Q

Does Morto worm prove inherent flaws in Windows RDP security?

The recent Morto worm had unusual success spreading via Windows Remote Desktop Protocol. Does that mean RDP is security too weak? Nick Lewis explains.

What’s your take on the recent Morto worm? Do you think its growth is more of a result of issues with Windows Remote Desktop Protocol, configuration errors or simply weak passwords? Do you think it’s likely the state of a move away from Trojans and bots and back toward “old-school” Internet worms?

The Morto worm, discovered in August, was a new way to attack insecurely configured Windows systems. Essentially an evolved Windows networking worm, it spread from an infected system using the Windows Remote Desktop Protocol (RDP), an element of the Windows Remote Desktop Connection service that allows a Windows PC or server to be controlled remotely.  However, the worm did not exploit vulnerabilities in RDP the way previous Windows networking worms did.

The rapid propagation of the Morto worm was a result of RDP being allowed through many firewalls. Most generic firewalls could not block this attack without blocking RDP, but firewalls or IPSes with application detection capabilities could be used to block the application-layer attack. The Morto worm was an application-layer attack, exploiting a common configuration error: a weak password. It would attempt a brute-force login as an administrator on a victim’s machine, using a series of common passwords.  Most systems should be configured to enforce strong passwords, yet Morto was still able to spread quite successfully.

The Morto worm was just an infection mechanism to load malware on the system. As far as malware goes, it didn’t seem to be terribly malicious, but Morto could have loaded Zeus or a number of different malware or bot software programs on its victim machines to profit from its rapid propagation. It's an important lesson for security practitioners, as broad use of strong passwords would have likely stopped the Morto worm in its tracks.

It is ironic that Windows systems configured with one of the least secure options, a blank administrative password, would have been protected from this worm since users with blank passwords are not allowed to connect via Remote Desktop Protocol. I wouldn't be surprised if the Morto worm paves the way for a future Windows worm to attack an application or protocol exposed by network or additional host-based firewalls, like the Witty worm did for BlackIce. There are security weaknesses in older versions of RDP (.pdf) and the remote desktop client, but Microsoft has improved the security of the protocol and the client application in the most recent versions, most notably with strong encryption for network connections.

This was first published in January 2012

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