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Man-in-the-middle attack sidesteps SSL
Published: 2009-02-18

ARLINGTON, VA -- A combination of poorly educated users, fewer security warnings in browsers, and sites that mix secured and unsecured content allow man-in-the-middle attacks that can sidestep the ubiquitous secure sockets layer (SSL) encryption used to pass login credentials, a researcher told attendees on Wednesday at the Black Hat Security Briefings.

Using a proxy server sitting between the victim and the Internet, security researcher Moxie Marlinspike — his real name, he said — showed how Web requests for pages that included a login box, such as the home page of many banks, can be intercepted and forged. A program on the proxy server sends the request to the Web site, handles any redirect to an SSL-encrypted page and returns an exact duplicate to the user, without the encryption.

While telltale signs of the switch remain — the Web address starts with HTTP rather than HTTPS — most users do not even notice. As an experiment, Marlinspike placed his proxy software on a node in the Tor network and intercepted 200 requests for SSL encrypted pages over 20 hours, including 114 Yahoo! credentials, 50 Gmail credentials and 16 credit-card numbers. None of the users refused to enter their sensitive information into the unencrypted page, he said.

"It is suppose to post to a secure link, but there is no way to know that," Marlinspike said. "There is no disastrous warning."

The presentation demonstrated a practical attack using a collection of already understood weaknesses. In the past, cross-site scripting has been used to inject content into supposedly secure sites.

The security researcher stressed that the attack succeeds because browsers have moved from providing positive feedback when a site is secure to only providing negative feedback when the software detects something wrong. By providing additional cues, such as a locked icon as the favicon, an attacker could make a targeted user more likely to fall for the ruse. In addition, an attacker could use international domain names to create a URL that appears to be a valid address to a major Web site, but in reality, includes '.' and '/' characters from international character sets.

Marlinspike plans to release the code to his software, dubbed sslstrip, by the end of the week.

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Posted by: Robert Lemos
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