The DNS cache on the default DNS server used by China Netcom, one of the country's largest Internet service providers, has been poisoned, said computer security company Websense on Tuesday.

China Netcom customers who mistype a Web address and enter an invalid domain name get directed to a Web page with malicious code, the company said.

"When users mistype a domain name, they are sometimes directed by their ISPs to a placeholder Web site with generic advertisements," Websense explained in an online post. "This is typically an additional revenue source for the ISP. In the case of [China Netcom], customers of this prominent ISP are directed to a Web site under the control of an attacker."

The malicious destination page includes an iframe that points to a server in China that attempts to exploit Adobe's Flash player, MS06-014 (Microsoft Data Access Components), MS08-041 (Microsoft Snapshot Viewer), and RealPlayer, if present on the victim's computer.

According to Stephan Chenette, manager of Websense security labs, the DNS poisoning was carried out by exploiting the weakness in DNS software identified by security researcher Dan Kaminsky, who discussed the flaw at the recent Black Hat security conference in Las Vegas.

The issue is that many popular DNS software packages fail to randomize transmission ports sufficiently, which could allow a knowledgeable attacker to alter, or poison, DNS cache information. The result is that Internet users relying on compromised DNS servers could see their e-mail traffic hijacked or could be sent to a malicious Web site.

Chenette considers the China Netcom hack to be particularly clever because rather than redirecting all traffic, which would be quickly detected, it only redirects mistyped Web addresses. He also said that the exploits used arise from recently disclosed vulnerabilities, making it more likely that victims' computers will not have been patched yet.

"Literally all Microsoft users who haven't updated to the latest patch are vulnerable to this," said Chenette. "This is particularly dangerous in China because many users have pirated software or software that is not up-to-date."

Chenette said that Websense has reported the problem to China Netcom. He expects that the problem will be fixed soon if it hasn't been already.

MessageLabs, another computer security company, reports that it has seen a 52% increase in suspicious DNS traffic between July and August. It sees this as a sign that "the online underworld is poised to launch targeted attacks in the coming weeks."

That's a view Chenette shares. "I don't believe we've seen the end of the use of this vulnerability," he said. "I think we're going to see more attacks."

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