Security researchers have found a way to intercept data during online interactions with SSL-protected Web sites and plan to present their findings at the Black Hat security conference in Las Vegas next week.

Mike Zusman, principal consultant at Intrepidus Group, and Alex Sotirov, an independent security researcher, have identified a Web browser design flaw that allows an attacker to conduct a "Man-in-the-Middle" attack against Web sites with Extended Validation (EV) Secure Sockets Layer (SSL) certificates.

EV SSL certificates convey more information than standard SSL certificates and require a more extensive certification process by the issuing Certificate Authority. Their aim is to help Internet users feel confident that the Web sites they're doing business with are safe, and not phishing sites masquerading as legitimate sites.

The most recent versions of Apple Safari, Google Chrome, Microsoft Internet Explorer, Mozilla Firefox, and Opera all support EV SSL.

Zusman and Sotirov call their attack "SSL Rebinding" and claim that it can be used to sniff sensitive data as it leaves the user's browser or to conduct a browser cache poisoning attack against EV SSL Web sites.

The leading "S" in SSL may stand for "Secure," but security researchers are proving that's something of an overstatement. In February, at the Washington, D.C. Black Hat Conference, security researcher Moxie Marlinspike demonstrated the frailty of SSL by capturing 117 e-mail accounts, 16 credit card numbers, seven PayPal logins, and some 300 other miscellaneous secure login sessions in only 24 hours. He used software of his own design called "sslstrip," which he subsequently released.

The sslstrip software scans plain-text HTTP traffic for HTTPS links and rewrites them in the insecure HTTP protocol. It then adds fake security indicators, like the yellow lock icon, to create the illusion of security.

In an e-mail, Zusman said that the research he conducted with Sotirov differs from the work done by Marlinspike.

"Our attacks allow the browser to display the actual trust indicators that a browser shows during an EV SSL session, specifically the 'green badge' that displays the legal identity of the organization running the Web site," he said. "Like sslstrip, which downgrades the connection from HTTPS to HTTP, our tool downgrades the victim's connection from EV SSL to non-EV SSL. However, the similarities end there, as the methods the two tools use are very different."

Zusman and Sotirov are scheduled to present their findings on Thursday, July 30.

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