about_Signing

Article
09/19/2022
8 minutes to read

Short description

Explains how to sign scripts so that they comply with the PowerShell execution policies.

Long description

This information only applies to PowerShell running on Windows.

The Restricted execution policy does not permit any scripts to run. The AllSigned and RemoteSigned execution policies prevent PowerShell from running scripts that do not have a digital signature.

This topic explains how to run selected scripts that are not signed, even while the execution policy is RemoteSigned, and how to sign scripts for your own use.

For more information about PowerShell execution policies, see about_Execution_Policies.

To permit signed scripts to run

When you start PowerShell on a computer for the first time, the Restricted execution policy (the default) is likely to be in effect.

The Restricted policy does not permit any scripts to run.

To find the effective execution policy on your computer, type:

Get-ExecutionPolicy

To run unsigned scripts that you write on your local computer and signed scripts from other users, start PowerShell with the Run as Administrator option and then use the following command to change the execution policy on the computer to RemoteSigned:

Set-ExecutionPolicy RemoteSigned

For more information, see the help topic for the Set-ExecutionPolicy cmdlet.

Running unsigned scripts using the RemoteSigned execution policy

If your PowerShell execution policy is RemoteSigned, PowerShell will not run unsigned scripts that are downloaded from the internet, including unsigned scripts you receive through email and instant messaging programs.

If you try to run a downloaded script, PowerShell displays the following error message:

The file <file-name> cannot be loaded. The file <file-name> is not digitally
signed. The script will not execute on the system. Please see "Get-Help
about_Signing" for more details.

Before you run the script, review the code to be sure that you trust it. Scripts have the same effect as any executable program.

To run an unsigned script, use the Unblock-File cmdlet or use the following procedure.

Save the script file on your computer.
Click Start, click My Computer, and locate the saved script file.
Right-click the script file, and then click Properties.
Click Unblock.

If a script that was downloaded from the internet is digitally signed, but you have not yet chosen to trust its publisher, PowerShell displays the following message:

Do you want to run software from this untrusted publisher?
The file <file-name> is published by CN=<publisher-name>. This
publisher is not trusted on your system. Only run scripts
from trusted publishers.

[V] Never run  [D] Do not run  [R] Run once  [A] Always run
[?] Help (default is "D"):

If you trust the publisher, select "Run once" or "Always run." If you do not trust the publisher, select either "Never run" or "Do not run." If you select "Never run" or "Always run," PowerShell will not prompt you again for this publisher.

Methods of signing scripts

You can sign the scripts that you write and the scripts that you obtain from other sources. Before you sign any script, examine each command to verify that it is safe to run.

For best practices about code signing, see Code-Signing Best Practices.

For more information about how to sign a script file, see Set-AuthenticodeSignature.

The New-SelfSignedCertificate cmdlet, introduced in the PKI module in PowerShell 3.0, creates a self-signed certificate that is Appropriate for testing. For more information, see the help topic for the New-SelfSignedCertificate cmdlet.

To add a digital signature to a script, you must sign it with a code signing certificate. Two types of certificates are suitable for signing a script file:

Certificates that are created by a certification authority: For a fee, a public certification authority verifies your identity and gives you a code signing certificate. When you purchase your certificate from a reputable certification authority, you are able to share your script with users on other computers that are running Windows because those other computers trust the certification authority.
Certificates that you create: You can create a self-signed certificate for which your computer is the authority that creates the certificate. This certificate is free of charge and enables you to write, sign, and run scripts on your computer. However, a script signed by a self-signed certificate will not run on other computers.

Typically, you would use a self-signed certificate only to sign scripts that you write for your own use and to sign scripts that you get from other sources that you have verified to be safe. It is not appropriate for scripts that will be shared, even within an enterprise.

If you create a self-signed certificate, be sure to enable strong private key protection on your certificate. This prevents malicious programs from signing scripts on your behalf. The instructions are included at the end of this topic.

Create a self-signed certificate

To create a self-signed certificate, use the New-SelfSignedCertificate cmdlet in the PKI module. This module is introduced in PowerShell 3.0 and is included in Windows 8 and Windows Server 2012. For more information, see the help topic for the New-SelfSignedCertificate cmdlet.

$params = @{
    Subject = 'CN=PowerShell Code Signing Cert'
    Type = 'CodeSigning'
    CertStoreLocation = 'Cert:\CurrentUser\My'
    HashAlgorithm = 'sha256'
}
$cert = New-SelfSignedCertificate @params

Using Makecert.exe

To create a self-signed certificate in earlier versions of Windows, use the Certificate Creation tool MakeCert.exe. This tool is included in the Microsoft .NET SDK (versions 1.1 and later) and in the Microsoft Windows SDK.

For more information about the syntax and the parameter descriptions of the MakeCert.exe tool, see Certificate Creation Tool (MakeCert.exe).

To use the MakeCert.exe tool to create a certificate, run the following commands in an SDK Command Prompt window.

Note

The first command creates a local certification authority for your computer. The second command generates a personal certificate from the certification authority. You can copy or type the commands exactly as they appear. No substitutions are necessary, although you can change the certificate name.

makecert -n "CN=PowerShell Local Certificate Root" -a sha256 `
-eku 1.3.6.1.5.5.7.3.3 -r -sv root.pvk root.cer `
-ss Root -sr localMachine

makecert -pe -n "CN=PowerShell User" -ss MY -a sha256 `
-eku 1.3.6.1.5.5.7.3.3 -iv root.pvk -ic root.cer

The MakeCert.exe tool will prompt you for a private key password. The password ensures that no one can use or access the certificate without your consent. Create and enter a password that you can remember. You will use this password later to retrieve the certificate.

To verify that the certificate was generated correctly, use the following command to get the certificate in the certificate store on the computer. You will not find a certificate file in the file system directory.

At the PowerShell prompt, type:

Get-ChildItem cert:\CurrentUser\my -codesigning

This command uses the PowerShell Certificate provider to view information about the certificate.

If the certificate was created, the output shows the thumbprint that identifies the certificate in a display that resembles the following:

Directory: Microsoft.PowerShell.Security\Certificate::CurrentUser\My

Thumbprint                                Subject
----------                                -------
4D4917CB140714BA5B81B96E0B18AAF2C4564FDF  CN=PowerShell User ]

Sign a script

After you create a self-signed certificate, you can sign scripts. If you use the AllSigned execution policy, signing a script permits you to run the script on your computer.

The following sample script, Add-Signature.ps1, signs a script. However, if you are using the AllSigned execution policy, you must sign the Add-Signature.ps1 script before you run it.

Important

Prior to PowerShell 7.2, the script must be saved using ASCII or UTF8NoBOM encoding. PowerShell 7.2 and higher supports signed scripts for any encoding format.

To use this script, copy the following text into a text file, and name it Add-Signature.ps1.

## Signs a file
param([string] $file=$(throw "Please specify a filename."))
$cert = @(Get-ChildItem cert:\CurrentUser\My -codesigning)[0]
Set-AuthenticodeSignature $file $cert

To sign the Add-Signature.ps1 script file, type the following commands at the PowerShell command prompt:

$cert = @(Get-ChildItem cert:\CurrentUser\My -codesigning)[0]
Set-AuthenticodeSignature add-signature.ps1 $cert

After the script is signed, you can run it on the local computer. However, the script will not run on computers on which the PowerShell execution policy requires a digital signature from a trusted authority. If you try, PowerShell displays the following error message:

The file C:\remote_file.ps1 cannot be loaded. The signature of the
certificate cannot be verified.
At line:1 char:15
+ .\ remote_file.ps1 <<<<

If PowerShell displays this message when you run a script that you did not write, treat the file as you would treat any unsigned script. Review the code to determine whether you can trust the script.

Enable strong protection for your private key

If you have a private key and certificate on your computer, malicious programs might be able to sign scripts on your behalf, which authorizes PowerShell to run them.

To prevent automated signing on your behalf, use Certificate Manager Certmgr.exe to export your signing key and certificate to a .pfx file. Certificate Manager is included in the Microsoft .NET SDK, the Microsoft Windows SDK, and in Internet Explorer.

To export the certificate:

Start Certificate Manager.
Select the certificate issued by PowerShell Local Certificate Root.
Click Export to start the Certificate Export Wizard.
Select "Yes, export the private key", and then click Next.
Select "Enable strong protection."
Type a password, and then type it again to confirm.
Type a file name that has the .pfx file name extension.
Click Finish.

To re-import the certificate:

Start Certificate Manager.
Click Import to start the Certificate Import Wizard.
Open to the location of the .pfx file that you created during the export process.
On the Password page, select "Enable strong private key protection", and then enter the password that you assigned during the export process.
Select the Personal certificate store.
Click Finish.

Prevent the signature from expiring

The digital signature in a script is valid until the signing certificate expires or as long as a timestamp server can verify that the script was signed while the signing certificate was valid.

Because most signing certificates are valid for one year only, using a time stamp server ensures that users can use your script for many years to come.