Miscreants are actively exploiting a gaping hole in the internet's address lookup system that can cause millions of web surfers to receive counterfeit pages when they try to access online banking services and other types of websites.

The first confirmed instance came on Tuesday, when security researcher H D Moore discovered a domain-name service server operated by AT&T had been compromised. The attack caused Moore and other AT&T subscribers to be redirected to a fake Google page that tried to push affiliate advertising sites.

According to Dan Kaminsky, the researcher who first warned of the DNS vulnerability, "there are definitely other confirmed attacks," but non-disclosure agreements prevent him from giving details.

Equally concerning, Kaminsky said, is the sophistication the AT&T attackers showed in carrying out their attack. Rather than use exploit code added last week to Metasploit, a penetration testing kit that just happens to be maintained by Moore, the miscreants fashioned their own program that stealthily redirected users trying to visit Google to an impostor site.

"That was a wildly mature attack," Kaminsky told The Register. "Someone had an entire infrastructure built to attack Google's click-fraud system. That's a significant amount of code."

For more than a week, other researchers pointed to an increase in queries to DNS servers and other evidence suggesting attacks, but the AT&T exploit is the first to be documented.

As we reported last week, AT&T was one of the many laggard internet service providers reported to be dragging their feet in applying patches that fix the devastating DNS flaw. Kaminsky says more ISPs appear to be getting the message. Last week, about 51 per cent of unique name servers tested on his site (see the "check my DNS" button to the right) showed up as vulnerable. Now, he says it's closer to 35 percent.

In most cases, installing the patch is a straight-forward affair, but not always. Paul Vixie, head of the organization that maintains Berkeley Internet Name Domain, the net's most popular DNS server software, recently said updates patching the flaw could cut performance under heavy loads. Vixie said he believed fixing the flaw was more important than suffering slower performance. An update improving performance is in the works.

Even still, it's been more than three weeks since Kaminsky, Vixie and a choir of other influential net figures began imploring organizations to run the patch. Now that attacks have been confirmed in the wild, it's hard to imagine a justification for not doing so.

To test whether your ISP is an offender, please run the tests here or here, and report the results in the comments section. Be sure to include the name server's IP address and the name of the ISP. ®