Hide password changes for managed SGU users

docs/architecture.md

@@ -62,6 +62,15 @@ plus six digits. It searches `BaseDn` by `sAMAccountName`, creates the user when
 absent, moves it to the mapped OU when required, sets `userPrincipalName`, and
 passes the submitted password directly to ADSI `SetPassword`.
 
+Before an account becomes usable, the broker applies explicit deny ACEs for the
+Active Directory `Change Password` extended right to SELF and Everyone. Users
+beneath `OU=Usuarios-SGU` therefore cannot replace the synchronized password
+from Windows, Ctrl+Alt+Delete, LDAP or another client. The broker's
+administrative `SetPassword` operation uses the separate `Reset Password` right
+and remains able to synchronize the current institutional credential after each
+successful SGU authentication. Repeated synchronizations detect the existing
+ACEs and do not duplicate them.
+
 When the authenticated HTML exposes recognized stable IDs, the broker also
 updates the applicable `displayName`, `givenName`, `sn`, `mail`, `title`,
 `department`, `employeeType`, `employeeID`, `streetAddress`, `l`, `st`, and

docs/release-0.6.8.md

@@ -0,0 +1,17 @@
+# SGU Credential Provider 0.6.8
+
+Esta versión declara al Auth Broker como la única autoridad de contraseñas para
+todas las cuentas administradas bajo `OU=Usuarios-SGU`.
+
+- Cada creación, movimiento o actualización de una cuenta agrega de forma
+  idempotente las denegaciones SELF y Everyone para el derecho extendido de
+  Active Directory `Change Password`.
+- El usuario no puede cambiar la contraseña desde Windows, Ctrl+Alt+Delete,
+  LDAP ni herramientas equivalentes.
+- El broker conserva el derecho administrativo separado `Reset Password` y
+  continúa sincronizando la contraseña institucional exacta mediante
+  `SetPassword` después de una autenticación SGU válida.
+- El despliegue del broker recorre todas las cuentas existentes en
+  `Usuarios-SGU` y corrige aquellas que todavía permiten cambios directos.
+
+No se modifica la contraseña institucional ni se almacena una copia adicional.

docs/release-0.6.9.md

@@ -0,0 +1,16 @@
+# SGU Credential Provider 0.6.9
+
+Esta versión completa la autoridad de contraseñas del Auth Broker en la
+interfaz de Windows.
+
+- La GPO `SGU - User session restrictions` habilita la directiva **Remove
+  Change Password** para todas las cuentas bajo `OU=Usuarios-SGU`.
+- La opción **Cambiar una contraseña** deja de aparecer en la pantalla de
+  seguridad de Ctrl+Alt+Supr.
+- La protección real continúa en Active Directory mediante las denegaciones
+  del derecho extendido `Change Password`; la GPO únicamente evita mostrar una
+  acción que esas cuentas no pueden completar.
+
+El Auth Broker conserva el derecho administrativo separado `Reset Password`
+para sincronizar la contraseña institucional después de una autenticación SGU
+válida.

docs/security.md

@@ -18,6 +18,13 @@ the same source list. RDP uses a separate allowlist. See
   `Marshal.ZeroFreeGlobalAllocUnicode`; managed references are released as soon
   as each request completes.
 - The broker uses the exact received value for both NTLM and AD `SetPassword`.
+- Managed `Usuarios-SGU` accounts deny the SELF and Everyone `Change Password`
+  extended right. Only an administrator or the broker through the separate
+  `Reset Password` right can replace the AD password.
+- The `SGU - User session restrictions` GPO also hides the **Change a
+  password** command from the Windows Ctrl+Alt+Delete security screen for
+  managed users. This is a user-interface complement to the directory ACL,
+  not a substitute for it.
 - There is no HMAC password, pepper, local password cache, Supabase password, or
   other derived credential in this Windows path.
 - Neither application logs request bodies or passwords. Deployment configuration

scripts/Deploy-AuthBroker.ps1

@@ -201,6 +201,19 @@ if ($RemoteDesktopGroupDn) {
     }
 }
 
+$managedUsersPasswordChangeCorrected = 0
+if ($PSCmdlet.ShouldProcess($usersOuDn, 'Deny direct password changes for every managed SGU user')) {
+    $managedUsers = @(Get-ADUser -Filter * -SearchBase $usersOuDn -SearchScope Subtree `
+        -Properties CannotChangePassword -Server $LdapHost -ErrorAction Stop)
+    foreach ($managedUser in $managedUsers) {
+        if (-not $managedUser.CannotChangePassword) {
+            Set-ADAccountControl -Identity $managedUser.DistinguishedName `
+                -CannotChangePassword $true -Server $LdapHost -Confirm:$false
+            $managedUsersPasswordChangeCorrected++
+        }
+    }
+}
+
 foreach ($file in @('SGU.AuthBroker.exe', 'SGU.AuthBroker.dll', 'appsettings.json')) {
     if (-not (Test-Path -LiteralPath (Join-Path $PublishPath $file))) {
         throw "PublishPath is missing $file."
@@ -356,4 +369,5 @@ if ($PSCmdlet.ShouldProcess($installPath, 'Install the SGU Authentication Broker
 }
 
 Get-Service -Name $serviceName | Select-Object Name, Status, StartType,
-    @{ Name = 'EventLog'; Expression = { $brokerEventLogName } }
+    @{ Name = 'EventLog'; Expression = { $brokerEventLogName } },
+    @{ Name = 'ExistingUsersPasswordChangeDenied'; Expression = { $managedUsersPasswordChangeCorrected } }

scripts/Set-SguDomainUserPolicies.ps1

@@ -10,6 +10,7 @@ param(
 $ErrorActionPreference = 'Stop'
 $policyKey = 'HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System'
 $policyValueName = 'DisableLockWorkstation'
+$disableChangePasswordValueName = 'DisableChangePassword'
 $desktopPolicyKey = 'HKCU\Software\Policies\Microsoft\Windows\Control Panel\Desktop'
 $themeKey = 'HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize'
 
@@ -76,6 +77,17 @@ if ($PSCmdlet.ShouldProcess($GpoName, 'Prevent SGU users from manually locking w
         -Type DWord `
         -Value 1 | Out-Null
 
+    # The directory ACL remains the security boundary. This user policy also
+    # removes the unusable Change a password action from Ctrl+Alt+Delete.
+    Set-GPRegistryValue `
+        -Name $GpoName `
+        -Domain $domainName `
+        -Server $DomainController `
+        -Key $policyKey `
+        -ValueName $disableChangePasswordValueName `
+        -Type DWord `
+        -Value 1 | Out-Null
+
     Set-GPRegistryValue `
         -Name $GpoName `
         -Domain $domainName `
@@ -135,6 +147,12 @@ $configuredValue = Get-GPRegistryValue `
     -Server $DomainController `
     -Key $policyKey `
     -ValueName $policyValueName
+$disableChangePasswordValue = Get-GPRegistryValue `
+    -Name $GpoName `
+    -Domain $domainName `
+    -Server $DomainController `
+    -Key $policyKey `
+    -ValueName $disableChangePasswordValueName
 $screenSaverValue = Get-GPRegistryValue `
     -Name $GpoName `
     -Domain $domainName `
@@ -175,6 +193,7 @@ if ($WallpaperPath) {
     TargetOu = $TargetOuDn
     LinkEnabled = [bool]$linkEnabled
     DisableLockWorkstation = [int]$configuredValue.Value
+    DisableChangePassword = [int]$disableChangePasswordValue.Value
     ScreenSaverDisabled = [string]$screenSaverValue.Value -eq '0'
     DarkMode = ([int]$appsThemeValue.Value -eq 0) -and ([int]$systemThemeValue.Value -eq 0)
     Wallpaper = $configuredWallpaper

src/SGU.AuthBroker/BrokerEventIds.cs

@@ -22,4 +22,5 @@ internal static class BrokerEventIds
     internal static readonly EventId DirectoryOptionalMetadataFailure = new(1301, nameof(DirectoryOptionalMetadataFailure));
     internal static readonly EventId DirectoryGroupMembershipFailure = new(1302, nameof(DirectoryGroupMembershipFailure));
     internal static readonly EventId DirectoryRoleGroupMembershipAdded = new(1303, nameof(DirectoryRoleGroupMembershipAdded));
+    internal static readonly EventId DirectoryPasswordChangeDenied = new(1304, nameof(DirectoryPasswordChangeDenied));
 }

src/SGU.AuthBroker/Services/ActiveDirectorySynchronizer.cs

@@ -1,5 +1,7 @@
 using System.Collections.Concurrent;
 using System.DirectoryServices;
+using System.Security.AccessControl;
+using System.Security.Principal;
 using SGU.AuthBroker.Core.Directory;
 using SGU.AuthBroker.Core.Identity;
 using SGU.AuthBroker.Core.Profiles;
@@ -15,6 +17,12 @@ public sealed class ActiveDirectorySynchronizer(
     private const int InfoAttributeMaximumLength = 1024;
     private const int AccountDisabled = 0x0002;
     private const int NormalAccount = 0x0200;
+    private static readonly Guid ChangePasswordExtendedRight =
+        new("AB721A53-1E2F-11D0-9819-00AA0040529B");
+    private static readonly SecurityIdentifier EveryoneSid =
+        new(WellKnownSidType.WorldSid, null);
+    private static readonly SecurityIdentifier SelfSid =
+        new(WellKnownSidType.SelfSid, null);
     private static readonly AuthenticationTypes BindFlags =
         AuthenticationTypes.Secure | AuthenticationTypes.Signing | AuthenticationTypes.Sealing;
 
@@ -117,6 +125,12 @@ public sealed class ActiveDirectorySynchronizer(
             // account without its required classification.
             EnsureRoleGroupMembership(user, identity);
 
+            // SGU remains the password authority. Deny the user's Change
+            // Password extended right before making the account usable. This
+            // does not deny the broker's administrative Reset Password right,
+            // which ADSI SetPassword uses for each successful authentication.
+            EnsureCannotChangePassword(user, identity.UserName);
+
             // The exact institutional password received by the broker is passed to AD.
             // It is not derived, transformed, written to disk, or included in logs.
             user.Invoke("SetPassword", [password]);
@@ -143,6 +157,52 @@ public sealed class ActiveDirectorySynchronizer(
         }
     }
 
+    private void EnsureCannotChangePassword(DirectoryEntry user, string institutionalUser)
+    {
+        user.Options!.SecurityMasks = SecurityMasks.Dacl;
+        ActiveDirectorySecurity security = user.ObjectSecurity;
+        if (!EnsureCannotChangePassword(security))
+        {
+            return;
+        }
+
+        user.ObjectSecurity = security;
+        user.CommitChanges();
+        logger.LogInformation(
+            BrokerEventIds.DirectoryPasswordChangeDenied,
+            "Denied direct password changes for managed Active Directory user {InstitutionalUser}; SGU Auth Broker remains the password authority.",
+            institutionalUser);
+    }
+
+    internal static bool EnsureCannotChangePassword(ActiveDirectorySecurity security)
+    {
+        bool changed = false;
+        foreach (SecurityIdentifier identity in new[] { EveryoneSid, SelfSid })
+        {
+            bool exists = security
+                .GetAccessRules(includeExplicit: true, includeInherited: false, typeof(SecurityIdentifier))
+                .OfType<ActiveDirectoryAccessRule>()
+                .Any(rule =>
+                    rule.AccessControlType == AccessControlType.Deny &&
+                    rule.IdentityReference.Equals(identity) &&
+                    rule.ObjectType == ChangePasswordExtendedRight &&
+                    (rule.ActiveDirectoryRights & ActiveDirectoryRights.ExtendedRight) != 0);
+            if (exists)
+            {
+                continue;
+            }
+
+            security.AddAccessRule(new ActiveDirectoryAccessRule(
+                identity,
+                ActiveDirectoryRights.ExtendedRight,
+                AccessControlType.Deny,
+                ChangePasswordExtendedRight));
+            changed = true;
+        }
+
+        return changed;
+    }
+
     private static void TryApplyProfile(
         DirectoryEntry user,
         UserIdentity identity,

tests/AuthBrokerPasswordAuthority.Tests.ps1

@@ -0,0 +1,28 @@
+$repositoryRoot = (Resolve-Path (Join-Path $PSScriptRoot '..')).Path
+$synchronizerPath = Join-Path $repositoryRoot 'src\SGU.AuthBroker\Services\ActiveDirectorySynchronizer.cs'
+$deployPath = Join-Path $repositoryRoot 'scripts\Deploy-AuthBroker.ps1'
+$userPolicyPath = Join-Path $repositoryRoot 'scripts\Set-SguDomainUserPolicies.ps1'
+$synchronizer = Get-Content -LiteralPath $synchronizerPath -Raw
+$deploy = Get-Content -LiteralPath $deployPath -Raw
+$userPolicy = Get-Content -LiteralPath $userPolicyPath -Raw
+
+Describe 'SGU Auth Broker password authority' {
+    It 'denies the Change Password extended right to SELF and Everyone before SetPassword' {
+        $synchronizer | Should Match 'AB721A53-1E2F-11D0-9819-00AA0040529B'
+        $synchronizer | Should Match 'WellKnownSidType\.WorldSid'
+        $synchronizer | Should Match 'WellKnownSidType\.SelfSid'
+        $synchronizer | Should Match 'AccessControlType\.Deny'
+        $synchronizer.IndexOf('EnsureCannotChangePassword(user', [StringComparison]::Ordinal) |
+            Should BeLessThan $synchronizer.IndexOf('user.Invoke("SetPassword"', [StringComparison]::Ordinal)
+    }
+
+    It 'repairs every existing account below Usuarios-SGU during broker deployment' {
+        $deploy | Should Match 'Get-ADUser.*-SearchBase \$usersOuDn.*-SearchScope Subtree'
+        $deploy | Should Match '(?s)Set-ADAccountControl.*-CannotChangePassword \$true'
+    }
+
+    It 'removes Change Password from the Windows security screen for managed users' {
+        $userPolicy | Should Match "disableChangePasswordValueName = 'DisableChangePassword'"
+        $userPolicy | Should Match '(?s)-ValueName \$disableChangePasswordValueName.*-Type DWord.*-Value 1'
+    }
+}

tests/SGU.AuthBroker.Tests/ActiveDirectorySynchronizerTests.cs

@@ -1,3 +1,6 @@
+using System.DirectoryServices;
+using System.Security.AccessControl;
+using System.Security.Principal;
 using SGU.AuthBroker.Core.Profiles;
 using SGU.AuthBroker.Services;
 using Xunit;
@@ -31,4 +34,29 @@ public sealed class ActiveDirectorySynchronizerTests
 
         Assert.Null(updated);
     }
+
+    [Fact]
+    public void CannotChangePasswordRulesAreCompleteAndIdempotent()
+    {
+        ActiveDirectorySecurity security = new();
+
+        Assert.True(ActiveDirectorySynchronizer.EnsureCannotChangePassword(security));
+        Assert.False(ActiveDirectorySynchronizer.EnsureCannotChangePassword(security));
+
+        Guid changePasswordRight = new("AB721A53-1E2F-11D0-9819-00AA0040529B");
+        ActiveDirectoryAccessRule[] rules = security
+            .GetAccessRules(includeExplicit: true, includeInherited: false, typeof(SecurityIdentifier))
+            .OfType<ActiveDirectoryAccessRule>()
+            .Where(rule =>
+                rule.AccessControlType == AccessControlType.Deny &&
+                rule.ObjectType == changePasswordRight &&
+                (rule.ActiveDirectoryRights & ActiveDirectoryRights.ExtendedRight) != 0)
+            .ToArray();
+
+        Assert.Equal(2, rules.Length);
+        Assert.Contains(rules, rule => rule.IdentityReference.Equals(
+            new SecurityIdentifier(WellKnownSidType.WorldSid, null)));
+        Assert.Contains(rules, rule => rule.IdentityReference.Equals(
+            new SecurityIdentifier(WellKnownSidType.SelfSid, null)));
+    }
 }