's Web site later today. Users, however, would first have to reconnect to the Internet by using one of the recommended workarounds, or download the file on another machine, then transfer it to the affected PC. ZoneAlarm is also working on ways to automatically push the fix to users, but that approach was still being tested as of mid-day Wednesday.

"This isn't about finger-pointing," said Yecies, when asked which company was responsible for the snafu, ZoneAlarm or Microsoft. When pressed, however, she acknowledged that Microsoft should have caught the problem before issuing its security update. "Clearly the lack of testing is significantly affecting our mutual users," she said. "This was not some small feature that was disabled. Users were really impacted, and we take that very seriously."

Microsoft did not reply to questions about the part its DNS patch may have played and whether it would issue a fix of its own.

This is the second Internet connection problem in two months involving a Microsoft update and third-party security software. In May, users of Symantec's consumer security programs said that they'd lost network and wireless connections after installing Windows XP Service Pack 3 (SP3).

"In 20-20 hindsight, we want to avoid problems like this," admitted Yecies. But the lesson to learn isn't that kernel hooking is bad, she said. "That's fundamental to how we provide security in XP. The lesson to learn is in collaboration and advanced testing."