New versions of the MyDoom worm that began spreading late Monday afternoon are among the fastest-ever exploits of an unpatched Microsoft vulnerability, security experts said Tuesday.

MyDoom.ag and MyDoom.ah, as McAfee dubs the worms, appeared just five days after a bug in Microsoft's Internet Explorer was disclosed by security firms, including Denmark's Secunia. The IE vulnerability, named IFRAME after the HTML tag that can cause a buffer overflow, is being used by the new MyDooms to infect machines. There is no patch available for the flaw in IE.

"We're not surprised to see [the new MyDooms,]," said Alfred Huger, the senior director of engineering for Symantec's security response team, "since it's really simple to exploit this vulnerability.

"It's not quite a zero-day exploit, but it's close," he added.

As of mid-morning Tuesday, the new MyDooms had infected only a modest number of machines, said security analysts, and for now, the worst may be already over. "It's probably not going to be much bigger than it is right now," said Craig Schmugar, the virus research manager at McAfee.

But it's likely that additional attacks will hit users, said Ken Dunham, the director of malicious code research at iDefense, in an e-mail to TechWeb. "Publicly available exploit code for the IFRAME vulnerability has enabled attackers to quickly add it to MyDoom code. As a result, several new codes exploiting the hole may emerge in the next few weeks, since there is no patch available."

These versions of MyDoom differ in some ways from the typical variants of the long-running family.

In fact, these differences have generated some disagreement among security researchers, some of whom refuse to go along with the crowd to dub them MyDoom variants. "Detailed analysis reveals that the similarities they have with the MyDoom family are outweighed by the differences," said Graham Cluley, a senior technology consultant for U.K.-based Sophos, in a statement. Finnish security firm F-Secure also doubted that these worms were MyDooms, reporting on its Web site that its analysis showed only a 49 percent correlation between the new worms and the last MyDooms.

Whatever they are, they infect machines differently than the usual MyDoom. Rather than include its payload in an attached file -- which must be opened by the user to infect the target PC -- MyDoom.ag and MyDoom.ah only include a Web site link in the transmitting message. Clicking on that link, which hypes an adult Web cam site or claims a $175 PayPal credit awaits, takes the user to a previously-compromised PC, which has had a Web server surreptitiously installed. That Web server uploads the worm to the new system, and the process starts all over again with the second PC launching e-mails with the malicious links.

"The MyDooms redirects you to a Web page that has the exploit," explained Symantec's Huger. "That Web page is hosted on another compromised machine. It's a little unusual -- there are easier ways to go about this -- but it's not unique." Other worms, he said, have used similar tactics.

By transferring itself from each infected machine, these MyDooms may be trying to avoid counterattack, said Schmugar. Worms that point to a central remote server for downloading additional code, or to download the actual worm, can be stymied by Internet providers shutting down the site. Spreading out the infection vector makes it harder to shut down the worm-spewing systems.

Because Windows XP Service Pack 2's edition of Internet Explorer doesn't suffer from the IFRAME vulnerability, XP users can prevent infection by updating to SP2.

Other defensive tactics include switching to another browser and not clicking on links within unsolicited e-mails, said iDefense's Dunham, who predicted that the quick action of hackers to exploit the vulnerability means that more attacks are on the horizon.

"Cyber criminals continue to compress the timeline for attack," he said. "MyDoom comes less than a week after the vulnerability was posted online, and more variants and other worms are likely to attack the IFRAME vulnerability as criminals attempt to take advantage of this situation prior to a patch being released."

Microsoft did not include a patch for Internet Explorer in its monthly security alerts, which debuted Tuesday.