PowerShell is a powerful interactive command-line interface and scripting environment included in the Windows operating system.  Adversaries can use PowerShell to perform a number of actions, including discovery of information and execution of code. Examples include the Start-Process cmdlet which can be used to run an executable and the Invoke-Command cmdlet which runs a command locally or on a remote computer.
PowerShell may also be used to download and run executables from the Internet, which can be executed from disk or in memory without touching disk.
Administrator permissions are required to use PowerShell to connect to remote systems.
PowerShell commands/scripts can also be executed without directly invoking the powershell.exe binary through interfaces to PowerShell's underlying System.Management.Automation assembly exposed through the .NET framework and Windows Common Language Interface (CLI).  
|Code Signing||Set PowerShell execution policy to execute only signed scripts.|
|Disable or Remove Feature or Program||It may be possible to remove PowerShell from systems when not needed, but a review should be performed to assess the impact to an environment, since it could be in use for many legitimate purposes and administrative functions.Disable/restrict the WinRM Service to help prevent uses of PowerShell for remote execution.|
|Privileged Account Management||When PowerShell is necessary, restrict PowerShell execution policy to administrators. Be aware that there are methods of bypassing the PowerShell execution policy, depending on environment configuration.|
Turla has used PowerShell to execute commands/scripts, in some cases via a custom executable or code from Empire's PSInject. Turla has also used PowerShell scripts to load and execute malware in memory.
If proper execution policy is set, adversaries will likely be able to define their own execution policy if they obtain administrator or system access, either through the Registry or at the command line. This change in policy on a system may be a way to detect malicious use of PowerShell. If PowerShell is not used in an environment, then simply looking for PowerShell execution may detect malicious activity.
It is also beneficial to turn on PowerShell logging to gain increased fidelity in what occurs during execution (which is applied to .NET invocations).  PowerShell 5.0 introduced enhanced logging capabilities, and some of those features have since been added to PowerShell 4.0. Earlier versions of PowerShell do not have many logging features.  An organization can gather PowerShell execution details in a data analytic platform to supplement it with other data.
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