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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 Thursday.

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, and then re-issue 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 posting to his blog Wednesday, 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 keyset 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 organisation's site.

Symantec 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 system."

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 13 May, 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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