A vulnerability in servers used by EarthLink to handle mistyped Web page requests may have allowed attackers to launch undetectable phishing attacks against any Internet site, according to a noted Internet security researcher.

The bug, which was patched earlier this week, underscores a fundamental security risk in the way that some ISPs are attempting to generate advertising revenue from mistyped Web addresses, said Dan Kaminsky, director of penetration testing with IOActive, a security consulting firm.

The vulnerability was in a service called Barefruit, which EarthLink has been using since August 2006 to return Web pages with search terms and advertising to customers who mistype a domain name in their browser.

[ Find out how phishers can use  DNS server redirection to catch unsuspecting users. ]

Barefruit, which is based in London, operates a service that works with Domain Name Servers (DNSes), which are used by the browser to translate domain names, such as yahoo.com, into numerical IP addresses. Typically, when a browser asks a DNS server for a nonexistent Internet address -- adsewrds.yahoo.com, for example -- the DNS server returns an error message indicating that no such address exists. With Barefruit's servers, the user is told that the address does exist, and is then sent to a Web page that displays advertising and suggested search terms.

Because of a bug in the software used to redirect users to these advertising and search pages, Kaminsky was able to get the pages to run his own JavaScript code. With the browser treating this code as if it were from a legitimate domain, Kaminsky was able to steal users' cookies, create fake Web sites that appeared to be hosted on legitimate domains, and even log into certain Web sites without authorization.

Generating revenue from domain name typos has generated controversy before. In 2003, domain name registrar VeriSign was forced to disable a similar system called SiteFinder, which redirected Web surfers who had typed nonexistent domains.

EarthLink is not the only ISP to be testing this system. Kaminsky said he found evidence of Barefruit or similar systems being tested on Verizon, Time Warner, Qwest, and Comcast, which outsources some of its network to EarthLink.

"The security of the entire Web for these ISPs is right now limited by the security of some random ad server run by a British company," he said. "Somebody running an ad server controls the security of whitehouse.gov. This is not a good situation."

A Verizon spokesman said that his company was not using the Barefruit service.

In a statement, EarthLink confirmed that it had patched Kaminsky's bug, but did not address the broader security concerns that Kaminsky believes are raised by this model.

However, the company did defend its use of the service in 2006, when it was first introduced.

"By presenting users with contextual help based upon the nonexistent domain the user entered, we believe we are improving the EarthLink user experience with a system that will not interfere with other network processes," EarthLink said in a blog posting at the time. "And DNS error handling presents an opportunity for EarthLink to generate additional revenue as well."

Originally, Kaminsky hadn't intended to go public with his findings, but after press reports last week that Network Solutions was engaging in a similar practice -- redirecting nonexistent subdomains of some of its Web hosting customers to Network Solutions pages containing advertisements -- he felt compelled to draw attention to the issue.

News of this Network Solutions policy was sure to encourage attackers to look for cross-site scripting flaws on the company's servers, similar to the bug he had found in Barefruit, Kaminsky said. "If anything happens to those servers, a lot of people are in trouble," he said.

Kaminsky will present a demonstration of his findings on Saturday at the Toorcon Seattle security conference.