diff --git a/docs/operations/CURRENT_STATE.md b/docs/operations/CURRENT_STATE.md
index 5bcc87e..bb91d55 100644
--- a/docs/operations/CURRENT_STATE.md
+++ b/docs/operations/CURRENT_STATE.md
@@ -1,5 +1,18 @@
 # Infrastructure current state
 
+## Windows ACL SID resolution — 2026-11-03
+
+- Read-only PowerShell 5.1 inspection found that display-name conversion of
+  ACL entries can throw `IdentityNotMappedException` even though the ACL itself
+  is readable. The installer and external-receiver guards now enumerate access
+  rules directly as `SecurityIdentifier` values and read owners as SIDs; unknown
+  principals remain visible to the existing fail-closed policy. Regression
+  coverage exercises synthetic unmapped SIDs through the actual installer guard
+  functions. These repository changes are pending review; no Windows ACL,
+  installer, task, VPS, or production state was changed. The native PowerShell
+  harness still requires execution on Windows before this behavior is confirmed
+  in that runtime.
+
 ## External deployment runtime binding — 2026-11-03
 
 - PR #59 is merged to `main`; it contains the root-only read-only status
diff --git a/infra/backup/receive_external.py b/infra/backup/receive_external.py
index 5bce695..65ef909 100644
--- a/infra/backup/receive_external.py
+++ b/infra/backup/receive_external.py
@@ -83,8 +83,9 @@ $allowed = @($owner, 'S-1-5-18', 'S-1-5-32-544')
 $aclOwner = $acl.GetOwner([System.Security.Principal.SecurityIdentifier]).Value
 if ($aclOwner -notin $allowed) { exit 6 }
 $ownerAllowed = $false
-foreach ($rule in $acl.Access) {
-  $sid = $rule.IdentityReference.Translate([System.Security.Principal.SecurityIdentifier]).Value
+$rules = $acl.GetAccessRules($true, $true, [System.Security.Principal.SecurityIdentifier])
+foreach ($rule in $rules) {
+  $sid = $rule.IdentityReference.Value
   if ($rule.AccessControlType -ne [System.Security.AccessControl.AccessControlType]::Allow -or $sid -notin $allowed) { exit 2 }
   if (($rule.FileSystemRights -band [System.Security.AccessControl.FileSystemRights]::FullControl) -ne [System.Security.AccessControl.FileSystemRights]::FullControl) { exit 3 }
   if ($sid -eq $owner) { $ownerAllowed = $true }
diff --git a/infra/backup/test_receive_external.py b/infra/backup/test_receive_external.py
index 229e138..4a1566b 100644
--- a/infra/backup/test_receive_external.py
+++ b/infra/backup/test_receive_external.py
@@ -203,6 +203,9 @@ class ExternalReceiverTests(unittest.TestCase):
         self.assertIn("System.IO.DriveInfo", run_mock.call_args.args[0][-1])
         self.assertNotIn("Get-Volume", run_mock.call_args.args[0][-1])
         self.assertIn("$acl.GetOwner(", run_mock.call_args.args[0][-1])
+        self.assertIn("$acl.GetAccessRules($true, $true, [System.Security.Principal.SecurityIdentifier])", run_mock.call_args.args[0][-1])
+        self.assertIn("$rule.IdentityReference.Value", run_mock.call_args.args[0][-1])
+        self.assertNotIn(".Translate(", run_mock.call_args.args[0][-1])
         self.assertIn("$aclOwner -notin $allowed", run_mock.call_args.args[0][-1])
 
     def test_private_acl_rejects_an_unapproved_acl_owner(self):
diff --git a/infra/deploy/install_windows_release_client.ps1 b/infra/deploy/install_windows_release_client.ps1
index 6491035..f754a97 100644
--- a/infra/deploy/install_windows_release_client.ps1
+++ b/infra/deploy/install_windows_release_client.ps1
@@ -50,7 +50,8 @@ function Assert-ProtectedPath([string]$Path, [bool]$AllowOwnerWrite = $false) {
     $item = Get-Item -LiteralPath $Path -Force
     if (($item.Attributes -band [IO.FileAttributes]::ReparsePoint) -ne 0) { throw "Refusing reparse path: $Path" }
     $acl = Get-Acl -LiteralPath $Path
-    $ownerSid = ([Security.Principal.NTAccount]$acl.Owner).Translate([Security.Principal.SecurityIdentifier]).Value
+    $ownerSid = $acl.GetOwner([Security.Principal.SecurityIdentifier]).Value
+    $rules = $acl.GetAccessRules($true, $true, [Security.Principal.SecurityIdentifier])
     $trustedInstaller = 'S-1-5-80-956008885-3418522649-1831038044-1853292631-2271478464'
     if ($AllowOwnerWrite) {
         $approvedOwners = @('S-1-5-18', 'S-1-5-32-544', $identity.User.Value)
@@ -60,24 +61,24 @@ function Assert-ProtectedPath([string]$Path, [bool]$AllowOwnerWrite = $false) {
     if ($ownerSid -notin $approvedOwners) { throw "Protected path has an unapproved owner: $Path" }
     if ($AllowOwnerWrite) {
         $allowed = @('S-1-5-18', 'S-1-5-32-544', $identity.User.Value)
-        foreach ($rule in $acl.Access) {
-            $sid = $rule.IdentityReference.Translate([Security.Principal.SecurityIdentifier]).Value
+        foreach ($rule in $rules) {
+            $sid = $rule.IdentityReference.Value
             if ($rule.AccessControlType -ne 'Allow' -or $sid -notin $allowed -or
                 ($rule.FileSystemRights -band [System.Security.AccessControl.FileSystemRights]::FullControl) -ne [System.Security.AccessControl.FileSystemRights]::FullControl) {
                 throw "Private state path ACL must grant only full control to owner/SYSTEM/Administrators: $Path"
             }
         }
-        $ownerHasFullControl = @($acl.Access | Where-Object {
+        $ownerHasFullControl = @($rules | Where-Object {
             $_.AccessControlType -eq 'Allow' -and
-            $_.IdentityReference.Translate([Security.Principal.SecurityIdentifier]).Value -eq $identity.User.Value -and
+            $_.IdentityReference.Value -eq $identity.User.Value -and
             ($_.FileSystemRights -band [System.Security.AccessControl.FileSystemRights]::FullControl) -eq [System.Security.AccessControl.FileSystemRights]::FullControl
         }).Count -gt 0
         if (-not $ownerHasFullControl) { throw "Task owner needs full control on protected state path: $Path" }
     } else {
-        $writableUntrusted = @($acl.Access | Where-Object {
+        $writableUntrusted = @($rules | Where-Object {
             $_.AccessControlType -eq 'Allow' -and
             (([int]$_.FileSystemRights -band [int]$mutationRights) -ne 0) -and
-            $_.IdentityReference.Translate([Security.Principal.SecurityIdentifier]).Value -notin @('S-1-5-18', 'S-1-5-32-544', $trustedInstaller)
+            $_.IdentityReference.Value -notin @('S-1-5-18', 'S-1-5-32-544', $trustedInstaller)
         })
         if ($writableUntrusted.Count -gt 0) { throw "Protected path grants write access outside SYSTEM/Administrators: $Path" }
     }
@@ -102,13 +103,14 @@ function Assert-TrustedPath([string]$Path, [bool]$ParentsOnly = $false) {
             throw "Protected path contains a reparse point: $($current.FullName)"
         }
         $acl = Get-Acl -LiteralPath $current.FullName
-        $ownerSid = ([Security.Principal.NTAccount]$acl.Owner).Translate([Security.Principal.SecurityIdentifier]).Value
+        $ownerSid = $acl.GetOwner([Security.Principal.SecurityIdentifier]).Value
         if ($ownerSid -notin @('S-1-5-18', 'S-1-5-32-544', $trustedInstaller)) {
             throw "Protected path has an untrusted owner: $($current.FullName)"
         }
         $checkedRights = if ($isLeaf) { $mutationRights } else { $pathReplacementRights }
-        foreach ($rule in $acl.Access) {
-            $sid = $rule.IdentityReference.Translate([Security.Principal.SecurityIdentifier]).Value
+        $rules = $acl.GetAccessRules($true, $true, [Security.Principal.SecurityIdentifier])
+        foreach ($rule in $rules) {
+            $sid = $rule.IdentityReference.Value
             if (($rule.PropagationFlags -band [System.Security.AccessControl.PropagationFlags]::InheritOnly) -ne 0) { continue }
             if ($rule.AccessControlType -eq 'Allow' -and
                 (([int]$rule.FileSystemRights -band [int]$checkedRights) -ne 0) -and
diff --git a/infra/deploy/test_windows_release_bundle_mode.ps1 b/infra/deploy/test_windows_release_bundle_mode.ps1
index 3d7a685..d21a418 100644
--- a/infra/deploy/test_windows_release_bundle_mode.ps1
+++ b/infra/deploy/test_windows_release_bundle_mode.ps1
@@ -17,8 +17,7 @@ if (($temporaryRootItem.Attributes -band [IO.FileAttributes]::ReparsePoint) -ne
     throw 'Temporary fixture root must not be a reparse point.'
 }
 $temporaryRootCreated = $temporaryRootItem.CreationTimeUtc
-$temporaryAclOwner = (Get-Acl -LiteralPath $temporaryRoot).Owner
-$temporaryOwnerSid = ([Security.Principal.NTAccount]$temporaryAclOwner).Translate(
+$temporaryOwnerSid = (Get-Acl -LiteralPath $temporaryRoot).GetOwner(
     [Security.Principal.SecurityIdentifier]).Value
 $currentSid = [Security.Principal.WindowsIdentity]::GetCurrent().User.Value
 if ($temporaryOwnerSid -cne $currentSid) { throw 'Temporary fixture root is not owned by the current user.' }
@@ -44,8 +43,7 @@ function Get-VerifiedFixtureRoot([string]$Candidate) {
         $item.CreationTimeUtc -ne $temporaryRootCreated) {
         throw 'Fixture root was replaced, moved, or resolved outside its original identity.'
     }
-    $ownerName = (Get-Acl -LiteralPath $candidatePath).Owner
-    $ownerSid = ([Security.Principal.NTAccount]$ownerName).Translate(
+    $ownerSid = (Get-Acl -LiteralPath $candidatePath).GetOwner(
         [Security.Principal.SecurityIdentifier]).Value
     if ($ownerSid -cne $temporaryOwnerSid -or $ownerSid -cne $currentSid) {
         throw 'Fixture root owner changed.'
@@ -63,8 +61,7 @@ function Get-VerifiedFixtureEntry([string]$Path, [string]$ResolvedRoot) {
     if (-not $resolved.StartsWith($ResolvedRoot + '\', [StringComparison]::OrdinalIgnoreCase)) {
         throw 'Fixture entry resolved outside its verified temporary root.'
     }
-    $ownerName = (Get-Acl -LiteralPath $resolved).Owner
-    $ownerSid = ([Security.Principal.NTAccount]$ownerName).Translate(
+    $ownerSid = (Get-Acl -LiteralPath $resolved).GetOwner(
         [Security.Principal.SecurityIdentifier]).Value
     if ($ownerSid -cne $temporaryOwnerSid -or $ownerSid -cne $currentSid) {
         throw 'Fixture entry is not owned by the current user.'
@@ -104,6 +101,88 @@ try {
     if ($null -eq $validatorAst) { throw 'Installer bundle-mode validator was not found.' }
     . ([scriptblock]::Create($validatorAst.Extent.Text))
 
+    $identity = [Security.Principal.WindowsIdentity]::GetCurrent()
+    $mutationRights = [System.Security.AccessControl.FileSystemRights](
+        [int][System.Security.AccessControl.FileSystemRights]::WriteData -bor
+        [int][System.Security.AccessControl.FileSystemRights]::AppendData -bor
+        [int][System.Security.AccessControl.FileSystemRights]::WriteExtendedAttributes -bor
+        [int][System.Security.AccessControl.FileSystemRights]::WriteAttributes -bor
+        [int][System.Security.AccessControl.FileSystemRights]::DeleteSubdirectoriesAndFiles -bor
+        [int][System.Security.AccessControl.FileSystemRights]::Delete -bor
+        [int][System.Security.AccessControl.FileSystemRights]::ChangePermissions -bor
+        [int][System.Security.AccessControl.FileSystemRights]::TakeOwnership
+    )
+    $pathReplacementRights = [System.Security.AccessControl.FileSystemRights](
+        [int][System.Security.AccessControl.FileSystemRights]::DeleteSubdirectoriesAndFiles -bor
+        [int][System.Security.AccessControl.FileSystemRights]::Delete -bor
+        [int][System.Security.AccessControl.FileSystemRights]::ChangePermissions -bor
+        [int][System.Security.AccessControl.FileSystemRights]::TakeOwnership
+    )
+    foreach ($guardName in @('Assert-ProtectedPath', 'Assert-TrustedPath')) {
+        $guardAst = $installerAst.Find({
+            param($node)
+            $node -is [System.Management.Automation.Language.FunctionDefinitionAst] -and
+                $node.Name -ceq $guardName
+        }, $true)
+        if ($null -eq $guardAst) { throw "Installer ACL guard was not found: $guardName" }
+        . ([scriptblock]::Create($guardAst.Extent.Text))
+    }
+
+    $unknownSid = [Security.Principal.SecurityIdentifier]::new(
+        'S-1-5-21-3141592653-2718281828-1618033988-987654')
+    $unmapped = $false
+    try { $null = $unknownSid.Translate([Security.Principal.NTAccount]) }
+    catch [Security.Principal.IdentityNotMappedException] { $unmapped = $true }
+    if (-not $unmapped) { throw 'The synthetic ACL SID unexpectedly resolves to an NTAccount.' }
+
+    $savedMockAcl = $script:akyldashAclFixture
+    function Get-Acl([string]$LiteralPath) { return $script:akyldashAclFixture }
+    try {
+        $sidSystem = [Security.Principal.SecurityIdentifier]::new('S-1-5-18')
+        $sidAdmins = [Security.Principal.SecurityIdentifier]::new('S-1-5-32-544')
+        $privateFixtureAcl = [System.Security.AccessControl.DirectorySecurity]::new()
+        $privateFixtureAcl.SetAccessRuleProtection($true, $false)
+        $privateFixtureAcl.SetOwner($identity.User)
+        foreach ($sid in @($sidSystem, $sidAdmins, $identity.User)) {
+            $null = $privateFixtureAcl.AddAccessRule([System.Security.AccessControl.FileSystemAccessRule]::new(
+                $sid, [System.Security.AccessControl.FileSystemRights]::FullControl,
+                [System.Security.AccessControl.AccessControlType]::Allow))
+        }
+        $null = $privateFixtureAcl.AddAccessRule([System.Security.AccessControl.FileSystemAccessRule]::new(
+            $unknownSid, [System.Security.AccessControl.FileSystemRights]::FullControl,
+            [System.Security.AccessControl.AccessControlType]::Allow))
+        $script:akyldashAclFixture = $privateFixtureAcl
+        $privateRejected = $false
+        try { Assert-ProtectedPath -Path $temporaryRoot -AllowOwnerWrite $true }
+        catch {
+            if ($_.Exception.Message -notlike 'Private state path ACL must grant only full control*') { throw }
+            $privateRejected = $true
+        }
+        if (-not $privateRejected) { throw 'Unmapped SID was not rejected by the protected-path guard.' }
+
+        $trustedFixtureAcl = [System.Security.AccessControl.DirectorySecurity]::new()
+        $trustedFixtureAcl.SetAccessRuleProtection($true, $false)
+        $trustedFixtureAcl.SetOwner($sidSystem)
+        $null = $trustedFixtureAcl.AddAccessRule([System.Security.AccessControl.FileSystemAccessRule]::new(
+            $sidSystem, [System.Security.AccessControl.FileSystemRights]::FullControl,
+            [System.Security.AccessControl.AccessControlType]::Allow))
+        $null = $trustedFixtureAcl.AddAccessRule([System.Security.AccessControl.FileSystemAccessRule]::new(
+            $unknownSid, [System.Security.AccessControl.FileSystemRights]::WriteData,
+            [System.Security.AccessControl.AccessControlType]::Allow))
+        $script:akyldashAclFixture = $trustedFixtureAcl
+        $trustedRejected = $false
+        try { Assert-TrustedPath -Path $temporaryRoot }
+        catch {
+            if ($_.Exception.Message -notlike 'Protected path grants replacement/write access*') { throw }
+            $trustedRejected = $true
+        }
+        if (-not $trustedRejected) { throw 'Unmapped SID was not rejected by the trusted-path guard.' }
+    } finally {
+        Remove-Item Function:\Get-Acl -ErrorAction SilentlyContinue
+        if ($null -eq $savedMockAcl) { Remove-Variable -Name akyldashAclFixture -Scope Script -ErrorAction SilentlyContinue }
+        else { $script:akyldashAclFixture = $savedMockAcl }
+    }
+
     Add-Type -AssemblyName System.IO.Compression
     $stream = [IO.File]::OpenRead($bundlePath)
     try {
diff --git a/infra/deploy/test_windows_release_launcher.py b/infra/deploy/test_windows_release_launcher.py
index cebdc12..ab28840 100644
--- a/infra/deploy/test_windows_release_launcher.py
+++ b/infra/deploy/test_windows_release_launcher.py
@@ -113,6 +113,10 @@ class WindowsReleaseBootstrapTests(unittest.TestCase):
                         script.index("$bundleStream = [IO.File]::Open"))
         self.assertIn("$approvedOwners = @('S-1-5-18', 'S-1-5-32-544', $trustedInstaller)", script)
         self.assertIn("$approvedOwners = @('S-1-5-18', 'S-1-5-32-544', $identity.User.Value)", script)
+        self.assertIn("$acl.GetOwner([Security.Principal.SecurityIdentifier]).Value", script)
+        self.assertGreaterEqual(script.count("$acl.GetAccessRules($true, $true, [Security.Principal.SecurityIdentifier])"), 2)
+        self.assertIn("$rule.IdentityReference.Value", script)
+        self.assertNotIn(".Translate(", script)
         self.assertIn("Assert-TrustedPath $appRoot", script)
         self.assertIn("Assert-TrustedPath $programData -ParentsOnly $true", script)
         self.assertIn("Set-TrustedCodeOwner $app", script)
@@ -120,6 +124,15 @@ class WindowsReleaseBootstrapTests(unittest.TestCase):
         for name in release_bundle.FILES:
             self.assertIn(f"files/{name}", script)
 
+    def test_windows_acl_harness_exercises_unmapped_sid_with_real_installer_guards(self):
+        harness = (ROOT / "infra/deploy/test_windows_release_bundle_mode.ps1").read_text(encoding="utf-8")
+        self.assertIn("Assert-ProtectedPath", harness)
+        self.assertIn("Assert-TrustedPath", harness)
+        self.assertIn("IdentityNotMappedException", harness)
+        self.assertIn("$unknownSid", harness)
+        self.assertIn("$guardAst.Extent.Text", harness)
+        self.assertIn("Unmapped SID was not rejected by the protected-path guard", harness)
+
     def test_release_bundle_uses_exact_commit_objects_not_dirty_worktree_or_git_hooks(self):
         with tempfile.TemporaryDirectory() as directory:
             root = Path(directory)