To demonstrate the fragility of the Secure Sockets Layer, which is used to protect sensitive transactions online, security researcher Moxie Marlinspike captured 117 e-mail accounts, 16 credit card numbers, seven PayPal logins, and some 300 other miscellaneous secure login sessions in only 24 hours.

On Wednesday at the Black Hat Conference in Washington, D.C., Marlinspike explained that he obtained such data by placing proxy software he'd written, called "sslstrip," on a node of a Tor network, to conduct what's known as a man-in-the-middle attack.

The proxy software intercepts HTTPS traffic, generates and signs security certificates, and mediates data passing between the client and server, capturing everything in the process. And though there are ways to detect the attack, like recognizing that a Web URL begins with HTTP rather than HTTPS, none of the test victims noticed.

The attack can also be augmented with the addition of a lock icon, which would suggest to most users that the session is secure, even if it's not. It can be further enhanced through the addition of a homograph attack, which uses letters from different character sets to spoof well-known Web sites. Security researcher Eric Johanson in 2005 described such an attack using a Cyrillic 'a' in "www.paypal.com" to create a PayPal doppelganger site.

Marlinspike's attack isn't so much technical as it is social engineering. It relies on users failing to recognize the distinction between HTTP and HTTPS sessions and on other insecure habits, like people's penchant for typing, say, "www.wellsfargo.com" without the HTTPS portion of the URL.

Such tendencies allow Marlinspike to bypass SSL entirely. "Lots of times the security of HTTPS comes down to the security of HTTP, and HTTP is not secure," he explains in his presentation slides.

Marlinspike plans to release sslstrip later this week.

Want to hear about security for rich Internet applications? Black Hat is hosting a virtual event on this topic on Thursday, Feb. 26. Find out more (registration required).