PowerShell Security Best Practices

Securely executing PowerShell scripts is crucial for maintaining system integrity and preventing unauthorized access. This document outlines essential security practices for PowerShell.

1. Execution Policy

The PowerShell Execution Policy controls the conditions under which PowerShell loads configuration files and runs scripts. It's the first line of defense against running untrusted scripts.

• Restricted: No scripts can be run. Only interactive mode commands are allowed. This is the default setting.
• AllSigned: Only scripts signed by a trusted publisher can be run.
• RemoteSigned: Downloads from the internet must be signed by a trusted publisher. Local scripts can be run without a signature. This is often a good balance for many environments.
• Unrestricted: All scripts can be run. Use with extreme caution, as this offers no protection against malicious scripts.

To view the current execution policy, use:

Get-ExecutionPolicy

To set the execution policy (requires administrative privileges):

Set-ExecutionPolicy RemoteSigned -Scope LocalMachine

It's recommended to use the -Scope LocalMachine to enforce the policy for all users on the machine. For specific user overrides, -Scope CurrentUser can be used.

2. Script Signing

Signing your scripts with a digital certificate ensures their integrity and authenticity. This allows you to verify that a script has not been tampered with and that it originates from a trusted source.

To sign a script:

1. Obtain a code-signing certificate (e.g., from a Certificate Authority or by creating a self-signed certificate for internal use).
2. Use the Set-AuthenticodeSignature cmdlet:

Set-AuthenticodeSignature -FilePath "C:\Scripts\MyScript.ps1" -Cert C:\Certificates\MyCodeSigningCert.pfx

When running signed scripts, PowerShell will prompt you to confirm the publisher if the script is from an untrusted source (unless your execution policy dictates otherwise).

3. Minimizing Script Privileges

Always run scripts with the minimum privileges necessary to perform their tasks. Avoid running scripts as an administrator unless absolutely required.

• Least Privilege Principle: Grant users and services only the permissions they need.
• Role-Based Access Control (RBAC): Use PowerShell's built-in RBAC features or implement your own to define specific roles and assign them to users.

4. Input Validation and Sanitization

Treat all external input (from users, files, network sources) as potentially malicious. Validate and sanitize any input before using it in your scripts.

• Check data types, lengths, and formats.
• Use regular expressions for strict pattern matching.
• Avoid directly embedding user input into commands or queries.

Example of validating a string to ensure it only contains alphanumeric characters:

$inputString = Read-Host "Enter a valid username"
            if ($inputString -match '^[a-zA-Z0-9]+$') {
                Write-Host "Valid input: $inputString"
            } else {
                Write-Host "Invalid input. Please use only letters and numbers."
            }

5. Secure Credential Management

Avoid hardcoding credentials directly within scripts. Use secure methods for handling sensitive information.

• Get-Credential: Prompt users for credentials securely.
• PowerShell Secrets Management Module: Store and retrieve secrets securely from a vault.
• Credential Objects: Pass credential objects around rather than plain text passwords.

$credential = Get-Credential "Enter credentials for remote server"
            Invoke-Command -ComputerName remoteServer -ScriptBlock { ... } -Credential $credential

6. Logging and Auditing

Enable PowerShell logging to track script execution, command history, and potential security events. This is invaluable for monitoring and forensic analysis.

• Module Logging: Logs all cmdlets and script blocks that are run.
• Script Block Logging: Logs the content of script blocks that are run.
• Transcription Logging: Records all input to and output from a PowerShell session.

These can be configured via Group Policy or registry settings.

7. Script Security Auditing

Regularly audit your PowerShell scripts for security vulnerabilities. Tools like PSScriptAnalyzer can help identify common security issues.

Install-Module PSScriptAnalyzer
            Invoke-ScriptAnalyzer -Path "C:\Scripts\" -IncludeSecurityRules

8. Regular Updates and Patching

Keep PowerShell and the underlying operating system up-to-date with the latest security patches. This ensures that you benefit from Microsoft's security fixes.

Last updated: October 26, 2023