Hacker Tool Targets SSL Flaw

A new tool designed to launch denial of service attacks can bring down Secure Sockets Layer servers using just a laptop computer and a standard DSL connection.

Developed by a German group called The Hacker’s Choice, THC-SSL-DOS tool is intended to be a proof-of-concept to disclose “fishy security” in the SSL protocol, the group wrote on The Hacker’s Choice blog on 24 October.

Widely known flaw

The tool implements the attack by establishing a large number of SSL connections with a given server to eventually consume all of the system resources and making the server unavailable. THC officially released the tool because a version had been leaked accidentally a few months earlier.

The tool exploits a widely known issue with how SSL connections work. A lot of server resources are required to successfully handle SSL “handshakes” at the beginning of a session to establish a secure connection. A client sending enough sessions requests can cause the server to fail. The issue is worsened if the server has the SSL renegotiation feature enabled to handle scenarios that involve processing the client-side certificate.

“It works great if the server supports SSL Renegotiation. It still works if SSL Renegotiation is not supported, but requires some modifications and more bots,” the group wrote.

The feature “renegotiates” the key material of an SSL connection, such as in cases where client certificates are used. The Tor Project, which offers users anonymous proxies to hide web activity, uses renegotiation, as does many smart cards used for web authentication, Marsh Ray, a developer at PhoneFactor, wrote on the Full Disclosure mailing list.

“A security feature that was supposed to make SSL more secure makes it indeed more vulnerable to this attack,” according to THC.

The attack can be mitigated by servers that have SSL acceleration hardware installed, which speeds up the processing of SSL operations.

The tool exists for both Windows and Unix and can be executed with one client on a typical home link. Experts have said in the past that a traditional distributed denial of service attack cannot be launched by home users because servers generally have superior bandwidth than the pipe available to DSL users. THC-SSL-DOS exploits the SSL handshakes to overcome the narrow bandwidth pipe and launch a successful attack without needing a large number of participants.

Evolving threats

Attackers can still use the tool successfully on servers where SSL Renegotiation has been disabled. But this approach requires more than a single laptop. Taking on a large server farms that use SSL load balancers required 20 “average-sized” laptops and a connection with 120kbit/sec of traffic capacity, according to THC.

With threats growing and evolving, companies are trying to protect their environments by deploying Secure-Socket-Layer (SSL) to encrypt connections, Carl Herberger, vice president of security solutions at Radware, told eWEEK. However, a number of vulnerabilities have been exposed in the protocol recently and “most security tools” are not well-equipped to handle the rise of attacks against SSL, Herberger said.

Many hacktivist organisations who are having “less desired” effects with their current attack tools are migrating towards SSL-based attacks as they can “achieve dramatic results” in this space, according to Herberger.

“Most companies are sitting ducks as vulnerabilities rise and security defenses are inadequate,” he said.

For example, the Low-Orbit Ion Cannon tool favored by hacker groups like Anonymous made it possible for a large number of users on residential network connections to band together to launch an attack successfully and overcome the DSL limitations.

However, the group recently released #RefRef, a new tool that was more powerful than LOIC to launch attacks.

“We are hoping that the fishy security in SSL does not go unnoticed. The industry should step in to fix the problem so that citizens are safe and secure again,” THC said.