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Windows-SRP-Policy-Generator

Generates a custom Registry.pol file for hash-based software restriction group policies

Purpose

Windows Software Restriction Policies can help prevent execution of common living off the land (LOL) tools, but the Group Policy editor is severely limited for hash-based restrictions. To add a hash-based rule, you need to have a copy of the file in question. For executables like Powershell.exe, this would require somehow obtaining a copy of every version published to ensure complete coverage.

This tool generates a custom Registry.pol file from a supplied hash list that can then be copied to your group policy object folder, bypassing this Group Policy editor limitation.

Important

Common guidance suggests that path-based restrictions will work, but an attacker can simply copy the contents of the restricted folder to a new location. Hash-based restrictions provide the highest assurance that your policy will function as expected.

Usage

  • Download the repository code ZIP
  • Right-click on the downloaded file, select "Properties" and click "Unblock"
  • Click "OK"
  • Unzip the file to your desired location
  • Following the .csv examples in the /hashes folder, add .csv files or edit the existing to include all hashes you would like in your Software Restriction Policy
    • All fields are required, hashes are case insensitive
    • Ensure MD5 values are 32 hexadecimal characters in length
    • Ensure SHA256 values are 64 hexadecimal characters in length
  • In a PowerShell window, navigate to the directory where Create-SRPPolicy.ps1 is located
  • Run .\Create-SRPPolicy.ps1
    • Note, the script will process ALL .csv files in the /hashes folder so make sure it contains only what you want in your custom SRP policy
  • Open Group Policy Management, create a new group policy object and edit this new policy
  • Create a Software Restriction Policy
    • Can be found under User Configuration/Windows Settings/Security Settings/Software Restriction Policies
  • Exit the GPO editor
  • Copy the Windows SRP Policy Generator-generated Registry.pol file over your newly created group policy Registry.pol file
    • The Registry.pol file can be found in "C:\Windows\SYSVOL\sysvol\your.domain.name\Policies{Unique GPO ID}\User" on your domain controller
    • Ensure the new Registry.pol file does not have the mark of the web (BEFORE copying, right-click, Properties, uncheck "Unblock")
  • Open Group Policy Management and browse to the GPO. If the Registry.pol file permissions are incorrect, you will be prompted to have them corrected automatically
  • Open the GPO in the GPO editor, navigate to the Software Restriction Policy and ensure it loads with no errors
  • Test your new GPO

Where do I get the hash values needed to make my own .csv entries?

  • In a Windows command prompt, issue the following commands and copy the value returned into the respective .csv data columns
  certutil -hashfile <path to file> MD5
  certutil -hashfile <path to file> SHA256
  dir /-C <path to file>
  • In a PowerShell command prompt, you can use the following command to output the result directly to a CSV file
  Get-ChildItem <path to file> | Select-Object -Property @{Name="MD5";expression={(Get-FileHash $_.FullName -Algorithm MD5).hash}},@{Name="SHA256";expression={(Get-FileHash $_.FullName).hash}},@{Name="SIZE";expression={$_.Length}} | export-csv <path to csv file>

Where do I get the Unique GPO ID?

  • Open Group Policy Management, select the GPO in question, click on the "Details" tab and find the "Unique ID" entry.

The policy is not working/applying

  1. Ensure the group policy is applying as expected to the user/computer as scoped
  2. Verify you are not also applying AppLocker policies to the machine, as this results in the SRP entries being completely ignored Microsoft Doc

Disclaimer

This tool generates a random identifier for each SRP entry. There is an infinitesimally small chance of an identifier value created by this script overlapping an existing SRP identifier.