diff --git a/tools/tht/internal/safeio/claim_windows.go b/tools/tht/internal/safeio/claim_windows.go
index e9f1be46..6a524f48 100644
--- a/tools/tht/internal/safeio/claim_windows.go
+++ b/tools/tht/internal/safeio/claim_windows.go
@@ -31,33 +31,25 @@ func (value *windowsPrivateRegular) Close() {
 
 func openWindowsPrivateRegular(path string, links uint32) (*windowsPrivateRegular, error) {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil || len(parents.handles) == 0 || validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 || validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil {
 		if parents != nil {
 			parents.Close()
 		}
 		return nil, ErrUnsafeFile
 	}
-	fullPath := filepath.Join(parents.directory, target)
-	handle, err := windows.CreateFile(
-		windows.StringToUTF16Ptr(fullPath),
-		windows.GENERIC_READ,
-		windowsRetainedHandleShareMode,
-		nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_OPEN_REPARSE_POINT|windows.FILE_ATTRIBUTE_NORMAL,
-		0,
-	)
+	value, err := openWindowsPrivateRegularAt(parents.handles[len(parents.handles)-1], target, windows.GENERIC_READ, links)
 	if err != nil {
 		parents.Close()
 		return nil, err
 	}
-	var info windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(handle, &info); err != nil || info.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 || info.FileAttributes&windows.FILE_ATTRIBUTE_DIRECTORY != 0 || info.NumberOfLinks != links || validateOwnerOnlyDACL(handle) != nil {
-		_ = windows.CloseHandle(handle)
-		parents.Close()
-		return nil, ErrUnsafeFile
-	}
-	return &windowsPrivateRegular{parents: parents, handle: handle, path: fullPath, info: info}, nil
+	handle := value.handle
+	value.handle = 0
+	return &windowsPrivateRegular{
+		parents: parents,
+		handle:  handle,
+		path:    filepath.Join(parents.directory, target),
+		info:    value.info,
+	}, nil
 }
 
 func claimCanonicalPrivateRegular(source, claim string) (bool, error) {
diff --git a/tools/tht/internal/safeio/files.go b/tools/tht/internal/safeio/files.go
index 36b9d11c..0ea06497 100644
--- a/tools/tht/internal/safeio/files.go
+++ b/tools/tht/internal/safeio/files.go
@@ -279,7 +279,7 @@ func WriteCanonicalNewPrivateFile(path string, contents []byte, mode os.FileMode
 // already private parent and returns a read/write handle for streamed contents. Callers must close
 // the returned handle and remove the file if their stream fails.
 func CreateCanonicalNewPrivateFile(path string) (*os.File, error) {
-	if err := validateCanonicalNewFile(path, true); err != nil {
+	if err := validateCanonicalNewFile(path); err != nil {
 		return nil, err
 	}
 	file, err := createCanonicalNewPrivateParentReadWriteFile(path, 0o600)
@@ -290,7 +290,7 @@ func CreateCanonicalNewPrivateFile(path string) (*os.File, error) {
 }
 
 func writeCanonicalNewFile(path string, contents []byte, mode os.FileMode, requirePrivateParent bool) error {
-	if err := validateCanonicalNewFile(path, requirePrivateParent); err != nil {
+	if err := validateCanonicalNewFile(path); err != nil {
 		return err
 	}
 	var (
@@ -307,40 +307,28 @@ func writeCanonicalNewFile(path string, contents []byte, mode os.FileMode, requi
 	}
 	if _, err := file.Write(contents); err != nil {
 		_ = file.Close()
-		_ = os.Remove(path)
+		_ = RemoveCanonicalPrivateRegular(path)
 		return ErrUnsafeFile
 	}
 	if err := file.Sync(); err != nil {
 		_ = file.Close()
-		_ = os.Remove(path)
+		_ = RemoveCanonicalPrivateRegular(path)
 		return ErrUnsafeFile
 	}
 	if err := file.Close(); err != nil {
-		_ = os.Remove(path)
+		_ = RemoveCanonicalPrivateRegular(path)
 		return ErrUnsafeFile
 	}
 	return nil
 }
 
-func validateCanonicalNewFile(path string, requirePrivateParent bool) error {
+// validateCanonicalNewFile performs only lexical validation. Platform creators retain and
+// inspect the actual parent handle before mutation; pathname existence or parent checks here
+// would be stale by the time an exclusive create reaches the filesystem.
+func validateCanonicalNewFile(path string) error {
 	if err := ValidateCanonicalPath(path); err != nil {
 		return err
 	}
-	parent := filepath.Dir(path)
-	if err := requireCanonicalDirectory(parent); err != nil {
-		return err
-	}
-	if requirePrivateParent && ValidatePrivateDirectory(parent) != nil {
-		return ErrUnsafeFile
-	}
-	if info, err := os.Lstat(path); err == nil {
-		if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || info.Mode()&os.ModeType != 0 {
-			return ErrUnsafeFile
-		}
-		return ErrUnsafeFile
-	} else if !errors.Is(err, os.ErrNotExist) {
-		return ErrUnsafeFile
-	}
 	return nil
 }
 
@@ -406,18 +394,3 @@ func randomTemporaryName() (string, error) {
 	}
 	return ".tht-auth-" + hex.EncodeToString(bytes) + ".tmp", nil
 }
-
-func requireCanonicalDirectory(path string) error {
-	if err := ValidateCanonicalPath(path); err != nil {
-		return err
-	}
-	resolved, err := filepath.EvalSymlinks(path)
-	if err != nil || resolved != path {
-		return ErrUnsafeFile
-	}
-	info, err := os.Stat(path)
-	if err != nil || !info.IsDir() {
-		return ErrUnsafeFile
-	}
-	return nil
-}
diff --git a/tools/tht/internal/safeio/files_unix_test.go b/tools/tht/internal/safeio/files_unix_test.go
index c644dbbd..657e46ed 100644
--- a/tools/tht/internal/safeio/files_unix_test.go
+++ b/tools/tht/internal/safeio/files_unix_test.go
@@ -159,3 +159,65 @@ func TestPrivateDirectoryCreationUsesThePinnedParentAfterAncestorSwap(t *testing
 		})
 	}
 }
+
+func TestCreateCanonicalNewPrivateFileUsesPinnedParentAfterAncestorSwap(t *testing.T) {
+	temporaryRoot, err := filepath.EvalSymlinks(os.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	root, err := os.MkdirTemp(temporaryRoot, "tht-safeio-private-file-openat-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = os.RemoveAll(root) })
+
+	parent := filepath.Join(root, "parent")
+	if err := os.Mkdir(parent, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(parent); err != nil {
+		t.Fatal(err)
+	}
+	outside := filepath.Join(root, "outside")
+	if err := os.Mkdir(outside, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	path := filepath.Join(parent, "archive.zip")
+	movedParent := parent + "-original"
+	swapped := false
+	restoreHook := SetPrivateDirectoryTestHookForTest(func(stage string) {
+		if stage != "after-canonical-private-file-parent-open" || swapped {
+			return
+		}
+		if err := os.Rename(parent, movedParent); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Symlink(outside, parent); err != nil {
+			t.Fatal(err)
+		}
+		swapped = true
+	})
+	t.Cleanup(restoreHook)
+
+	file, err := CreateCanonicalNewPrivateFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := file.Write([]byte("staged")); err != nil {
+		_ = file.Close()
+		t.Fatal(err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if !swapped {
+		t.Fatal("private file creator did not retain the parent before creation")
+	}
+	created := filepath.Join(movedParent, "archive.zip")
+	if err := ValidatePrivateRegular(created); err != nil {
+		t.Fatalf("pinned-parent private file validation = %v", err)
+	}
+	if _, err := os.Lstat(filepath.Join(outside, "archive.zip")); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("outside target was mutated: %v", err)
+	}
+}
diff --git a/tools/tht/internal/safeio/files_windows.go b/tools/tht/internal/safeio/files_windows.go
index 0040971d..9261d354 100644
--- a/tools/tht/internal/safeio/files_windows.go
+++ b/tools/tht/internal/safeio/files_windows.go
@@ -4,46 +4,28 @@ package safeio
 
 import (
 	"os"
-	"path/filepath"
-	"strings"
 
 	"golang.org/x/sys/windows"
 )
 
 const windowsRetainedHandleShareMode uint32 = windows.FILE_SHARE_READ | windows.FILE_SHARE_WRITE
 
-// ReadCanonicalRegular opens each component with FILE_FLAG_OPEN_REPARSE_POINT and rejects a
-// reparse point on the opened handle before opening the next component. Retained handles allow
-// ordinary read/write sharing but deny delete sharing, which blocks rename or deletion after a
-// component is opened and throughout the final read. Windows' Win32 API does not expose a
-// portable descriptor-relative equivalent of POSIX openat, so a hostile local actor can still
-// replace a not-yet-opened normal component between absolute-path opens. Installation directories
-// therefore need trusted local filesystem/ACL ownership on Windows.
+// ReadCanonicalRegular resolves every component under the already-opened parent with NT
+// RootDirectory-relative opens. Each retained handle rejects reparse points and denies delete
+// sharing, so a later ancestor replacement cannot redirect the final regular-file open.
 func ReadCanonicalRegular(path string, maximum int64) ([]byte, error) {
 	if err := ValidateCanonicalPath(path); err != nil {
 		return nil, err
 	}
-	volume := filepath.VolumeName(path)
-	root := volume + string(filepath.Separator)
-	components := strings.Split(strings.TrimPrefix(path, root), string(filepath.Separator))
-	if volume == "" || len(components) == 0 || components[0] == "" {
+	parents, target, err := openCanonicalWindowsParent(path)
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return nil, ErrUnsafeFile
 	}
-
-	current := root
-	parents := make([]windows.Handle, 0, len(components)-1)
-	defer func() { closeWindowsHandles(parents) }()
-	for _, component := range components[:len(components)-1] {
-		current = filepath.Join(current, component)
-		handle, err := openWindowsComponent(current, true)
-		if err != nil {
-			return nil, ErrUnsafeFile
-		}
-		parents = append(parents, handle)
-	}
-
-	current = filepath.Join(current, components[len(components)-1])
-	handle, err := openWindowsComponent(current, false)
+	defer parents.Close()
+	handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], target, false, windows.GENERIC_READ)
 	if err != nil {
 		return nil, ErrUnsafeFile
 	}
@@ -93,6 +75,34 @@ func openWindowsComponentWithAccess(path string, directory bool, access uint32)
 	return handle, nil
 }
 
+// openWindowsRelativeComponent is the generic NT equivalent of openat for one canonical leaf.
+// Its root handle has already pinned all lexical ancestors; FILE_OPEN_REPARSE_POINT makes a
+// reparse point observable so it can be rejected rather than followed.
+func openWindowsRelativeComponent(parent windows.Handle, name string, directory bool, access uint32) (windows.Handle, error) {
+	options := uint32(windows.FILE_SYNCHRONOUS_IO_NONALERT | windows.FILE_OPEN_REPARSE_POINT)
+	if directory {
+		options |= windows.FILE_DIRECTORY_FILE
+	} else {
+		options |= windows.FILE_NON_DIRECTORY_FILE
+	}
+	handle, err := openWindowsRelativeObject(parent, name, access, windows.FILE_OPEN, options, nil)
+	if err != nil {
+		return 0, err
+	}
+	var information windows.ByHandleFileInformation
+	if err := windows.GetFileInformationByHandle(handle, &information); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, err
+	}
+	if information.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 ||
+		(directory && information.FileAttributes&windows.FILE_ATTRIBUTE_DIRECTORY == 0) ||
+		(!directory && (information.FileAttributes&windows.FILE_ATTRIBUTE_DIRECTORY != 0 || information.NumberOfLinks != 1)) {
+		_ = windows.CloseHandle(handle)
+		return 0, ErrUnsafeFile
+	}
+	return handle, nil
+}
+
 func closeWindowsHandles(handles []windows.Handle) {
 	for _, handle := range handles {
 		windows.CloseHandle(handle)
diff --git a/tools/tht/internal/safeio/preflight_windows.go b/tools/tht/internal/safeio/preflight_windows.go
index e127721a..57a6989d 100644
--- a/tools/tht/internal/safeio/preflight_windows.go
+++ b/tools/tht/internal/safeio/preflight_windows.go
@@ -2,22 +2,20 @@
 
 package safeio
 
-import (
-	"errors"
-	"path/filepath"
-
-	"golang.org/x/sys/windows"
-)
+import "golang.org/x/sys/windows"
 
 func preflightPrivateDirectory(path string) (bool, error) {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return false, ErrUnsafeFile
 	}
 	defer parents.Close()
-	handle, err := openWindowsComponent(filepath.Join(parents.directory, target), true)
+	handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], target, true, windows.GENERIC_READ)
 	if err != nil {
-		if errors.Is(err, windows.ERROR_FILE_NOT_FOUND) {
+		if isWindowsRelativeNotFound(err) {
 			writableParent, accessErr := openWindowsComponentWithAccess(
 				parents.directory,
 				true,
diff --git a/tools/tht/internal/safeio/private_root.go b/tools/tht/internal/safeio/private_root.go
index c1e9459b..aac603f4 100644
--- a/tools/tht/internal/safeio/private_root.go
+++ b/tools/tht/internal/safeio/private_root.go
@@ -55,7 +55,7 @@ func SetPrivateDirectoryTestHookForTest(hook func(string)) func() {
 	}
 }
 
-// NotifyPrivateDirectoryTestHookForTest marks an internal retained-root boundary. It is called
+// NotifyPrivateDirectoryTestHookForTest marks an internal retained-handle boundary. It is called
 // only by storage code and lets tests install deterministic directory replacement races.
 func NotifyPrivateDirectoryTestHookForTest(stage string) {
 	privateDirectoryTestHook.RLock()
diff --git a/tools/tht/internal/safeio/private_root_windows.go b/tools/tht/internal/safeio/private_root_windows.go
index c86697e3..2d38e11a 100644
--- a/tools/tht/internal/safeio/private_root_windows.go
+++ b/tools/tht/internal/safeio/private_root_windows.go
@@ -395,6 +395,13 @@ func openWindowsPrivateRegularAtAllowedLinks(
 }
 
 func createWindowsPrivateRegularAt(parent windows.Handle, name string) (*windowsPrivateRegularAt, error) {
+	return createWindowsPrivateRegularAtWithAccess(parent, name, windows.FILE_GENERIC_READ|windows.FILE_GENERIC_WRITE|windows.DELETE)
+}
+
+// createWindowsPrivateRegularAtWithAccess creates the final leaf beneath an already-retained
+// parent with an owner-only DACL in the same NtCreateFile operation. The caller never re-resolves
+// an absolute pathname after the parent is pinned.
+func createWindowsPrivateRegularAtWithAccess(parent windows.Handle, name string, access uint32) (*windowsPrivateRegularAt, error) {
 	security, err := newOwnerOnlySecurityDescriptor()
 	if err != nil {
 		return nil, ErrUnsafeFile
@@ -403,7 +410,7 @@ func createWindowsPrivateRegularAt(parent windows.Handle, name string) (*windows
 	handle, err := openWindowsRelativeObject(
 		parent,
 		name,
-		windows.FILE_GENERIC_READ|windows.FILE_GENERIC_WRITE|windows.DELETE,
+		access|windows.DELETE,
 		windows.FILE_CREATE,
 		windows.FILE_NON_DIRECTORY_FILE|windows.FILE_SYNCHRONOUS_IO_NONALERT|windows.FILE_OPEN_REPARSE_POINT,
 		security,
diff --git a/tools/tht/internal/safeio/private_unix.go b/tools/tht/internal/safeio/private_unix.go
index 322f1ce9..f7566c7f 100644
--- a/tools/tht/internal/safeio/private_unix.go
+++ b/tools/tht/internal/safeio/private_unix.go
@@ -5,7 +5,6 @@ package safeio
 import (
 	"errors"
 	"os"
-	"path/filepath"
 
 	"golang.org/x/sys/unix"
 )
@@ -56,22 +55,32 @@ func createPrivateDirectoryAt(parents *unixParentHandles) error {
 
 // 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 {
+	parents, descriptor, err := openCanonicalUnixPrivateDirectory(path)
+	if err != nil {
 		return ErrUnsafeFile
 	}
-	return ValidatePrivateDirectory(path)
+	defer parents.Close()
+	defer unix.Close(descriptor)
+	if unix.Fchmod(descriptor, 0o700) != nil {
+		return ErrUnsafeFile
+	}
+	var stat unix.Stat_t
+	if unix.Fstat(descriptor, &stat) != nil || !privateUnixDirectoryStat(&stat) {
+		return ErrUnsafeFile
+	}
+	return nil
 }
 
 // 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
+	parents, descriptor, err := openCanonicalUnixPrivateDirectory(path)
+	if err != nil {
+		return ErrUnsafeFile
 	}
-	info, err := os.Lstat(path)
-	if err != nil || !info.IsDir() || info.Mode()&os.ModeSymlink != 0 || !isExactPrivateMode(info.Mode(), 0o700) {
+	defer parents.Close()
+	defer unix.Close(descriptor)
+	var stat unix.Stat_t
+	if unix.Fstat(descriptor, &stat) != nil || !privateUnixDirectoryStat(&stat) {
 		return ErrUnsafeFile
 	}
 	return nil
@@ -79,61 +88,125 @@ func ValidatePrivateDirectory(path string) error {
 
 // 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 {
+	parents, descriptor, err := openCanonicalUnixPrivateRegular(path)
+	if err != nil {
 		return ErrUnsafeFile
 	}
-	return ValidatePrivateRegular(path)
-}
-
-func createCanonicalNewPrivateFile(path string, mode os.FileMode) (*os.File, error) {
-	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_WRONLY)
-}
-
-func createCanonicalNewPrivateFileWithFlags(path string, mode os.FileMode, flags int) (*os.File, error) {
-	file, err := os.OpenFile(path, flags|os.O_CREATE|os.O_EXCL, mode)
-	if err != nil {
-		return nil, err
+	defer parents.Close()
+	defer unix.Close(descriptor)
+	if unix.Fchmod(descriptor, 0o600) != nil {
+		return ErrUnsafeFile
 	}
-	if err := ProtectPrivateRegular(path); err != nil {
-		_ = file.Close()
-		_ = os.Remove(path)
-		return nil, ErrUnsafeFile
-	}
-	return file, nil
-}
-
-func createCanonicalNewPrivateParentFile(path string, mode os.FileMode) (*os.File, error) {
-	if err := ValidatePrivateDirectory(filepath.Dir(path)); err != nil {
-		return nil, ErrUnsafeFile
-	}
-	return createCanonicalNewPrivateFile(path, mode)
-}
-
-func createCanonicalNewPrivateParentReadWriteFile(path string, mode os.FileMode) (*os.File, error) {
-	if err := ValidatePrivateDirectory(filepath.Dir(path)); err != nil {
-		return nil, ErrUnsafeFile
-	}
-	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_RDWR)
-}
-
-// 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) {
+	var stat unix.Stat_t
+	if unix.Fstat(descriptor, &stat) != nil || !privateUnixRegularStat(&stat) {
 		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
+func createCanonicalNewPrivateFile(path string, mode os.FileMode) (*os.File, error) {
+	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_WRONLY, false)
+}
+
+func createCanonicalNewPrivateParentFile(path string, mode os.FileMode) (*os.File, error) {
+	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_WRONLY, true)
+}
+
+func createCanonicalNewPrivateParentReadWriteFile(path string, mode os.FileMode) (*os.File, error) {
+	return createCanonicalNewPrivateFileWithFlags(path, mode, os.O_RDWR, true)
+}
+
+// createCanonicalNewPrivateFileWithFlags resolves and validates every parent once, then creates
+// the final name through that retained parent descriptor. No later lexical ancestor replacement
+// can redirect the create, mode repair, metadata check, or failure cleanup.
+func createCanonicalNewPrivateFileWithFlags(path string, mode os.FileMode, flags int, requirePrivateParent bool) (*os.File, error) {
+	parents, err := openCanonicalUnixParent(path)
+	if err != nil {
+		return nil, ErrUnsafeFile
+	}
+	defer parents.Close()
+	if requirePrivateParent {
+		var parentStat unix.Stat_t
+		if unix.Fstat(parents.parent, &parentStat) != nil || !privateUnixDirectoryStat(&parentStat) {
+			return nil, ErrUnsafeFile
+		}
+	}
+	NotifyPrivateDirectoryTestHookForTest("after-canonical-private-file-parent-open")
+
+	// The final 0600 is applied with fchmod rather than trusting process umask or a path lookup.
+	// mode remains accepted for the existing helper contract; private files are always exactly 0600.
+	_ = mode
+	descriptor, err := unix.Openat(parents.parent, parents.target,
+		flags|unix.O_CREAT|unix.O_EXCL|unix.O_CLOEXEC|unix.O_NOFOLLOW, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	failed := true
+	defer func() {
+		if failed {
+			_ = unix.Close(descriptor)
+			_ = unix.Unlinkat(parents.parent, parents.target, 0)
+		}
+	}()
+	if unix.Fchmod(descriptor, 0o600) != nil {
+		return nil, ErrUnsafeFile
+	}
+	var stat unix.Stat_t
+	if unix.Fstat(descriptor, &stat) != nil || !privateUnixRegularStat(&stat) {
+		return nil, ErrUnsafeFile
+	}
+	file := os.NewFile(uintptr(descriptor), "tht-safeio-private")
+	if file == nil {
+		return nil, ErrUnsafeFile
+	}
+	failed = false
+	return file, nil
+}
+
+// ValidatePrivateRegular requires a canonical, single-link private regular file.
+func ValidatePrivateRegular(path string) error {
+	parents, descriptor, err := openCanonicalUnixPrivateRegular(path)
+	if err != nil {
+		return ErrUnsafeFile
+	}
+	defer parents.Close()
+	defer unix.Close(descriptor)
+	var stat unix.Stat_t
+	if unix.Fstat(descriptor, &stat) != nil || !privateUnixRegularStat(&stat) {
+		return ErrUnsafeFile
+	}
+	return nil
+}
+
+func openCanonicalUnixPrivateDirectory(path string) (*unixParentHandles, int, error) {
+	parents, err := openCanonicalUnixParent(path)
+	if err != nil {
+		return nil, -1, ErrUnsafeFile
+	}
+	descriptor, err := unix.Openat(parents.parent, parents.target,
+		unix.O_RDONLY|unix.O_CLOEXEC|unix.O_DIRECTORY|unix.O_NOFOLLOW, 0)
+	if err != nil {
+		parents.Close()
+		return nil, -1, ErrUnsafeFile
+	}
+	return parents, descriptor, nil
+}
+
+func openCanonicalUnixPrivateRegular(path string) (*unixParentHandles, int, error) {
+	parents, err := openCanonicalUnixParent(path)
+	if err != nil {
+		return nil, -1, ErrUnsafeFile
+	}
+	descriptor, err := unix.Openat(parents.parent, parents.target,
+		unix.O_RDONLY|unix.O_CLOEXEC|unix.O_NOFOLLOW|unix.O_NONBLOCK, 0)
+	if err != nil {
+		parents.Close()
+		return nil, -1, ErrUnsafeFile
+	}
+	return parents, descriptor, nil
+}
+
+func privateUnixRegularStat(stat *unix.Stat_t) bool {
+	return stat != nil && stat.Mode&unix.S_IFMT == unix.S_IFREG && stat.Nlink == 1 &&
+		stat.Uid == uint32(os.Geteuid()) && stat.Mode&0o7777 == 0o600
 }
diff --git a/tools/tht/internal/safeio/private_windows.go b/tools/tht/internal/safeio/private_windows.go
index 18a30744..5376aeda 100644
--- a/tools/tht/internal/safeio/private_windows.go
+++ b/tools/tht/internal/safeio/private_windows.go
@@ -3,7 +3,6 @@
 package safeio
 
 import (
-	"errors"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -15,38 +14,37 @@ import (
 
 func createPrivateDirectory(path string) error {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	security, err := newOwnerOnlySecurityDescriptor()
-	if err != nil {
-		return ErrUnsafeFile
-	}
-	defer security.Close()
-	attributes := &windows.SecurityAttributes{
-		Length:             uint32(unsafe.Sizeof(windows.SecurityAttributes{})),
-		SecurityDescriptor: security.descriptor,
-	}
-	err = windows.CreateDirectory(windows.StringToUTF16Ptr(filepath.Join(parents.directory, target)), attributes)
-	runtime.KeepAlive(security)
-	if errors.Is(err, windows.ERROR_ALREADY_EXISTS) {
+	handle, err := createWindowsRelativePrivateDirectory(parents.handles[len(parents.handles)-1], target)
+	if isWindowsRelativeCollision(err) {
 		return os.ErrExist
 	}
 	if err != nil {
 		return ErrUnsafeFile
 	}
-	return ValidatePrivateDirectory(path)
+	if err := windows.CloseHandle(handle); err != nil {
+		return ErrUnsafeFile
+	}
+	return nil
 }
 
 // ProtectPrivateDirectory sets a protected DACL containing only the current owner.
 func ProtectPrivateDirectory(path string) error {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	handle, err := openWindowsComponentWithAccess(filepath.Join(parents.directory, target), true, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
+	handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], target, true, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
 	if err != nil {
 		return ErrUnsafeFile
 	}
@@ -60,11 +58,14 @@ func ProtectPrivateDirectory(path string) error {
 // ValidatePrivateDirectory requires a canonical directory protected for its current owner only.
 func ValidatePrivateDirectory(path string) error {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	handle, err := openWindowsComponent(filepath.Join(parents.directory, target), true)
+	handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], target, true, windows.GENERIC_READ)
 	if err != nil {
 		return ErrUnsafeFile
 	}
@@ -78,11 +79,14 @@ func ValidatePrivateDirectory(path string) error {
 // ProtectPrivateRegular sets a protected DACL containing only the current owner.
 func ProtectPrivateRegular(path string) error {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	handle, err := openWindowsComponentWithAccess(filepath.Join(parents.directory, target), false, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
+	handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], target, false, windows.GENERIC_READ|windows.WRITE_DAC|windows.WRITE_OWNER)
 	if err != nil {
 		return ErrUnsafeFile
 	}
@@ -109,62 +113,45 @@ func createCanonicalNewPrivateParentReadWriteFile(path string, mode os.FileMode)
 
 func createCanonicalNewFile(path string, mode os.FileMode, requirePrivateParent bool, access uint32) (*os.File, error) {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil || len(parents.handles) == 0 || (requirePrivateParent && validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil) {
+	if err != nil || parents == nil || len(parents.handles) == 0 || (requirePrivateParent && validateOwnerOnlyDACL(parents.handles[len(parents.handles)-1]) != nil) {
 		if parents != nil {
 			parents.Close()
 		}
 		return nil, ErrUnsafeFile
 	}
 	defer parents.Close()
-	security, err := newOwnerOnlySecurityDescriptor()
-	if err != nil {
-		return nil, ErrUnsafeFile
-	}
-	defer security.Close()
-	attributes := &windows.SecurityAttributes{
-		Length:             uint32(unsafe.Sizeof(windows.SecurityAttributes{})),
-		SecurityDescriptor: security.descriptor,
-	}
-	handle, err := windows.CreateFile(
-		windows.StringToUTF16Ptr(filepath.Join(parents.directory, target)),
-		access,
-		windowsRetainedHandleShareMode,
-		attributes,
-		windows.CREATE_NEW,
-		windows.FILE_ATTRIBUTE_NORMAL,
-		0,
-	)
-	runtime.KeepAlive(security)
+	NotifyPrivateDirectoryTestHookForTest("after-canonical-private-file-parent-open")
+	// mode remains accepted for the existing helper contract; Windows installs the owner-only
+	// DACL in the NtCreateFile call below rather than relying on inherited file attributes.
+	_ = mode
+	value, err := createWindowsPrivateRegularAtWithAccess(parents.handles[len(parents.handles)-1], target, access)
 	if err != nil {
 		return nil, err
 	}
-	if err := validateOwnerOnlyDACL(handle); err != nil {
-		_ = windows.CloseHandle(handle)
-		_ = os.Remove(path)
-		return nil, ErrUnsafeFile
-	}
-	file := os.NewFile(uintptr(handle), "tht-safeio-private")
+	file := os.NewFile(uintptr(value.handle), "tht-safeio-private")
 	if file == nil {
-		_ = windows.CloseHandle(handle)
-		_ = os.Remove(path)
+		_ = closeAndDeleteWindowsPrivateRegular(value)
 		return nil, ErrUnsafeFile
 	}
+	value.handle = 0
 	return file, nil
 }
 
 // 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 {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	handle, err := openWindowsComponent(filepath.Join(parents.directory, target), false)
+	value, err := openWindowsPrivateRegularAt(parents.handles[len(parents.handles)-1], target, windows.GENERIC_READ, 1)
 	if err != nil {
 		return ErrUnsafeFile
 	}
-	defer windows.CloseHandle(handle)
-	if err := validateOwnerOnlyDACL(handle); err != nil {
+	if err := value.Close(); err != nil {
 		return ErrUnsafeFile
 	}
 	return nil
@@ -182,8 +169,8 @@ func (parents *windowsParentHandles) Close() {
 }
 
 // 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.
+// target parent without FILE_SHARE_DELETE. Every component after the volume root is resolved
+// through the prior retained handle's NT RootDirectory, never by re-opening an absolute prefix.
 func openCanonicalWindowsParent(path string) (*windowsParentHandles, string, error) {
 	if err := ValidateCanonicalPath(path); err != nil {
 		return nil, "", err
@@ -201,12 +188,12 @@ func openCanonicalWindowsParent(path string) (*windowsParentHandles, string, 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)
+		handle, err := openWindowsRelativeComponent(parents.handles[len(parents.handles)-1], component, true, windows.GENERIC_READ)
 		if err != nil {
 			parents.Close()
 			return nil, "", err
 		}
+		parents.directory = filepath.Join(parents.directory, component)
 		parents.handles = append(parents.handles, handle)
 	}
 	return parents, components[len(components)-1], nil
diff --git a/tools/tht/internal/safeio/private_windows_test.go b/tools/tht/internal/safeio/private_windows_test.go
index 6d70b590..742c2bd6 100644
--- a/tools/tht/internal/safeio/private_windows_test.go
+++ b/tools/tht/internal/safeio/private_windows_test.go
@@ -216,6 +216,47 @@ func TestOpenCanonicalWindowsParentBlocksParentRename(t *testing.T) {
 	}
 }
 
+func TestCreateCanonicalNewPrivateFilePinsWindowsParentBeforeCreate(t *testing.T) {
+	root := t.TempDir()
+	parent := filepath.Join(root, "auth")
+	if err := os.Mkdir(parent, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := ProtectPrivateDirectory(parent); err != nil {
+		t.Fatal(err)
+	}
+	path := filepath.Join(parent, "archive.zip")
+	attemptedSwap := false
+	restoreHook := SetPrivateDirectoryTestHookForTest(func(stage string) {
+		if stage != "after-canonical-private-file-parent-open" || attemptedSwap {
+			return
+		}
+		attemptedSwap = true
+		if err := os.Rename(parent, parent+"-renamed"); err == nil {
+			t.Fatal("parent rename succeeded while private file creation retained its handle")
+		}
+	})
+	t.Cleanup(restoreHook)
+
+	file, err := CreateCanonicalNewPrivateFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := file.Write([]byte("staged")); err != nil {
+		_ = file.Close()
+		t.Fatal(err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if !attemptedSwap {
+		t.Fatal("private file creator did not retain the parent before creation")
+	}
+	if err := ValidatePrivateRegular(path); err != nil {
+		t.Fatalf("ValidatePrivateRegular() = %v, want owner-private staged file", err)
+	}
+}
+
 func setPermissiveDACL(path string) error {
 	world, err := windows.StringToSid("S-1-1-0")
 	if err != nil {
diff --git a/tools/tht/internal/safeio/replace_windows.go b/tools/tht/internal/safeio/replace_windows.go
index 838ccca9..76345883 100644
--- a/tools/tht/internal/safeio/replace_windows.go
+++ b/tools/tht/internal/safeio/replace_windows.go
@@ -49,14 +49,18 @@ func replaceCanonicalRegular(path string, contents []byte) error {
 
 func removeCanonicalPrivateRegular(path string) error {
 	parents, target, err := openCanonicalWindowsParent(path)
-	if err != nil {
+	if err != nil || parents == nil || len(parents.handles) == 0 {
+		if parents != nil {
+			parents.Close()
+		}
 		return ErrUnsafeFile
 	}
 	defer parents.Close()
-	if err := ValidatePrivateRegular(path); err != nil {
+	value, err := openWindowsPrivateRegularAt(parents.handles[len(parents.handles)-1], target, windows.GENERIC_READ|windows.DELETE, 1)
+	if err != nil {
 		return ErrUnsafeFile
 	}
-	if err := windows.DeleteFile(windows.StringToUTF16Ptr(filepath.Join(parents.directory, target))); err != nil {
+	if err := closeAndDeleteWindowsPrivateRegular(value); err != nil {
 		return ErrUnsafeFile
 	}
 	return nil