The DNS cache poisoning that first struck more than a month ago and led to users being redirected from popular Web sites to malicious sites that infected their machines with spyware, is continuing, said the Internet Storm Center (ISC) Wednesday. The attacks are taking advantage of vulnerabilities and design flaws in Microsoft server software.

DNS cache poisoning occurs when an attacker hacks into a domain name server, one of the machines that translate URLs such as www.techweb.com into the appropriate IP address. The attacker then "poisons" the server by planting counterfeit data in the cache of the name server. When a user requests, say, techweb.com, and the IP address is resolved by the hacked domain server, the bogus data is fed back to the browser and the user is directed to another Web site, not the intended destination.

To highlight the danger, the ISC raised its Homeland Security-esque alert color code from Green to Yellow. According to ISC, Yellow represents that "we are currently tracking a significant new threat. The impact is either unknown expected to be minor to the infrastructure. However, local impact would be significant."

To set the DNS cache poisoning threat in perspective, Yellow is the same alert color code that ISC used during the SQL Slammer, MSBlast, and Sasser worm outbreaks, three of the nastiest in the last two years.

The newest attack, said Kyle Haugsness, one of the ISC analysts, is actually the third since March 4. Like the initial attack, the motivation is certainly money, since the result is again the installation of mass quantities of spyware on victims' PCs.

"The motivation for these attacks is very simple: money," Haugsness said. "The end goal of the first attack was to install spyware/adware on as many Windows machines as possible."

The second attack, he continued, "seems to have been launched by a known spammer," said Haugsness. That second attack, which took place starting March 24, redirected users from legit sites to sites selling prescription drugs.

Initially, Haugsness and the other ISC analysts thought that a DNS cache poisoning attack was beyond the skills of most spammers -- and so might be proof that the original attackers were contracting their services, but now he said "they might be completely unrelated. In fact, one of the things we discovered after looking into these attacks is just how easy they are to carry off."

The third, and still-ongoing attack, which began March 25, has the same goal -- install spyware -- as the first, said Haugsness. One of the DNS servers involved in the early-March attack wasn't cleaned up properly, and the attacker returned and changed the poisoning tool.

"Right now this is still going on," said Haugsness. "The attackers are changing IP addresses around and poisoning other DNS servers [to stay ahead of security authorities]."

Among the domains included in one of the poisoned DNS servers during the first attack were major sites such as americanexpress.com, cnn.com, redhat.com, and msn.com. "These [665] domains organizations did not have their DNS cache's poisonedthese organizations were not compromised, although it is possible that customers of these sites unknowingly gave out login information or personal information to the malicious servers," wrote Haugsness in a long report posted on the ISC site about the attacks.

Although there's essentially nothing an end-user can do to protect him- or herself -- other than to regularly sweep the system for spyware and/or have real-time anti-spyware defenses up and running -- DNS server administrators, particularly those in enterprises, should scramble.

Windows-based DNS servers are particularly vulnerable, since Windows NT Server 4.0 and Windows 2000 Server prior to SP3 are insecure against DNS cache poisoning attacks. Windows 2000 Server SP3 and later, as well as Windows Server 2003, are configured securely by default. (For more information, see this Microsoft Knowledgebase article.)

Other users that are vulnerable are those running various Symantec gateway security products who haven't patched bugs the Cupertino, Calif.-based vendor released in mid-March.

But the entire Windows server software platform -- including properly configured NT/2000 and 2003 systems -- seems to have an architectural design flaw, said Haugsness, that makes them vulnerable to cache poisoning attacks. He said ISC was working with Microsoft to pin down the exact cause.

"This is a lot easier to do than we thought," said Haugsness, who noted that cache poisoning isn't new. "That's the main reason we went out there with this, and bumped up to Yellow.

"What's scarier is that this could be used in lot more subtle fashion, to make it difficult, or even impossible to detect."