Screensavers are programs that execute after a configurable time of user inactivity and consist of Portable Executable (PE) files with a .scr file extension.  The Windows screensaver application scrnsave.exe is located in
C:\Windows\System32\ along with screensavers included with base Windows installations. The following screensaver settings are stored in the Registry (
HKCU\Control Panel\Desktop\) and could be manipulated to achieve persistence:
SCRNSAVE.exe- set to malicious PE path
ScreenSaveActive- set to '1' to enable the screensaver
ScreenSaverIsSecure- set to '0' to not require a password to unlock
ScreenSaverTimeout- sets user inactivity timeout before screensaver is executed
Adversaries can use screensaver settings to maintain persistence by setting the screensaver to run malware after a certain timeframe of user inactivity. 
Block .scr files from being executed from non-standard locations. Set Group Policy to force users to have a dedicated screensaver where local changes should not override the settings to prevent changes. Use Group Policy to disable screensavers if they are unnecessary. 
Monitor process execution and command-line parameters of .scr files. Monitor changes to screensaver configuration changes in the Registry that may not correlate with typical user behavior.
Tools such as Sysinternals Autoruns can be used to detect changes to the screensaver binary path in the Registry. Suspicious paths and PE files may indicate outliers among legitimate screensavers in a network and should be investigated.