Stating the obvious - VNC is insecure

If you need to provide remote desktop access to your organization's Windows machines, and care even a little bit about IT security, do NOT use VNC. The Windows built-in Remote Assistance / Remote Desktop tools are a far better choice. This may seem obvious, but I still find organizations running VNC servers all over the place, usually for the helpdesk to provide users with remote troubleshooting. This can leave an environment ripe for compromise.

VNC has a few fundamental weaknesses:

• Most implementations do not require a username for authentication, and the password is stored in the registry using a trivial hashing mechanism. VNCcrack or Cain will instantly crack these hashes.
• There is typically no client-side confirmation or acknowledgment required for a VNC session to be established between a server and viewer, which can leave a user unaware that their activity is being monitored.
• Most significantly, because there's no simple way to centrally manage VNC servers, administrators will typically use the same VNC password for every host to facilitate configuration and deployment. So if you compromise one Windows box through local vulnerabilities and gain access to its registry, you can obtain a VNC password that will work on every other machine in the environment. Not good.

For a pen-tester's perspective, let's throw a few other variables into the mix. Several Windows tools will scan a set of systems via NetBIOS and tell you which users are logged on to each. And if the domain controllers are configured to allow certain anonymous NetBIOS queries, you can enumerate all the members of key groups like "Domain Administrators". Analyze your scan results and combine the data to produce a list of workstations used by privileged domain accounts. VNC into each system and look for an opportunity to take control of the keyboard and mouse. (Yet another reason to lock your desktop when away from the keyboard!)

It takes less than 15 seconds to open the command shell, execute your "net user /add /domain", and close the window. Bam - you now have a domain account. Not exactly an elegant attack, but it gets the job done.

VNC is an excellent tool that can serve a number of purposes in both home and enterprise environments. However, it is not an ideal solution for remote access across a large number of Windows hosts. It often takes a good penetration test to drive that point home.