Credentials Options in Configuration Data

Applies To: Windows PowerShell 5.0

Plain Text Passwords and Domain Users

DSC configurations containing a credential without encryption will generate an error message about plain text passwords. Also, DSC will generate a warning when using domain credentials. To suppress these error and warning messages use the DSC configuration data keywords:

• PsDscAllowPlainTextPassword
• PsDscAllowDomainUser

Handling Credentials in DSC

DSC configuration resources run as Local System by default. However, some resources need a credential, for example when the Package resource needs to install software under a specific user account.

Earlier resources used a hard-coded Credential property name to handle this. WMF 5.0 added an automatic PsDscRunAsCredential property for all resources. For information about using PsDscRunAsCredential, see Running DSC with user credentials. Newer resources and custom resources can use this automatic property instead of creating their own property for credentials.

To find the available credential properties on a resource use either Get-DscResource -Name ResourceName -Syntax or the Intellisense in the ISE (CTRL+SPACE).

Get-DscResource -Name Group -Syntax

Group [String] #ResourceName
{
    GroupName = [string]
    [Credential = [PSCredential]]
    [DependsOn = [string[]]]
    [Description = [string]]
    [Ensure = [string]{ Absent | Present }]
    [Members = [string[]]]
    [MembersToExclude = [string[]]]
    [MembersToInclude = [string[]]]
    [PsDscRunAsCredential = [PSCredential]]
}

This example uses a Group resource from the PSDesiredStateConfiguration built-in DSC resource module. It can create local groups and add or remove members. It accepts both the Credential property and the automatic PsDscRunAsCredential property. However, the resource only uses the Credential property.

For more information about the PsDscRunAsCredential property, see Running DSC with user credentials.

Example: The Group resource Credential property

DSC runs under Local System, so it already has permissions to change local users and groups. If the member added is a local account, then no credential is necessary. If the Group resource adds a domain account to the local group, then a credential is necessary.

Anonymous queries to Active Directory are not allowed. The Credential property of the Group resource is the domain account used to query Active Directory. For most purposes this could be a generic user account, because by default users can read most of the objects in Active Directory.

Example Configuration

The following example code uses DSC to populate a local group with a domain user:

Configuration DomainCredentialExample
{
    param
    (
        [PSCredential] $DomainCredential
    )
    Import-DscResource -ModuleName PSDesiredStateConfiguration

    node localhost
    {
        Group DomainUserToLocalGroup
        {
            GroupName        = 'ApplicationAdmins'
            MembersToInclude = 'contoso\alice'
            Credential       = $DomainCredential
        }
    }
}

$cred = Get-Credential -UserName contoso\genericuser -Message "Password please"
DomainCredentialExample -DomainCredential $cred

This code generates both an error and warning message:

ConvertTo-MOFInstance : System.InvalidOperationException error processing property 'Credential' OF
TYPE 'Group': Converting and storing encrypted passwords as plain text is not recommended.
For more information on securing credentials in MOF file, please refer to MSDN blog:
https://go.microsoft.com/fwlink/?LinkId=393729

At line:11 char:9
+   Group
At line:341 char:16
+     $aliasId = ConvertTo-MOFInstance $keywordName $canonicalizedValue
+                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [Write-Error], InvalidOperationException
    + FullyQualifiedErrorId : FailToProcessProperty,ConvertTo-MOFInstance
WARNING: It is not recommended to use domain credential for node 'localhost'. In order to suppress
the warning, you can add a property named 'PSDscAllowDomainUser' with a value of $true to your DSC
configuration data for node 'localhost'.

Compilation errors occurred while processing configuration
'DomainCredentialExample'. Please review the errors reported in error stream and modify your
configuration code appropriately.
At C:\WINDOWS\system32\WindowsPowerShell\v1.0\Modules\PSDesiredStateConfiguration\PSDesiredStateConfiguration.psm1:3917 char:5
+     throw $ErrorRecord
+     ~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (DomainCredentialExample:String) [], InvalidOperationException
    + FullyQualifiedErrorId : FailToProcessConfiguration

This example has two issues:

1. An error explains that plain text passwords are not recommended
2. A warning advises against using a domain credential

The flags PSDSCAllowPlainTextPassword and PSDSCAllowDomainUser suppress the error and warning informing the user of the risk involved.

PSDSCAllowPlainTextPassword

The first error message has a URL with documentation. This link explains how to encrypt passwords using a ConfigurationData structure and a certificate. For more information on certificates and DSC read this post.

To force a plain text password, the resource requires the PsDscAllowPlainTextPassword keyword in the configuration data section as follows:

$password = "ThisIsAPlaintextPassword" | ConvertTo-SecureString -asPlainText -Force
$username = "contoso\Administrator"
[PSCredential] $credential = New-Object System.Management.Automation.PSCredential($username,$password)

Configuration DomainCredentialExample
{
    Import-DscResource -ModuleName PSDesiredStateConfiguration

    node localhost
    {
        Group DomainUserToLocalGroup
        {
            GroupName        = 'ApplicationAdmins'
            MembersToInclude = 'contoso\alice'
            Credential       = $credential
        }
    }
}

$cd = @{
    AllNodes = @(
        @{
            NodeName = 'localhost'
            PSDscAllowPlainTextPassword = $true
        }
    )
}

DomainCredentialExample -ConfigurationData $cd

localhost.mof

The PSDSCAllowPlainTextPassword flag requires that the user acknowledge the risk of storing plain text passwords in a MOF file. In the generated MOF file, even though a PSCredential object containing a SecureString was used, the passwords still appear as plain text. This is the only time the credentials are exposed. Gaining access to this MOF file gives anyone access to the Administrator account.

/*
@TargetNode='localhost'
@GeneratedBy=Administrator
@GenerationDate=01/31/2019 06:43:13
@GenerationHost=Server01
*/

instance of MSFT_Credential as $MSFT_Credential1ref
{
Password = "ThisIsAPlaintextPassword";
 UserName = "Administrator";

};

instance of MSFT_GroupResource as $MSFT_GroupResource1ref
{
ResourceID = "[Group]DomainUserToLocalGroup";
 MembersToInclude = {
    "contoso\\alice"
};
 Credential = $MSFT_Credential1ref;
 SourceInfo = "::11::9::Group";
 GroupName = "ApplicationAdmins";
 ModuleName = "PSDesiredStateConfiguration";

ModuleVersion = "1.0";

 ConfigurationName = "DomainCredentialExample";

};

Credentials in transit and at rest

• The PSDscAllowPlainTextPassword flag allows the compilation of MOF files that contain passwords in clear text. Take precautions when storing MOF files containing clear text passwords.
• When the MOF file is delivered to a Node in Push mode, WinRM encrypts the communication to protect the clear text password unless you override the default with the AllowUnencrypted parameter.
  • Encrypting the MOF with a certificate protects the MOF file at rest before it has been applied to a node.
• In Pull mode, you can configure Windows pull server to use HTTPS to encrypt traffic using the protocol specified in Internet Information Server. For more information, see the articles Setting up a DSC pull client and Securing MOF files with Certificates.
  • In the Azure Automation State Configuration service, Pull traffic is always encrypted.
• On the Node, MOF files are encrypted at rest Beginning in PowerShell 5.0.
  • In PowerShell 4.0 MOF files are unencrypted at rest unless they are encrypted with a certificate when they pushed or pulled to the Node.

Microsoft advises to avoid plain text passwords due to the significant security risk.

Domain Credentials

Running the example configuration script again (with or without encryption), still generates the warning that using a domain account for a credential is not recommended. Using a local account eliminates potential exposure of domain credentials that could be used on other servers.

When using credentials with DSC resources, prefer a local account over a domain account when possible.

If there is a '\' or '@' in the Username property of the credential, then DSC will treat it as a domain account. There is an exception for "localhost", "127.0.0.1", and "::1" in the domain portion of the user name.

PSDscAllowDomainUser

In the DSC Group resource example above, querying an Active Directory domain requires a domain account. In this case add the PSDscAllowDomainUser property to the ConfigurationData block as follows:

$password = "ThisIsAPlaintextPassword" | ConvertTo-SecureString -asPlainText -Force
$username = "contoso\Administrator"
[PSCredential] $credential = New-Object System.Management.Automation.PSCredential($username,$password)

Configuration DomainCredentialExample
{
    Import-DscResource -ModuleName PSDesiredStateConfiguration

    node localhost
    {
        Group DomainUserToLocalGroup
        {
            GroupName        = 'ApplicationAdmins'
            MembersToInclude = 'contoso\alice'
            Credential       = $credential
        }
    }
}

$cd = @{
    AllNodes = @(
        @{
            NodeName = 'localhost'
            PSDscAllowDomainUser = $true
            PSDscAllowPlainTextPassword = $true
        }
    )
}

DomainCredentialExample -ConfigurationData $cd

Now the configuration script will generate the MOF file with no errors or warnings.