There is a default system policy in Windows 7 and above that prevents system services from sending Ctrl-Alt-Del (the Secure Attention Sequence, "SAS"). Note that only the VNC Server in Service Mode may process this key sequence sent from a connected VNC Viewer.
There are two ways to solve this, either by setting the group policy to allow services to simulate SAS, or by overriding the group policy in place.
How do I resolve this?
- Check that the "Disable or enable software Secure Attention Sequence" policy is set to allow "Services" on the VNC Server PC, or;
- Set SimulateSAS to 2 in the Expert tab of the VNC Server Options dialog will override the domain policy for VNC Server.
Since the server runs as a local system account with administrator privileges, the server is able to momentarily change the group policy to allow it to inject the SAS whenever it is sent from a connected VNC Viewer.