To prevent intruders from using an idle session, be sure to set timeouts for the ESXi Shell and vSphere Web Client.
ESXi Shell Timeout
For the ESXi Shell, you can set the following timeouts from the vSphere Web Client and from the Direct Console User Interface (DCUI).
The availability timeout setting is the amount of time that can elapse before you must log in after the ESXi Shell is enabled. After the timeout period, the service is disabled and users are not allowed to log in.
The idle timeout is the amount of time that can elapse before the user is logged out of an idle interactive sessions. Changes to the idle timeout apply the next time a user logs in to the ESXi Shell and do not affect existing sessions.
vSphere Web Client Timeout
vSphere Web Client sessions terminate after 120 minutes by default. You can change this default in the webclient.properties file, as discussed in the vCenter Server and Host Management documentation.