Nearly a month after a critical flaw in the Internet's Domain Name System was first reported, vendors of some of the most widely used firewall software are scrambling to fix a problem that can essentially undo part of the patches that address this bug.

The DNS flaw affects server software made by many vendors, including Microsoft, Cisco Systems, and the Internet Systems Consortium.

Some firewall software undoes a source port randomization feature that was introduced in the DNS patches. While this change doesn't completely negate the DNS patch, it could make it easier for attackers to pull off a cache-poisoning attack against the DNS server, security experts say.

This could lead to virtually undetectable phishing attacks against users of those DNS servers.

Firewalls that do IP (Internet Protocol) address translation -- converting the IP addresses used by computers on their internal networks to different IP addresses that are used by the other computers on the Internet -- can sometimes undo the source port randomization, security experts say.

The scope of the problem initially took some DNS experts by surprise, said Dan Kaminsky, the IOActive researcher who first discovered the DNS bug. "This is to some degree our fault," he said in an e-mail interview. "We underestimated the number of firewalls out there that were deployed in front of DNS servers."

"Cisco, Juniper, Citrix, and a number of other firewall vendors have been absolutely scrambling to update their equipment," he added.

These vendors say it could still be weeks before all products are fixed.

Last Wednesday, Cisco updated its Security Advisory on the DNS problem to give customers guidance on how to handle the issue, said Russ Smoak, a director with Cisco's Product Security Incident Response Team. The issue affects Cisco customers who are using the firewall to do port address translation (PAT), he said. "If you have a PAT firewall, the best thing you can do is look through our document, understand how our network is configured, and if you need the fix that's provided, then install the fix."

In the interim, network administrators can forward their DNS lookups to servers whose port addresses are not being translated or simply reconfigure the firewall, Kaminsky said.

Juniper Networks expects to deliver a random source port option for its products in the coming weeks, said Juniper spokeswoman Cindy Ta, via e-mail.

Not all firewall vendors are affected, however. Check Point Software, for example, says its firewalls don't have this problem.

Kaminsky's DNS flaw affects such a wide variety of products that it's not surprising that there have been some glitches in the patch process. Early last week, DNS experts reported that the patch they'd created was slowing down performance on some high-traffic servers -- those that were being hit with more than 10,000 queries per second. On Friday, security vendor nCircle reported that Apple's fix for the DNS issue did not work properly.

Internet Systems Consortium President Paul Vixie calls the port translation problem a "big deal," but he said that despite some early skepticism, users are starting to understand the seriousness of the situation. When Kaminsky first discussed the problem, some security experts had said that the problem appeared to be simply a rehash of a known issue.

But after the bug was inadvertently disclosed last week, some skeptics changed their tune.

"This continues to be a mess," he said via e-mail. "But at least there are no more deniers out there muddying the waters with the 'overblown, not urgent' message."

PC World's Will Schultz contributed to this story.