diff --git a/tools/tht/internal/authconfig/store.go b/tools/tht/internal/authconfig/store.go
index 7264c2b8..c770ecef 100644
--- a/tools/tht/internal/authconfig/store.go
+++ b/tools/tht/internal/authconfig/store.go
@@ -6,7 +6,6 @@ import (
 	"io"
 	"os"
 	"path/filepath"
-	"runtime"
 
 	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
 	"github.com/gofrs/flock"
@@ -112,26 +111,15 @@ func decodeStrictYAML(contents []byte, destination any) error {
 }
 
 func requirePrivateDirectory(directory string) error {
-	if err := safeio.ValidateCanonicalPath(directory); err != nil {
-		return err
-	}
-	info, err := os.Lstat(directory)
-	if err != nil || !info.IsDir() || info.Mode()&os.ModeSymlink != 0 {
-		return safeio.ErrUnsafeFile
-	}
-	resolved, err := filepath.EvalSymlinks(directory)
-	if err != nil || resolved != directory {
-		return safeio.ErrUnsafeFile
-	}
-	if runtime.GOOS != "windows" && info.Mode().Perm() != 0o700 {
-		return safeio.ErrUnsafeFile
-	}
-	return nil
+	return safeio.ValidatePrivateDirectory(directory)
 }
 
 func readPrivateFile(path string) ([]byte, error) {
+	if err := safeio.ValidatePrivateRegular(path); err != nil {
+		return nil, err
+	}
 	before, err := os.Lstat(path)
-	if err != nil || !before.Mode().IsRegular() || before.Mode()&os.ModeSymlink != 0 || (runtime.GOOS != "windows" && before.Mode().Perm() != 0o600) {
+	if err != nil {
 		return nil, safeio.ErrUnsafeFile
 	}
 	contents, err := safeio.ReadCanonicalRegular(path, maxYAMLBytes)
@@ -139,7 +127,10 @@ func readPrivateFile(path string) ([]byte, error) {
 		return nil, err
 	}
 	after, err := os.Lstat(path)
-	if err != nil || !after.Mode().IsRegular() || after.Mode()&os.ModeSymlink != 0 || (runtime.GOOS != "windows" && after.Mode().Perm() != 0o600) || !os.SameFile(before, after) {
+	if err != nil || !os.SameFile(before, after) {
+		return nil, safeio.ErrUnsafeFile
+	}
+	if err := safeio.ValidatePrivateRegular(path); err != nil {
 		return nil, safeio.ErrUnsafeFile
 	}
 	return contents, nil
@@ -149,14 +140,18 @@ func acquireLock(directory string) (*flock.Flock, error) {
 	path := filepath.Join(directory, lockFileName)
 	if _, err := os.Lstat(path); errors.Is(err, os.ErrNotExist) {
 		if err := safeio.WriteCanonicalNewFile(path, nil, 0o600); err != nil {
-			info, statErr := os.Lstat(path)
-			if statErr != nil || !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || (runtime.GOOS != "windows" && info.Mode().Perm() != 0o600) {
+			// A competing mutation may have created the lock after our Lstat. Accept only
+			// that exact safe/private lock; every other creation failure remains unsafe.
+			if validateErr := safeio.ValidatePrivateRegular(path); validateErr != nil {
 				return nil, safeio.ErrUnsafeFile
 			}
 		}
 	} else if err != nil {
 		return nil, safeio.ErrUnsafeFile
 	}
+	if err := safeio.ValidatePrivateRegular(path); err != nil {
+		return nil, err
+	}
 	lock := flock.New(path, flock.SetPermissions(0o600))
 	if err := lock.Lock(); err != nil {
 		return nil, errInvalidAuthenticationConfig
diff --git a/tools/tht/internal/authconfig/store_windows_test.go b/tools/tht/internal/authconfig/store_windows_test.go
new file mode 100644
index 00000000..c2bc6513
--- /dev/null
+++ b/tools/tht/internal/authconfig/store_windows_test.go
@@ -0,0 +1,92 @@
+//go:build windows
+
+package authconfig
+
+import (
+	"errors"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
+	"golang.org/x/sys/windows"
+)
+
+func TestLoadRejectsPermissiveWindowsDirectoryAuthAndUsersDACLs(t *testing.T) {
+	for name, makePermissive := range map[string]func(t *testing.T, directory string){
+		"directory": func(t *testing.T, directory string) {
+			setPermissiveAuthDACL(t, directory)
+		},
+		"auth file": func(t *testing.T, directory string) {
+			setPermissiveAuthDACL(t, filepath.Join(directory, "auth.yaml"))
+		},
+		"users file": func(t *testing.T, directory string) {
+			setPermissiveAuthDACL(t, filepath.Join(directory, "users.yaml"))
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			directory := writePrivateWindowsAuthFiles(t)
+			makePermissive(t, directory)
+			if _, _, err := Load(directory); !errors.Is(err, safeio.ErrUnsafeFile) {
+				t.Fatalf("Load() error = %v, want ErrUnsafeFile", err)
+			}
+		})
+	}
+}
+
+func TestMutateUsersCreatesAndRejectsPermissiveWindowsLockDACL(t *testing.T) {
+	directory := writePrivateWindowsAuthFiles(t)
+	if err := MutateUsers(directory, func(*Registry) error { return nil }); err != nil {
+		t.Fatal(err)
+	}
+	lockPath := filepath.Join(directory, lockFileName)
+	if err := safeio.ValidatePrivateRegular(lockPath); err != nil {
+		t.Fatalf("lock privacy error = %v", err)
+	}
+	setPermissiveAuthDACL(t, lockPath)
+	if err := MutateUsers(directory, func(*Registry) error { return nil }); !errors.Is(err, safeio.ErrUnsafeFile) {
+		t.Fatalf("MutateUsers() error = %v, want ErrUnsafeFile", err)
+	}
+}
+
+func writePrivateWindowsAuthFiles(t *testing.T) string {
+	t.Helper()
+	directory := writeAuthFiles(t, defaultAuthYAML, registryYAML(adminUserYAML("admin", "Admin", true, "admin")))
+	if err := safeio.ProtectPrivateDirectory(directory); err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range []string{"auth.yaml", "users.yaml"} {
+		if err := safeio.ProtectPrivateRegular(filepath.Join(directory, name)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	return directory
+}
+
+func setPermissiveAuthDACL(t *testing.T, path string) {
+	t.Helper()
+	world, err := windows.StringToSid("S-1-1-0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var pinner runtime.Pinner
+	pinner.Pin(world)
+	defer pinner.Unpin()
+	acl, err := windows.ACLFromEntries([]windows.EXPLICIT_ACCESS{{
+		AccessPermissions: windows.GENERIC_READ | windows.GENERIC_WRITE,
+		AccessMode:        windows.GRANT_ACCESS,
+		Trustee: windows.TRUSTEE{
+			TrusteeForm:  windows.TRUSTEE_IS_SID,
+			TrusteeType:  windows.TRUSTEE_IS_GROUP,
+			TrusteeValue: windows.TrusteeValueFromSID(world),
+		},
+	}}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := windows.SetNamedSecurityInfo(path, windows.SE_FILE_OBJECT,
+		windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION,
+		nil, nil, acl, nil); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/tools/tht/internal/safeio/files.go b/tools/tht/internal/safeio/files.go
index 8a0a0fa1..0ea916f3 100644
--- a/tools/tht/internal/safeio/files.go
+++ b/tools/tht/internal/safeio/files.go
@@ -77,6 +77,10 @@ func WriteCanonicalNewFile(path string, contents []byte, mode os.FileMode) error
 		return ErrUnsafeFile
 	}
 	defer file.Close()
+	if err := ProtectPrivateRegular(path); err != nil {
+		_ = os.Remove(path)
+		return ErrUnsafeFile
+	}
 	if _, err := file.Write(contents); err != nil {
 		_ = os.Remove(path)
 		return ErrUnsafeFile
diff --git a/tools/tht/internal/safeio/files_windows.go b/tools/tht/internal/safeio/files_windows.go
index 020a86c7..0040971d 100644
--- a/tools/tht/internal/safeio/files_windows.go
+++ b/tools/tht/internal/safeio/files_windows.go
@@ -57,6 +57,10 @@ func ReadCanonicalRegular(path string, maximum int64) ([]byte, error) {
 }
 
 func openWindowsComponent(path string, directory bool) (windows.Handle, error) {
+	return openWindowsComponentWithAccess(path, directory, windows.GENERIC_READ)
+}
+
+func openWindowsComponentWithAccess(path string, directory bool, access uint32) (windows.Handle, error) {
 	flags := uint32(windows.FILE_FLAG_OPEN_REPARSE_POINT)
 	if directory {
 		flags |= windows.FILE_FLAG_BACKUP_SEMANTICS
@@ -65,7 +69,7 @@ func openWindowsComponent(path string, directory bool) (windows.Handle, error) {
 	}
 	handle, err := windows.CreateFile(
 		windows.StringToUTF16Ptr(path),
-		windows.GENERIC_READ,
+		access,
 		windowsRetainedHandleShareMode,
 		nil,
 		windows.OPEN_EXISTING,
diff --git a/tools/tht/internal/safeio/private_unix.go b/tools/tht/internal/safeio/private_unix.go
new file mode 100644
index 00000000..8630a63c
--- /dev/null
+++ b/tools/tht/internal/safeio/private_unix.go
@@ -0,0 +1,61 @@
+//go:build !windows
+
+package safeio
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// ProtectPrivateDirectory sets the private directory mode used for local authentication state.
+func ProtectPrivateDirectory(path string) error {
+	if err := ValidateCanonicalPath(path); err != nil {
+		return err
+	}
+	if err := os.Chmod(path, 0o700); err != nil {
+		return ErrUnsafeFile
+	}
+	return ValidatePrivateDirectory(path)
+}
+
+// ValidatePrivateDirectory requires a canonical, non-symlinked directory with no group or world access.
+func ValidatePrivateDirectory(path string) error {
+	if err := requireCanonicalDirectory(path); err != nil {
+		return err
+	}
+	info, err := os.Lstat(path)
+	if err != nil || !info.IsDir() || info.Mode()&os.ModeSymlink != 0 || !isExactPrivateMode(info.Mode(), 0o700) {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+// ProtectPrivateRegular sets the private file mode used for local authentication files.
+func ProtectPrivateRegular(path string) error {
+	if err := ValidateCanonicalPath(path); err != nil {
+		return err
+	}
+	if err := os.Chmod(path, 0o600); err != nil {
+		return ErrUnsafeFile
+	}
+	return ValidatePrivateRegular(path)
+}
+
+// ValidatePrivateRegular requires a canonical, single-link private regular file.
+func ValidatePrivateRegular(path string) error {
+	if err := ValidateCanonicalPath(path); err != nil {
+		return err
+	}
+	if err := requireCanonicalDirectory(filepath.Dir(path)); err != nil {
+		return err
+	}
+	info, err := os.Lstat(path)
+	if err != nil || !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || !hasSingleLink(info) || !isExactPrivateMode(info.Mode(), 0o600) {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+func isExactPrivateMode(mode os.FileMode, permissions os.FileMode) bool {
+	return mode.Perm() == permissions && mode&(os.ModeSetuid|os.ModeSetgid|os.ModeSticky) == 0
+}
diff --git a/tools/tht/internal/safeio/private_windows.go b/tools/tht/internal/safeio/private_windows.go
new file mode 100644
index 00000000..49ca9283
--- /dev/null
+++ b/tools/tht/internal/safeio/private_windows.go
@@ -0,0 +1,192 @@
+//go:build windows
+
+package safeio
+
+import (
+	"path/filepath"
+	"runtime"
+	"strings"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// ProtectPrivateDirectory sets a protected DACL containing only the current owner.
+func ProtectPrivateDirectory(path string) error {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer parents.Close()
+	handle, err := openWindowsComponentWithAccess(filepath.Join(parents.directory, target), true, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer windows.CloseHandle(handle)
+	if err := setOwnerOnlyDACL(handle); err != nil {
+		return ErrUnsafeFile
+	}
+	return validateOwnerOnlyDACL(handle)
+}
+
+// ValidatePrivateDirectory requires a canonical directory protected for its current owner only.
+func ValidatePrivateDirectory(path string) error {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer parents.Close()
+	handle, err := openWindowsComponent(filepath.Join(parents.directory, target), true)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer windows.CloseHandle(handle)
+	if err := validateOwnerOnlyDACL(handle); err != nil {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+// ProtectPrivateRegular sets a protected DACL containing only the current owner.
+func ProtectPrivateRegular(path string) error {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer parents.Close()
+	handle, err := openWindowsComponentWithAccess(filepath.Join(parents.directory, target), false, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer windows.CloseHandle(handle)
+	if err := setOwnerOnlyDACL(handle); err != nil {
+		return ErrUnsafeFile
+	}
+	return validateOwnerOnlyDACL(handle)
+}
+
+// ValidatePrivateRegular requires a canonical, single-link file protected for its current owner only.
+func ValidatePrivateRegular(path string) error {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer parents.Close()
+	handle, err := openWindowsComponent(filepath.Join(parents.directory, target), false)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer windows.CloseHandle(handle)
+	if err := validateOwnerOnlyDACL(handle); err != nil {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+type windowsParentHandles struct {
+	directory string
+	handles   []windows.Handle
+}
+
+func (parents *windowsParentHandles) Close() {
+	for index := len(parents.handles) - 1; index >= 0; index-- {
+		_ = windows.CloseHandle(parents.handles[index])
+	}
+}
+
+// openCanonicalWindowsParent retains every directory handle from the volume root through the
+// target parent without FILE_SHARE_DELETE. The resulting parent cannot be renamed or replaced by
+// a reparse point while an operation uses its absolute child paths.
+func openCanonicalWindowsParent(path string) (*windowsParentHandles, string, error) {
+	if err := ValidateCanonicalPath(path); err != nil {
+		return nil, "", err
+	}
+	volume := filepath.VolumeName(path)
+	root := volume + `\`
+	components := strings.Split(strings.TrimPrefix(path, root), `\`)
+	if volume == "" || len(components) == 0 || components[0] == "" {
+		return nil, "", ErrUnsafeFile
+	}
+	parents := &windowsParentHandles{directory: root}
+	rootHandle, err := openWindowsComponent(root, true)
+	if err != nil {
+		return nil, "", err
+	}
+	parents.handles = append(parents.handles, rootHandle)
+	for _, component := range components[:len(components)-1] {
+		parents.directory = filepath.Join(parents.directory, component)
+		handle, err := openWindowsComponent(parents.directory, true)
+		if err != nil {
+			parents.Close()
+			return nil, "", err
+		}
+		parents.handles = append(parents.handles, handle)
+	}
+	return parents, components[len(components)-1], nil
+}
+
+func setOwnerOnlyDACL(handle windows.Handle) error {
+	sid, err := currentOwnerSID()
+	if err != nil {
+		return err
+	}
+	var pinner runtime.Pinner
+	pinner.Pin(sid)
+	defer pinner.Unpin()
+	acl, err := windows.ACLFromEntries([]windows.EXPLICIT_ACCESS{{
+		AccessPermissions: windows.GENERIC_ALL,
+		AccessMode:        windows.GRANT_ACCESS,
+		Trustee: windows.TRUSTEE{
+			TrusteeForm:  windows.TRUSTEE_IS_SID,
+			TrusteeType:  windows.TRUSTEE_IS_USER,
+			TrusteeValue: windows.TrusteeValueFromSID(sid),
+		},
+	}}, nil)
+	if err != nil {
+		return err
+	}
+	return windows.SetSecurityInfo(handle, windows.SE_FILE_OBJECT,
+		windows.OWNER_SECURITY_INFORMATION|windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION,
+		sid, nil, acl, nil)
+}
+
+func validateOwnerOnlyDACL(handle windows.Handle) error {
+	ownerSID, err := currentOwnerSID()
+	if err != nil {
+		return err
+	}
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT,
+		windows.OWNER_SECURITY_INFORMATION|windows.DACL_SECURITY_INFORMATION)
+	if err != nil || descriptor == nil {
+		return ErrUnsafeFile
+	}
+	owner, _, err := descriptor.Owner()
+	if err != nil || owner == nil || !windows.EqualSid(owner, ownerSID) {
+		return ErrUnsafeFile
+	}
+	control, _, err := descriptor.Control()
+	if err != nil || control&windows.SE_DACL_PROTECTED == 0 {
+		return ErrUnsafeFile
+	}
+	dacl, defaulted, err := descriptor.DACL()
+	if err != nil || defaulted || dacl == nil || dacl.AceCount != 1 {
+		return ErrUnsafeFile
+	}
+	var ace *windows.ACCESS_ALLOWED_ACE
+	if err := windows.GetAce(dacl, 0, &ace); err != nil || ace == nil || ace.Header.AceType != windows.ACCESS_ALLOWED_ACE_TYPE || ace.Header.AceFlags != 0 || ace.Mask != windows.GENERIC_ALL {
+		return ErrUnsafeFile
+	}
+	aceSID := (*windows.SID)(unsafe.Pointer(&ace.SidStart))
+	if !windows.EqualSid(aceSID, ownerSID) {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+func currentOwnerSID() (*windows.SID, error) {
+	user, err := windows.GetCurrentProcessToken().GetTokenUser()
+	if err != nil || user == nil || user.User.Sid == nil {
+		return nil, ErrUnsafeFile
+	}
+	return user.User.Sid, nil
+}
diff --git a/tools/tht/internal/safeio/private_windows_test.go b/tools/tht/internal/safeio/private_windows_test.go
new file mode 100644
index 00000000..2be8812c
--- /dev/null
+++ b/tools/tht/internal/safeio/private_windows_test.go
@@ -0,0 +1,169 @@
+//go:build windows
+
+package safeio
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+func TestPrivateWindowsDACLRejectsPermissiveDirectoryAndRegularFile(t *testing.T) {
+	directory := filepath.Join(t.TempDir(), "auth")
+	if err := os.Mkdir(directory, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(directory); err != nil {
+		t.Fatal(err)
+	}
+	if err := ValidatePrivateDirectory(directory); err != nil {
+		t.Fatalf("ValidatePrivateDirectory() protected directory error = %v", err)
+	}
+
+	path := filepath.Join(directory, "users.yaml")
+	if err := os.WriteFile(path, []byte("private"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateRegular(path); err != nil {
+		t.Fatal(err)
+	}
+	if err := ValidatePrivateRegular(path); err != nil {
+		t.Fatalf("ValidatePrivateRegular() protected file error = %v", err)
+	}
+
+	for name, path := range map[string]string{"directory": directory, "regular file": path} {
+		t.Run(name, func(t *testing.T) {
+			if err := setPermissiveDACL(path); err != nil {
+				t.Fatal(err)
+			}
+			var err error
+			if name == "directory" {
+				err = ValidatePrivateDirectory(path)
+			} else {
+				err = ValidatePrivateRegular(path)
+			}
+			if !errors.Is(err, ErrUnsafeFile) {
+				t.Fatalf("private validation error = %v, want ErrUnsafeFile", err)
+			}
+		})
+	}
+}
+
+func TestReplaceCanonicalRegularCreatesPrivateTemporaryAndReplacement(t *testing.T) {
+	directory := filepath.Join(t.TempDir(), "auth")
+	if err := os.Mkdir(directory, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(directory); err != nil {
+		t.Fatal(err)
+	}
+	path := filepath.Join(directory, "users.yaml")
+	if err := os.WriteFile(path, []byte("old"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateRegular(path); err != nil {
+		t.Fatal(err)
+	}
+
+	temporary, err := writePrivateTemporary(directory, []byte("temporary"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = os.Remove(temporary) })
+	if err := ValidatePrivateRegular(temporary); err != nil {
+		t.Fatalf("temporary DACL error = %v", err)
+	}
+
+	if err := ReplaceCanonicalRegular(path, []byte("replacement"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := ValidatePrivateRegular(path); err != nil {
+		t.Fatalf("replacement DACL error = %v", err)
+	}
+}
+
+func TestReplaceCanonicalRegularRejectsReparseParent(t *testing.T) {
+	root := t.TempDir()
+	realDirectory := filepath.Join(root, "real")
+	if err := os.Mkdir(realDirectory, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(realDirectory); err != nil {
+		t.Fatal(err)
+	}
+	target := filepath.Join(realDirectory, "users.yaml")
+	if err := os.WriteFile(target, []byte("old"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateRegular(target); err != nil {
+		t.Fatal(err)
+	}
+
+	parentLink := filepath.Join(root, "reparse-parent")
+	if err := os.Symlink(realDirectory, parentLink); err != nil {
+		t.Skipf("Windows host does not permit test symlink creation: %v", err)
+	}
+	if err := ReplaceCanonicalRegular(filepath.Join(parentLink, "users.yaml"), []byte("new"), 0o600); !errors.Is(err, ErrUnsafeFile) {
+		t.Fatalf("reparse-parent replacement error = %v, want ErrUnsafeFile", err)
+	}
+}
+
+func TestOpenCanonicalWindowsParentBlocksParentRename(t *testing.T) {
+	root := t.TempDir()
+	realDirectory := filepath.Join(root, "real")
+	if err := os.Mkdir(realDirectory, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(realDirectory); err != nil {
+		t.Fatal(err)
+	}
+	target := filepath.Join(realDirectory, "users.yaml")
+	if err := os.WriteFile(target, []byte("old"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateRegular(target); err != nil {
+		t.Fatal(err)
+	}
+	parents, _, err := openCanonicalWindowsParent(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	renamed := realDirectory + "-renamed"
+	if err := os.Rename(realDirectory, renamed); err == nil {
+		parents.Close()
+		t.Fatal("parent rename succeeded while canonical replacement handles were retained")
+	}
+	parents.Close()
+	if err := os.Rename(realDirectory, renamed); err != nil {
+		t.Fatalf("parent rename after closing canonical replacement handles: %v", err)
+	}
+}
+
+func setPermissiveDACL(path string) error {
+	world, err := windows.StringToSid("S-1-1-0")
+	if err != nil {
+		return err
+	}
+	var pinner runtime.Pinner
+	pinner.Pin(world)
+	defer pinner.Unpin()
+	acl, err := windows.ACLFromEntries([]windows.EXPLICIT_ACCESS{{
+		AccessPermissions: windows.GENERIC_READ | windows.GENERIC_WRITE,
+		AccessMode:        windows.GRANT_ACCESS,
+		Trustee: windows.TRUSTEE{
+			TrusteeForm:  windows.TRUSTEE_IS_SID,
+			TrusteeType:  windows.TRUSTEE_IS_GROUP,
+			TrusteeValue: windows.TrusteeValueFromSID(world),
+		},
+	}}, nil)
+	if err != nil {
+		return err
+	}
+	return windows.SetNamedSecurityInfo(path, windows.SE_FILE_OBJECT,
+		windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION,
+		nil, nil, acl, nil)
+}
diff --git a/tools/tht/internal/safeio/replace_windows.go b/tools/tht/internal/safeio/replace_windows.go
index 257c8060..5b1302fd 100644
--- a/tools/tht/internal/safeio/replace_windows.go
+++ b/tools/tht/internal/safeio/replace_windows.go
@@ -14,38 +14,41 @@ import (
 const windowsReplaceMoveFlags = windows.MOVEFILE_REPLACE_EXISTING | windows.MOVEFILE_WRITE_THROUGH
 
 func replaceCanonicalRegular(path string, contents []byte) error {
-	directory := filepath.Dir(path)
-	if err := requireCanonicalDirectory(directory); err != nil || !safeExistingRegular(path) {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil {
 		return ErrUnsafeFile
 	}
-	temporary, err := writePrivateTemporary(directory, contents)
+	defer parents.Close()
+	if len(parents.handles) == 0 || validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil || !safeExistingRegular(filepath.Join(parents.directory, target)) {
+		return ErrUnsafeFile
+	}
+	temporary, err := writePrivateTemporary(parents.directory, contents)
 	if err != nil {
 		return ErrUnsafeFile
 	}
 	defer func() { _ = os.Remove(temporary) }()
-	if !safeExistingRegular(path) {
+	if !safeExistingRegular(filepath.Join(parents.directory, target)) {
 		return ErrUnsafeFile
 	}
 	from, err := windows.UTF16PtrFromString(temporary)
 	if err != nil {
 		return ErrUnsafeFile
 	}
-	to, err := windows.UTF16PtrFromString(path)
+	to, err := windows.UTF16PtrFromString(filepath.Join(parents.directory, target))
 	if err != nil {
 		return ErrUnsafeFile
 	}
 	if err := windows.MoveFileEx(from, to, windowsReplaceMoveFlags); err != nil {
 		return ErrUnsafeFile
 	}
+	if err := ValidatePrivateRegular(filepath.Join(parents.directory, target)); err != nil {
+		return ErrUnsafeFile
+	}
 	return nil
 }
 
 func safeExistingRegular(path string) bool {
-	handle, err := openWindowsComponent(path, false)
-	if err != nil {
-		return false
-	}
-	return windows.CloseHandle(handle) == nil
+	return ValidatePrivateRegular(path) == nil
 }
 
 func writePrivateTemporary(directory string, contents []byte) (string, error) {
@@ -62,6 +65,11 @@ func writePrivateTemporary(directory string, contents []byte) (string, error) {
 		if err != nil {
 			return "", err
 		}
+		if err := ProtectPrivateRegular(path); err != nil {
+			_ = file.Close()
+			_ = os.Remove(path)
+			return "", err
+		}
 		if err := file.Chmod(0o600); err == nil {
 			var written int
 			written, err = file.Write(contents)