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This chapter is from the book

This chapter is from the book

Understanding Security

When WSH was released with Windows 98, it was a godsend for Windows administrators who wanted the same automation capabilities as their UNIX brethren. At the same time, virus writers quickly discovered that WSH also opened up a large attack vector against Windows systems.

Almost anything on a Windows system can be automated and controlled by using WSH, which is an advantage for administrators. However, WSH doesn't provide any security in script execution. If given a script, WSH runs it. Where the script comes from or its purpose doesn't matter. With this behavior, WSH became known more as a security vulnerability than an automation tool.

Execution Policies

Because of past criticisms of WSH's security, when the PowerShell team set out to build a Microsoft shell, the team decided to include an execution policy to mitigate the security threats posed by malicious code. An execution policy defines restrictions on how PowerShell allows scripts to run or what configuration files can be loaded. PowerShell has four execution policies, discussed in more detail in the following sections: Restricted, AllSigned, RemoteSigned, and Unrestricted.

Restricted

By default, PowerShell is configured to run under the Restricted execution policy. This execution policy is the most secure because it allows PowerShell to operate only in an interactive mode. This means no scripts can be run, and only configuration files digitally signed by a trusted publisher are allowed to run or load.

AllSigned

The AllSigned execution policy is a notch under Restricted. When this policy is enabled, only scripts or configuration files that are digitally signed by a publisher you trust can be run or loaded. Here's an example of what you might see if the AllSigned policy has been enabled:

PS C:\Scripts> .\evilscript.ps1
The file C:\Scripts\evilscript.ps1 cannot be loaded. The file
C:\Scripts\evilscript.ps1 is not digitally signed. The script will not
execute on the system. Please see "get-help about_signing" for more
details.
At line:1 char:16
+ .\evilscript.ps1 <<<<
PS C:\Scripts>
               

Signing a script or configuration file requires a code-signing certificate. This certificate can come from a trusted certificate authority (CA), or you can generate one with the Certificate Creation Tool (Makecert.exe). Usually, however, you want a valid code-signing certificate from a well-known trusted CA, such as Verisign, Thawte, or your corporation's internal public key infrastructure (PKI). Otherwise, sharing your scripts or configuration files with others might be difficult because your computer isn't a trusted CA by default.

RemoteSigned

The RemoteSigned execution policy is designed to prevent remote PowerShell scripts and configuration files that aren't digitally signed by a trusted publisher from running or loading automatically. Scripts and configuration files that are locally created can be loaded and run without being digitally signed, however.

A remote script or configuration file can be obtained from a communication application, such as Microsoft Outlook, Internet Explorer, Outlook Express, or Windows Messenger. Running or loading a file downloaded from any of these applications results in the following error message:

PS C:\Scripts> .\interscript.ps1
The file C:\Scripts\interscript.ps1 cannot be loaded. The file
C:\Scripts\interscript.ps1 is not digitally signed. The script will
not execute on the system. Please see "get-help about_signing" for
more details..
At line:1 char:17
+ .\interscript.ps1 <<<<
PS C:\Scripts>
               

To run or load an unsigned remote script or configuration file, you must specify whether to trust the file. To do this, right-click the file in Windows Explorer and click Properties. In the General tab, click the Unblock button (see Figure 3.1).

Figure 3.1

Figure 3.1 Trusting a remote script or configuration file

After you trust the file, the script or configuration file can be run or loaded. If it's digitally signed but the publisher isn't trusted, PowerShell displays the following prompt:

PS C:\Scripts> .\signed.ps1

Do you want to run software from this untrusted publisher?
File C:\Scripts\signed.ps1 is published by CN=companyabc.com, OU=IT,
O=companyabc.com, L=Oakland, S=California, C=US and 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"):
               

In this case, you must choose whether to trust the file content.

Unrestricted

As the name suggests, the Unrestricted execution policy removes almost all restrictions for running scripts or loading configuration files. All local or signed trusted files can run or load, but for remote files, PowerShell prompts you to choose an option for running or loading that file, as shown here:

PS C:\Scripts> .\remotescript.ps1

Security Warning
Run only scripts that you trust. While scripts from the Internet can
be useful, this script can potentially harm your computer. Do you want
to run
C:\Scripts\remotescript.ps1?
[D] Do not run  [R] Run once  [S] Suspend  [?] Help (default is "D"):
               

Setting the Execution Policy

To change the execution policy, you use the Set-ExecutionPolicy cmdlet, shown here:

PS C:\> set-executionpolicy AllSigned
PS C:\>
               

If you want to know the current execution policy, use the Get-ExecutionPolicy cmdlet:

PS C:\> get-executionpolicy
AllSigned
PS C:\>
               

By default, when PowerShell is first installed, the execution policy is set to Restricted. As you know, default settings never stay default for long. In addition, if PowerShell is installed on many machines, the likelihood of its execution policy being set to Unrestricted increases.

Fortunately, you can control the PowerShell execution policy through a Registry setting. This setting is a REG_SZ value named ExecutionPolicy, which is located in the HKLM\SOFTWARE\Microsoft\PowerShell\1\ShellIds\Microsoft.PowerShell key. Controlling the execution policy through the Registry means you can enforce a policy setting across many machines managed by a Group Policy Object (GPO).

In the past, creating a GPO to control the execution policy was simple because the PowerShell installation includes a Group Policy Administrative Template (ADM). However, as of the PowerShell RC2 release, the ADM is no longer part of the installation and may or may not be available in a separate PowerShell download. If Microsoft doesn't provide an ADM to control the execution policy, you can always create your own, as shown in the following example:

CLASS MACHINE

CATEGORY !!PowerShell
    POLICY !!Security
        KEYNAME "SOFTWARE\Microsoft\PowerShell\1\ShellIds\Microsoft.PowerShell"

        EXPLAIN !!PowerShell_ExecutionPolicy

        PART !!ExecutionPolicy EDITTEXT REQUIRED
            VALUENAME "ExecutionPolicy"
        END PART
    END POLICY
END CATEGORY

[strings]
PowerShell=PowerShell
Security=Security Settings
PowerShell_ExecutionPolicy=If enabled, this policy will set the PowerShell
execution policy on a machine to the defined value.  Execution policy values can be
Restricted, AllSigned, RemoteSigned, and Unrestricted.
Executionpolicy=Execution Policy

You can find a working version of this ADM on the PowerShell Unleashed Reference Web site: www.samspublishing.com. Although the PowerShellExecutionPolicy.adm file has been tested and should work in your environment, note that the execution policy settings in this file are considered preference settings. Preference settings are GPOs that are Registry values found outside the approved Group Policy Registry trees. When a GPO containing preference settings goes out of scope, the preference settings aren't removed from the Registry.

To configure the PowerShellExecutionPolicy.adm file, follow these steps:

  1. Log on to a GPO management machine as the GPO administrator.
  2. Using the Group Policy MMC, create a GPO named PowerShell.
  3. In the console tree, click to expand Computer Configuration and then Administrative Templates.
  4. Right-click Administrative Templates and click Add/Remove Templates in the shortcut menu.
  5. Navigate to the folder with the PowerShellExecutionPolicy.adm file. Select the file, click Open, and then click Close. The PowerShell node is then displayed under the Administrative Templates node.
  6. Click the Administrative Templates node, and then click View, Filtering from the Group Policy MMC menu. Click to clear the Only show policy settings that can be fully managed checkbox. Clearing this option allows you to manage preference settings.
  7. Next, click the PowerShell node under Administrative Templates.
  8. In the details pane, right-click Security Settings and click Properties in the shortcut menu.
  9. Click Enabled.
  10. Set the Execution Policy to one of these values: Restricted, AllSigned, RemoteSigned, or Unrestricted.
  11. Close the GPO, and then close the Group Policy MMC.

Controlling the execution policy through a GPO preference setting might seem like a less then perfect solution. After all, a preference setting doesn't offer the same level of security as an execution policy setting, so users with the necessary rights can modify it easily. This lack of security is probably why Microsoft removed the original ADM file from PowerShell. A future release of PowerShell might allow controlling the execution policy with a valid GPO policy setting.

Additional Security Measures

Execution policies aren't the only security layer Microsoft implemented in PowerShell. PowerShell script files with the .ps1 extension can't be run from Windows Explorer because they are associated with Notepad. In other words, you can't just double-click a .ps1 file to run it. Instead, PowerShell scripts must run from a PowerShell session by using the relative or absolute path or through the cmd command prompt by using the PowerShell executable.

Another security measure, explained in Chapter 2, is that to run or open a file in the current directory from the PowerShell console, you must prefix the command with .\ or ./. This feature prevents PowerShell users from accidentally running a command or PowerShell script without specifying its execution explicitly.

Last, by default, there's no method for connecting to or calling PowerShell remotely. However, that doesn't mean you can't write an application that allows remote PowerShell connections. In fact, it has been done. If you're interested in learning how, download the PowerShell Remoting beta from www.gotdotnet.com/workspaces/
workspace.aspx?id=ce09cdaf-7da2-4f1c-bed3-f8cb35de5aea
.

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