By Gregg Keizer
May 15, 2008
A recently disclosed vulnerability in widely used Linux distributions
can be exploited by attackers to guess cryptographic keys, possibly
leading to the forgery of digital signatures and theft of confidential
information, a noted security researcher said today.
HD Moore, best known as the exploit researcher who created the
Metasploit penetration testing framework, called the vulnerability in
Debian and Ubuntu systems "ugly" and said it will be a big job for
administrators to find every flawed key, then reissue them.
The bug, noted Tuesday by the Debian Project, is in the random number
generator used to produce a variety of digital keys, including SSH
(Secure Shell) keys and SSL (Secure Socket Layer) certificates. The
latter are widely used to secure traffic between users and secure sites
on the Internet.
According to Moore, the bug makes it relatively easy to "guess" keys. In
a blog post yesterday, Moore claimed he was able to generate 1024- and
2048-bit keys in about two hours.
Stronger keys, however, take considerably longer to create. He estimated
that an 8192-bit RSA key set would take some 3,100 hours (about 129
days) to generate.
Moore also published several key-generating tools -- collectively dubbed
"Toys" -- that included a shared library and a key generation script.
With that information out in the wild, other researchers banged the
warning drum. "This is very, very, very serious and scary," said Bojan
Zdrnja, an analyst at the Internet Storm Center (ISC) in a warning
posted on the organization's site today.
Symantec Corp. also warned customers of its DeepSight threat network of
the vulnerability and Moore's follow-on information and tools
disclosures. The California-based company also noted that another
hacker, "Markus M," published a tool that automates brute-force attacks
of the key weakness to the Full Disclosure security mailing list.
That revelation pushed the ISC to up its INFOCon threat status to
"yellow," a relatively rare occurrence. "The development of automated
scripts exploiting keys looks like a real threat to SSH servers around
the world," said Zdrnja in a later posting to the group's site.
It's not just users running Debian-based systems -- which includes the
popular Ubuntu Linux distribution -- who are at risk, Moore cautioned,
but virtually anyone. If data copied to other platforms has been secured
by keys generated on a Debian distribution, that data could be snatched.
"There's a lot of different areas that you're going to have to look, not
just within Debian," Moore said. "Administrators will have to audit
every single key. Even systems that do not use the Debian software need
to be audited in case any key is being used that was created on a Debian
Moore, ISC's Zdrnja and others have recommended that Debian and Ubuntu
users patch their systems -- updates are available -- and that users and
administrators regenerate all keys produced on a Debian system between
September 2006 and May 13, 2008. The September 2006 date, said Moore,
was when the first builds that included the flaw were made available.
Although he said the situation is serious, Moore doubted that there
would be general and widespread attacks. Instead, he said the most
likely outcome would be targeted attacks on systems that administered
large numbers of Debian users.
Moore also discounted any connection between the Debian vulnerability
and his disclosures, and brute-force attacks some vendors, including
Symantec, have been tracking the last 24 hours. "The timing is
definitely funny," he acknowledged, but said the differences -- the
attacks have been against user-generated passwords, not authentication
keys -- means the two events are probably just coincidental.
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