A hacker group has released a proof-of-concept tool that exploits how encryption keys can be renegotiated to launch a distributed denial of service attack against Secure Sockets Layer servers.
A 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 Oct. 24.
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
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.
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 size" 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,
Many hacktivist organizations 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.