Adversaries may implement hidden windows to conceal malicious activity from the plain sight of users. In some cases, windows that would typically be displayed when an application carries out an operation can be hidden. This may be utilized by system administrators to avoid disrupting user work environments when carrying out administrative tasks. Adversaries may abuse operating system functionality to hide otherwise visible windows from users so as not to alert the user to adversary activity on the system.
There are a variety of features in scripting languages in Windows, such as PowerShell, Jscript, and VBScript to make windows hidden. One example of this is powershell.exe -WindowStyle Hidden.
The configurations for how applications run on macOS are listed in the property list (plist) files. One of the tags in these files can be apple.awt.UIElement, which allows for Java applications to prevent the application’s icon from appearing in the Dock. A common use for this is when applications run in the system tray, but don’t also want to show up in the Dock. However, adversaries can abuse this feature and hide their running window.
Tactic: Defense Evasion
Platform: macOS, Windows
Permissions Required: User
Data Sources: Windows event logs, PowerShell logs, Process command-line parameters, Process monitoring, File monitoring
APT19: APT19 used -W Hidden to conceal PowerShell windows by setting the WindowStyle parameter to hidden
APT28: APT28 has used the WindowStyle parameter to conceal PowerShell windows
Execution Prevention: Limit or restrict program execution using anti-virus software. On MacOS, whitelist programs that are allowed to have the plist tag. All other programs should be considered suspicious.
Monitor processes and command-line arguments for actions indicative of hidden windows. In Windows, enable and configure event logging and PowerShell logging to check for the hidden window style. In MacOS, plist files are ASCII text files with a specific format, so they’re relatively easy to parse. File monitoring can check for the apple.awt.UIElement or any other suspicious plist tag in plist files and flag them.
index=windows source=”WinEventLog:Microsoft-Windows-Sysmon/Operational” (EventCode=1 Image=”*\\powershell.exe” CommandLine=”*-WindowStyle Hidden*”)