Adversaries may establish persistence by executing malicious content triggered by user inactivity. Screensavers are programs that execute after a configurable time of user inactivity and consist of Portable Executable (PE) files with a .scr file extension.[1] The Windows screensaver application scrnsave.scr is located in C:\Windows\System32\, and C:\Windows\sysWOW64\ on 64-bit Windows systems, 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 pathScreenSaveActive - set to '1' to enable the screensaverScreenSaverIsSecure - set to '0' to not require a password to unlockScreenSaveTimeout - sets user inactivity timeout before screensaver is executedAdversaries can use screensaver settings to maintain persistence by setting the screensaver to run malware after a certain timeframe of user inactivity.[2]
| ID | Name | Description |
|---|---|---|
| S0168 | Gazer |
Gazer can establish persistence through the system screensaver by configuring it to execute the malware.[2] |
| ID | Mitigation | Description |
|---|---|---|
| M1042 | Disable or Remove Feature or Program |
Use Group Policy to disable screensavers if they are unnecessary.[3] |
| M1038 | Execution Prevention |
Block .scr files from being executed from non-standard locations. |
| ID | Data Source | Data Component |
|---|---|---|
| DS0017 | Command | Command Execution |
| DS0022 | File | File Creation |
| File Modification | ||
| DS0009 | Process | Process Creation |
| DS0024 | Windows Registry | Windows Registry Key Modification |
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.