Original URL: http://www.theregister.co.uk/2009/05/19/open_ssh_hack/

OpenSSH chink bares encrypted data packets

One in 262,144 chance = good odds

By Dan Goodin

Posted in Security, 19th May 2009 04:28 GMT

Cryptographers are urging users of a widely employed network protocol to make sure they're running the latest version after discovering a flaw that could allow attackers to read data that's supposed to remain encrypted.

All programs that incorporate the OpenSSH implementation of SSH, short for Secure Shell, should make sure they use version 5.2, which provides several countermeasures to prevent the attacks. Other SSH implementations may be vulnerable as well, the researchers from the Information Security Group at the University of London's Royal Holloway said.

The attack exploits subtle differences in the way SSH software reacts when encountering errors during cryptographic processing. By directing specially manipulated packets at the application, an attacker has a one in 262,144 chance of recovering 32 bits of plaintext from an arbitrary chunk of ciphertext.

While those are extremely limited odds, the design flaw still poses a significant threat given the way many applications that employ SSH work. VPNs, or virtual private networks, for example, repeatedly reconnect to a server extremely rapidly each time they are disconnected. With some programs reconnecting several times per second, a determined attacker might find ample opportunity to succeed.

The research team tested their attack against OpenSSH, which powers the vast majority SSH applications. They believe the vulnerability resides in other implementations of SSH as well. The researchers, who were expected to release their findings Monday at a research conference in California, included Martin Albrecht, Kenny Paterson, and Gaven Watson.

Over the past decade, SSH has become a mainstay among network administrators looking for a secure way to securely access servers and transfer files across the internet. But this is by no means the first time the protocol has been found to be vulnerable. Last year, weaknesses were discovered in OpenSSL implementations included in the Debian distribution of Linux that have required encryption keys to be regenerated.

In the late 1990s, OpenSSH suffered from several devastating design flaws, including one that allowed attackers to inject keystrokes or entire commands into an SSH session, said Nate Lawson, a cryptographer who is principal of security consultancy Root Labs in San Francisco. The vulnerability was only purged when OpenSSH upgraded to version 2.

"The good news is that it appears that flaws in the SSH protocol are getting smaller and smaller over time and harder to exploit," Lawson said. "Given the circumstances, it shows that the reviews of SSH and the open design process are definitely resulting in bugs having less and less impact over time." ®