The ongoing attack of multiple bot worm families stepped up Wednesday, said security experts, who noted that so far more than 175 corporations have been hit with malicious code exploiting Windows 2000's Plug and Play vulnerability.

A new wave of bots, including two that are particularly malicious, have struck scores of corporations, including the Associated Press, the Canadian Imperial Bank of Commerce, Caterpillar, Inc., CNN, Daimler/Chrysler, General Electric, SBC Communications, and United Parcel Service.

"The bots are not affecting the Internet at large," said Oliver Friedrichs, the senior manager of Symantec’s security response team. "But they are affecting a number of world's biggest business. So far we know of 175 corporations that have been hit by the attacks."

Although by some counts there have been more than a dozen bots released that exploit the Windows 2000 Plug and Play vulnerability disclosed last week, Symantec sees two of them as especially potent: Zotob.e and Esbot.a.

(Like so many malicious code outbreaks, nomenclature can be deceptive. Zotob.e, for instance, is also known as Rbot.cbq, while Esbot.a is tagged as various versions of Rbot, Sbot, or even IRCbot. Confusion reigns.)

Both these bots not only use the Plug and Play vulnerability to gain access to a Windows 2000 PC, but they also include code that opens an IRC channel back to designated servers, from which they can download additional code to further compromise the machine, or turn it into a denial-of-service (DoS) or spam zombie.

The rate of infection by Zotob and its brethren has certainly increased, said Gunter Ollmann, the director of Internet Security Systems' (ISS) X-force research group. But it's hard to tell by how much, since the botnet operators have taken to putting their host in invisible mode so that eavesdropping researchers can't get an idea of how many PCs have been compromised.

Microsoft continued to downplay the outbreak, with a spokesperson on Wednesday again referring to it as a low level threat, a stance the Redmond, Wash.-based developer has maintained since the outbreak began Sunday. "Zotob has thus far had a low rate of infection. Zotob only targets Windows 2000," said the spokesperson.

Some security experts disagreed.

"This is one of the most significant threats we've seen in 2005," said Friedrichs. "These two are only the fourth and fifth level '3' threats this year." In 2004, Friedrichs noted in comparison, Symantec dubbed 33 threats as level 3 or higher.

The outages at corporations worldwide was likely caused by Zotob.e or Esbot.a, or another of the Zotob variants, said Friedrichs. "What we're seeing is an evolution since Sunday of the attacks, where propagation techniques have been improved.

"Once one of these bots gets inside the perimeter, it spreads rather quickly in the soft interior. We've seen as many as 2,000 hosts infected in just one hour."

Friedrichs said it was possible that many of the compromised corporations were hit when an infected laptop was re-connected to the company network. But he wouldn't rule out a straight-forward attack on the firewall in some of the cases.

"Anti-virus software is certainly effective once a threat has been identified, but an intrusion prevention system would have prevented this out the door, since they look for attacks against the vulnerability, not for a specific exploit."

Other analyst say the blame isn't on a lack of defensive systems, but a lax attitude toward patching. As Microsoft has noted repeatedly in its nearly-constantly-updated security advisory, patched systems are invulnerable to the bot attacks.

"I see Microsoft's situation, as damned if it does, damned if it doesn't," wrote JupiterResearch analyst Joe Wilcox on his blog. "If the company doesn't disclose vulnerabilities, customers are put at risk. If it discloses, customers are put at risk. My recommendation is if you're damned either way, than be damned if you do. Better Microsoft releases patches than not. But that means customers must patch as soon as possible."