A new Windows worm is working its way through company networks by taking advantage of weak passwords, security researchers said over the weekend.
The worm, dubbed "Morto" by Microsoft and F-Secure, has been circulating since at least last week, when company administrators noticed systems generating large numbers of unexplained connections to the Internet.
"Although the overall numbers of computers reporting detections are low in comparison to more established malware families, the traffic it generates is noticeable," said Hil Gradascevic, a researcher with the Microsoft Malware Protection Center (MMPC).
Morto spreads using RDP, or Remote Desktop Protocol, the Microsoft-developed protocol for controlling one computer by connecting to it from another.
All versions of Windows from XP on include client software that uses RDP to remotely access machines. The software, called Remote Desktop Connection (RDC) in XP, Vista and Windows 7, requires a username and password to log in to a remote system.
Windows PCs infected with Morto scan the local network for other machines that have RDC switched on, then try to log in to a Remote Desktop server using a preset list of common passwords, said F-Secure. If one of the passwords works, the worm then downloads additional malware components to the server and kills security software to remain hidden.
The scanning for potential targets generates significant traffic on TCP port 3389, the port a Remote Desktop server monitors for incoming access requests. That traffic caught the attention of puzzled network administrators.
"Every 10 minutes or so, a flood of TCP 3389 connection attempts out to seemingly random IP addresses," reported a user identified as "BarrySDCA" in a message posted to a Microsoft support forum. "Our firewall is blocking it from getting out and it keeps trying."
That thread currently has nearly 70 messages and has been viewed by others almost 6,000 times, both large numbers for a discussion that started only days ago.
Analyses done by Microsoft and F-Secure identified the list of weak passwords the worm tries, which includes such easy examples as "password," "123456" and "abc123."
"This particular worm highlights the importance of setting strong system passwords," said Microsoft's Gradascevic. "The ability of attackers to exploit weak passwords shouldn't be underestimated."
Morto's purpose may be to crank out denial-of-service attacks against hacker-designated targets, said Microsoft in the write-up published Sunday. Although Microsoft patched RDP just three weeks ago as part of August's monthly security update, Morto does not exploit that vulnerability, or any other in the protocol.