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Tool lets low-end PC crash much more powerful webserver

Denial-of-service attack targets SSL

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Hackers have released software that they say allows a single computer to knock servers offline by targeting a well-documented flaw in secure sockets layer implementations.

A German group known as The Hacker's Choice released the tool on Monday, in part to bring attention to what they said were a series of long-running deficiencies in SSL, which websites use to prevent social security numbers and other sensitive data from being monitored as they travel between servers and end-user computers.

“We are hoping that the fishy security in SSL does not go unnoticed,” an unnamed member of the group said in a blog post. “The industry should step in to fix the problem so that citizens are safe and secure again. SSL is using an aging method of protecting private data which is complex, unnecessary and not fit for the 21st century.”

The THC-SSL-DOS tool allows a single computer with a modest internet connection to crash a much more powerful server with vastly more bandwidth, but only when the server supports what's known as SSL renegotiation, Monday's postings claimed. Renegotiation is used to establish a new secret key securing communications after an encrypted session has already commenced. Renegotiation was at the heart of a flaw in the SSL protocol discovered in 2009 that allowed attackers to inject text into encrypted traffic passing between two endpoints.

“Interesting here is that a security feature that was supposed to make SSL more secure makes it indeed more vulnerable to this attack,” a member said.

The tool allows a single laptop with a standard DSL connection to take down an average webserver. Bringing down a larger server farm that makes use of an SSL load balancer required about 20 laptops and about 120Kbps of bandwidth. Even when websites don't support SSL renegotiation, they can still be toppled by THC-SSL-DOS, although the attack must be modified.

The tool is available as a Windows binary and Unix source code. ®

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