EarthLink redirect service poses security risk, expert says

April 20, 2008 —

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.

Barefruit, which is based in London, operates a service that works with Domain
Name Servers, (DNS) which are used by the browser to translate domain names,
such as yahoo.com, into numerical Internet Protocol 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.

IDG News Service