Adversaries may use Valid Accounts to remotely control machines using Virtual Network Computing (VNC). VNC is a platform-independent desktop sharing system that uses the RFB ("remote framebuffer") protocol to enable users to remotely control another computer’s display by relaying the screen, mouse, and keyboard inputs over the network.
VNC differs from Remote Desktop Protocol as VNC is screen-sharing software rather than resource-sharing software. By default, VNC uses the system's authentication, but it can be configured to use credentials specific to VNC.
Adversaries may abuse VNC to perform malicious actions as the logged-on user such as opening documents, downloading files, and running arbitrary commands. An adversary could use VNC to remotely control and monitor a system to collect data and information to pivot to other systems within the network. Specific VNC libraries/implementations have also been susceptible to brute force attacks and memory usage exploitation.
Inventory workstations for unauthorized VNC server software.
|M1042||Disable or Remove Feature or Program||
Uninstall any VNC server software where not required.
|M1037||Filter Network Traffic||
VNC defaults to TCP ports 5900 for the server, 5800 for browser access, and 5500 for a viewer in listening mode. Filtering or blocking these ports will inhibit VNC traffic utilizing default ports.
|M1033||Limit Software Installation||
Restrict software installation to user groups that require it. A VNC server must be manually installed by the user or adversary.
|ID||Data Source||Data Component|
|DS0028||Logon Session||Logon Session Creation|
|DS0029||Network Traffic||Network Connection Creation|
Use of VNC may be legitimate depending on the environment and how it’s used. Other factors, such as access patterns and activity that occurs after a remote login, may indicate suspicious or malicious behavior using VNC.
On macOS systems
log show --predicate 'process = "screensharingd" and eventMessage contains "Authentication:"' can be used to review incoming VNC connection attempts for suspicious activity.
Monitor for use of built-in debugging environment variables (such as those containing credentials or other sensitive information) as well as test/default users on VNC servers, as these can leave openings for adversaries to abuse.