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Making the second hop in PowerShell Remoting

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The "second hop problem" refers to a situation like the following:

  1. You are logged in to ServerA.
  2. From ServerA, you start a remote PowerShell session to connect to ServerB.
  3. A command you run on ServerB via your PowerShell Remoting session attempts to access a resource on ServerC.
  4. Access to the resource on ServerC is denied, because the credentials you used to create the PowerShell Remoting session aren't passed from ServerB to ServerC.

There are several ways to address this problem. The following table lists the methods in order of preference.

Configuration Note
CredSSP Balances ease of use and security
Resource-based Kerberos constrained delegation Higher security with simpler configuration
Kerberos constrained delegation High security but requires Domain Administrator
Kerberos delegation (unconstrained) Not recommended
Just Enough Administration (JEA) Can provide the best security but requires more detailed configuration
PSSessionConfiguration using RunAs Simpler to configure but requires credential management
Pass credentials inside an Invoke-Command script block Simplest to use but you must provide credentials

CredSSP

You can use the Credential Security Support Provider (CredSSP) for authentication. CredSSP caches credentials on the remote server (ServerB), so using it opens you up to credential theft attacks. If the remote computer is compromised, the attacker has access to the user's credentials. CredSSP is disabled by default on both client and server computers. You should enable CredSSP only in the most trusted environments. For example, a domain administrator connecting to a domain controller because the domain controller is highly trusted.

For more information about security concerns when using CredSSP for PowerShell Remoting, see Accidental Sabotage: Beware of CredSSP.

For more information about credential theft attacks, see Mitigating Pass-the-Hash (PtH) Attacks and Other Credential Theft.

For an example of how to enable and use CredSSP for PowerShell remoting, see Enable PowerShell "Second-Hop" Functionality with CredSSP.

Pros

  • It works for all servers with Windows Server 2008 or later.

Cons

  • Has security vulnerabilities.
  • Requires configuration of both client and server roles.
  • doesn't work with the Protected Users group. For more information, see Protected Users Security Group.

Kerberos constrained delegation

You can use legacy constrained delegation (not resource-based) to make the second hop. Configure Kerberos constrained delegation with the option "Use any authentication protocol" to allow protocol transition.

Pros

  • Requires no special coding
  • Credentials aren't stored.

Cons

  • Doesn't support the second hop for WinRM.
  • Requires Domain Administrator access to configure.
  • Must be configured on the Active Directory object of the remote server (ServerB).
  • Limited to one domain. Can't cross domains or forests.
  • Requires rights to update objects and Service Principal Names (SPNs).
  • ServerB can acquire a Kerberos ticket to ServerC on behalf of the user without user intervention.

Note

Active Directory accounts that have the Account is sensitive and can't be delegated property set can't be delegated. For more information, see Security Focus: Analysing 'Account is sensitive and can't be delegated' for Privileged Accounts and Kerberos Authentication Tools and Settings.

Resource-based Kerberos constrained delegation

Using resource-based Kerberos constrained delegation (introduced in Windows Server 2012), you configure credential delegation on the server object where resources reside. In the second hop scenario described above, you configure ServerC to specify from where it accepts delegated credentials.

Pros

  • Credentials aren't stored.
  • Configured using PowerShell cmdlets. No special coding required.
  • Doesn't require Domain Administrator access to configure.
  • Works across domains and forests.

Cons

  • Requires Windows Server 2012 or later.
  • Doesn't support the second hop for WinRM.
  • Requires rights to update objects and Service Principal Names (SPNs).

Note

Active Directory accounts that have the Account is sensitive and can't be delegated property set can't be delegated. For more information, see Security Focus: Analysing 'Account is sensitive and can't be delegated' for Privileged Accounts and Kerberos Authentication Tools and Settings.

Example

Let's look at a PowerShell example that configures resource-based constrained delegation on ServerC to allow delegated credentials from a ServerB. This example assumes that all servers are running supported versions of Windows Server, and that there is at least one Windows domain controller for each trusted domain.

Before you can configure constrained delegation, you must add the RSAT-AD-PowerShell feature to install the Active Directory PowerShell module, and then import that module into your session:

Add-WindowsFeature RSAT-AD-PowerShell
Import-Module ActiveDirectory
Get-Command -ParameterName PrincipalsAllowedToDelegateToAccount

Several available cmdlets now have a PrincipalsAllowedToDelegateToAccount parameter:

CommandType Name                 ModuleName
----------- ----                 ----------
Cmdlet      New-ADComputer       ActiveDirectory
Cmdlet      New-ADServiceAccount ActiveDirectory
Cmdlet      New-ADUser           ActiveDirectory
Cmdlet      Set-ADComputer       ActiveDirectory
Cmdlet      Set-ADServiceAccount ActiveDirectory
Cmdlet      Set-ADUser           ActiveDirectory

The PrincipalsAllowedToDelegateToAccount parameter sets the Active Directory object attribute msDS-AllowedToActOnBehalfOfOtherIdentity, which contains an access control list (ACL) that specifies which accounts have permission to delegate credentials to the associated account (in our example, it will be the machine account for ServerA).

Now let's set up the variables we'll use to represent the servers:

# Set up variables for reuse
$ServerA = $env:COMPUTERNAME
$ServerB = Get-ADComputer -Identity ServerB
$ServerC = Get-ADComputer -Identity ServerC

WinRM (and therefore PowerShell remoting) runs as the computer account by default. You can see this by looking at the StartName property of the winrm service:

Get-CimInstance Win32_Service -Filter 'Name="winrm"' | Select-Object StartName

StartName
---------
NT AUTHORITY\NetworkService

For ServerC to allow delegation from a PowerShell remoting session on ServerB, we must set the PrincipalsAllowedToDelegateToAccount parameter on ServerC to the computer object of ServerB:

# Grant resource-based Kerberos constrained delegation
Set-ADComputer -Identity $ServerC -PrincipalsAllowedToDelegateToAccount $ServerB

# Check the value of the attribute directly
$x = Get-ADComputer -Identity $ServerC -Properties msDS-AllowedToActOnBehalfOfOtherIdentity
$x.'msDS-AllowedToActOnBehalfOfOtherIdentity'.Access

# Check the value of the attribute indirectly
Get-ADComputer -Identity $ServerC -Properties PrincipalsAllowedToDelegateToAccount

The Kerberos Key Distribution Center (KDC) caches denied-access attempts (negative cache) for 15 minutes. If ServerB has previously attempted to access ServerC, you need to clear the cache on ServerB by invoking the following command:

Invoke-Command -ComputerName $ServerB.Name -Credential $cred -ScriptBlock {
    klist purge -li 0x3e7
}

You could also restart the computer, or wait at least 15 minutes to clear the cache.

After clearing the cache, you can successfully run code from ServerA through ServerB to ServerC:

# Capture a credential
$cred = Get-Credential Contoso\Alice

# Test kerberos double hop
Invoke-Command -ComputerName $ServerB.Name -Credential $cred -ScriptBlock {
    Test-Path \\$($using:ServerC.Name)\C$
    Get-Process lsass -ComputerName $($using:ServerC.Name)
    Get-EventLog -LogName System -Newest 3 -ComputerName $($using:ServerC.Name)
}

In this example, the $using variable is used to make the $ServerC variable visible to ServerB. For more information about the $using variable, see about_Remote_Variables.

To allow multiple servers to delegate credentials to ServerC, set the value of the PrincipalsAllowedToDelegateToAccount parameter on ServerC to an array:

# Set up variables for each server
$ServerB1 = Get-ADComputer -Identity ServerB1
$ServerB2 = Get-ADComputer -Identity ServerB2
$ServerB3 = Get-ADComputer -Identity ServerB3
$ServerC  = Get-ADComputer -Identity ServerC

$servers = @(
    $ServerB1,
    $ServerB2,
    $ServerB3
)

# Grant resource-based Kerberos constrained delegation
Set-ADComputer -Identity $ServerC -PrincipalsAllowedToDelegateToAccount $servers

If you want to make the second hop across domains, use the Server parameter to specify fully-qualified domain name (FQDN) of the domain controller of the domain to which ServerB belongs:

# For ServerC in Contoso domain and ServerB in other domain
$ServerB = Get-ADComputer -Identity ServerB -Server dc1.alpineskihouse.com
$ServerC = Get-ADComputer -Identity ServerC
Set-ADComputer -Identity $ServerC -PrincipalsAllowedToDelegateToAccount $ServerB

To remove the ability to delegate credentials to ServerC, set the value of the PrincipalsAllowedToDelegateToAccount parameter on ServerC to $null:

Set-ADComputer -Identity $ServerC -PrincipalsAllowedToDelegateToAccount $null

Information on resource-based Kerberos constrained delegation

Kerberos delegation (unconstrained)

You can also use Kerberos unconstrained delegation to make the second hop. Like all Kerberos scenarios, credentials aren't stored. This method doesn't support the second hop for WinRM.

Warning

This method provides no control of where delegated credentials are used. It's less secure than CredSSP. This method should only be used for testing scenarios.

Just Enough Administration (JEA)

JEA allows you to restrict what commands an administrator can run during a PowerShell session. It can be used to solve the second hop problem.

For information about JEA, see Just Enough Administration.

Pros

  • No password maintenance when using a virtual account.

Cons

  • Requires WMF 5.0 or later.
  • Requires configuration on every intermediate server (ServerB).

PSSessionConfiguration using RunAs

You can create a session configuration on ServerB and set its RunAsCredential parameter.

For information about using PSSessionConfiguration and RunAs to solve the second hop problem, see Another solution to multi-hop PowerShell remoting.

Pros

  • Works with any server with WMF 3.0 or later.

Cons

  • Requires configuration of PSSessionConfiguration and RunAs on every intermediate server (ServerB).
  • Requires password maintenance when using a domain RunAs account

Pass credentials inside an Invoke-Command script block

You can pass credentials inside the ScriptBlock parameter of a call to the Invoke-Command cmdlet.

Pros

  • Doesn't require special server configuration.
  • Works on any server running WMF 2.0 or later.

Cons

  • Requires an awkward code technique.
  • If running WMF 2.0, requires different syntax for passing arguments to a remote session.

Example

The following example shows how to pass credentials in a script block:

# This works without delegation, passing fresh creds
# Note $Using:Cred in nested request
$cred = Get-Credential Contoso\Administrator
Invoke-Command -ComputerName ServerB -Credential $cred -ScriptBlock {
    hostname
    Invoke-Command -ComputerName ServerC -Credential $Using:cred -ScriptBlock {hostname}
}

See also

PowerShell Remoting Security Considerations