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 | Detects |
|---|---|---|---|
| DS0017 | Command | Command Execution |
Monitor executed commands and arguments of .scr files. |
| DS0022 | File | File Creation |
Monitor newly constructed files that may establish persistence by executing malicious content triggered by user inactivity. Analytic 1 - Created on disk that are being used as Screensaver files
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| File Modification |
Monitor for changes made to files that may establish persistence by executing malicious content triggered by user inactivity. Note: Although there are no standard events for file modification, Windows Event ID 4663 (An Attempt Was Made to Access An Object) can be used to alert on attempted accesses of screensaver files (typically ending in a file extension of .scr). |
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| DS0009 | Process | Process Creation |
Monitor newly executed processes that may establish persistence by executing malicious content triggered by user inactivity. Analytic 1 - HKCU\Control Panel\Desktop registry key
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| DS0024 | Windows Registry | Windows Registry Key Modification |
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. Default screen saver files are stored in C:\Windows\System32. Use these files as a reference when defining list of not suspicious screen saver files. Analytic 1 - Registry Edit from Screensaver
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