From 9804aba7e189e5f9c2dcbf0f557f9c465af07c09 Mon Sep 17 00:00:00 2001
From: Paul Higinbotham <paulhi@microsoft.com>
Date: Mon, 8 Jul 2019 19:12:11 +0000
Subject: [PATCH] Merged PR 8969: Disable Enter-PSHostProcess cmdlet when
 system in lock down mode

This is based on an issue, where Enter-PSHostProcess on a locked down (WDAC enforced) machine allows any admin to connect to any another local hosted PowerShell process and execute commands as that user.  This amounts to privilege escalation on the policy locked down machine and something we want to prevent.

Fix is to check for system lock down and disable Enter-PSHostProcess cmdlet with an error message.
---
 .../commands/EnterPSHostProcessCommand.cs     | 14 +++++++++++
 .../resources/RemotingErrorIdStrings.resx     |  3 +++
 .../ConstrainedLanguageRestriction.Tests.ps1  | 25 +++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/src/System.Management.Automation/engine/remoting/commands/EnterPSHostProcessCommand.cs b/src/System.Management.Automation/engine/remoting/commands/EnterPSHostProcessCommand.cs
index 38169d67b9..51e5e2e9b3 100644
--- a/src/System.Management.Automation/engine/remoting/commands/EnterPSHostProcessCommand.cs
+++ b/src/System.Management.Automation/engine/remoting/commands/EnterPSHostProcessCommand.cs
@@ -13,6 +13,7 @@ using System.Management.Automation.Host;
 using System.Management.Automation.Internal;
 using System.Management.Automation.Remoting;
 using System.Management.Automation.Runspaces;
+using System.Management.Automation.Security;
 using System.Text;
 
 namespace Microsoft.PowerShell.Commands
@@ -126,6 +127,19 @@ namespace Microsoft.PowerShell.Commands
         /// </summary>
         protected override void EndProcessing()
         {
+            // Check if system is in locked down mode, in which case this cmdlet is disabled.
+            if (SystemPolicy.GetSystemLockdownPolicy() == SystemEnforcementMode.Enforce)
+            {
+                WriteError(
+                    new ErrorRecord(
+                        new PSSecurityException(RemotingErrorIdStrings.EnterPSHostProcessCmdletDisabled),
+                        "EnterPSHostProcessCmdletDisabled",
+                        ErrorCategory.SecurityError,
+                        null));
+
+                return;
+            }
+
             // Check for host that supports interactive remote sessions.
             _interactiveHost = this.Host as IHostSupportsInteractiveSession;
             if (_interactiveHost == null)
diff --git a/src/System.Management.Automation/resources/RemotingErrorIdStrings.resx b/src/System.Management.Automation/resources/RemotingErrorIdStrings.resx
index da1b1c6154..42be53c219 100644
--- a/src/System.Management.Automation/resources/RemotingErrorIdStrings.resx
+++ b/src/System.Management.Automation/resources/RemotingErrorIdStrings.resx
@@ -1693,4 +1693,7 @@ All WinRM sessions connected to PowerShell session configurations, such as Micro
   <data name="PSCoreRemotingEnableWarning" xml:space="preserve">
     <value>PowerShell remoting has been enabled only for PowerShell Core configurations and does not affect Windows PowerShell remoting configurations. Run this cmdlet in Windows PowerShell to affect all PowerShell remoting configurations.</value>
   </data>
+  <data name="EnterPSHostProcessCmdletDisabled" xml:space="preserve">
+    <value>Enter-PSHostProcess cmdlet is disabled because an application control policy such as 'AppLocker' or 'Windows Defender Application Control' is in enforcement.</value>
+  </data>
 </root>
diff --git a/test/powershell/Modules/Microsoft.PowerShell.Security/ConstrainedLanguageRestriction.Tests.ps1 b/test/powershell/Modules/Microsoft.PowerShell.Security/ConstrainedLanguageRestriction.Tests.ps1
index e5fe6d081c..19722d62ff 100644
--- a/test/powershell/Modules/Microsoft.PowerShell.Security/ConstrainedLanguageRestriction.Tests.ps1
+++ b/test/powershell/Modules/Microsoft.PowerShell.Security/ConstrainedLanguageRestriction.Tests.ps1
@@ -1164,6 +1164,31 @@ try
         }
     }
 
+    Describe "Enter-PSHostProcess cmdlet should be disabled on locked down systems" -Tags 'Feature','RequireAdminOnWindows' {
+
+        It "Verifies that Enter-PSHostProcess is disabled with lock down policy" {
+
+            $expectedError = $null
+            try
+            {
+                Invoke-LanguageModeTestingSupportCmdlet -SetLockdownMode
+                $ExecutionContext.SessionState.LanguageMode = "ConstrainedLanguage"
+
+                Enter-PSHostProcess -Id 5555 -ErrorAction Stop
+            }
+            catch
+            {
+                $expectedError = $_
+            }
+            finally
+            {
+                Invoke-LanguageModeTestingSupportCmdlet -RevertLockdownMode -EnableFullLanguageMode
+            }
+
+            $expectedError.FullyQualifiedErrorId | Should -BeExactly 'EnterPSHostProcessCmdletDisabled,Microsoft.PowerShell.Commands.EnterPSHostProcessCommand'
+        }
+    }
+
     # End Describe blocks
 }
 finally