diff --git a/internal/client/spool/identity.go b/internal/client/spool/identity.go
index eb0945d..3613887 100644
--- a/internal/client/spool/identity.go
+++ b/internal/client/spool/identity.go
@@ -50,7 +50,7 @@ func writeIdentityAtomically(path string, data []byte) (result error) {
 			_ = os.Remove(temporaryPath)
 		}
 	}()
-	if err := temporary.Chmod(0o600); err != nil {
+	if err := securePrivateFile(temporaryPath); err != nil {
 		_ = temporary.Close()
 		return fmt.Errorf("secure client identity temporary file: %w", err)
 	}
diff --git a/internal/client/spool/private_unix.go b/internal/client/spool/private_unix.go
new file mode 100644
index 0000000..84fca0d
--- /dev/null
+++ b/internal/client/spool/private_unix.go
@@ -0,0 +1,52 @@
+//go:build !windows
+
+package spool
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+func ensurePrivateFile(path string) error {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
+	if err == nil {
+		return file.Close()
+	}
+	if !errors.Is(err, os.ErrExist) {
+		return err
+	}
+	info, err := os.Lstat(path)
+	if err != nil {
+		return err
+	}
+	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 {
+		return fmt.Errorf("%w: %s is not a regular file", ErrUnsafeDataDirectory, path)
+	}
+	if info.Mode().Perm()&0o077 != 0 {
+		return fmt.Errorf("%w: %s permissions %04o expose private state", ErrUnsafeDataDirectory, path, info.Mode().Perm())
+	}
+	return nil
+}
+
+func ensurePrivateDirectory(path string) error {
+	info, err := os.Lstat(path)
+	if os.IsNotExist(err) {
+		if err := os.MkdirAll(path, 0o700); err != nil {
+			return err
+		}
+		info, err = os.Lstat(path)
+	}
+	if err != nil {
+		return err
+	}
+	if info.Mode()&os.ModeSymlink != 0 || !info.IsDir() {
+		return fmt.Errorf("%w: %s is not a real directory", ErrUnsafeDataDirectory, path)
+	}
+	if info.Mode().Perm()&0o077 != 0 {
+		return fmt.Errorf("%w: %s permissions %04o expose private state", ErrUnsafeDataDirectory, path, info.Mode().Perm())
+	}
+	return nil
+}
+
+func securePrivateFile(path string) error { return os.Chmod(path, 0o600) }
diff --git a/internal/client/spool/private_windows.go b/internal/client/spool/private_windows.go
new file mode 100644
index 0000000..cb65927
--- /dev/null
+++ b/internal/client/spool/private_windows.go
@@ -0,0 +1,76 @@
+//go:build windows
+
+package spool
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+const privateStateSDDL = "D:P(A;;FA;;;SY)(A;;FA;;;BA)"
+
+// Windows service state is owned by LocalSystem. The DACL is protected against
+// parent inheritance and grants full access only to SYSTEM and Administrators;
+// unprivileged execution contexts access it solely through the service.
+func ensurePrivateFile(path string) error {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
+	if err == nil {
+		err = file.Close()
+	} else if !errors.Is(err, os.ErrExist) {
+		return err
+	}
+	if err != nil {
+		return err
+	}
+	info, err := os.Lstat(path)
+	if err != nil {
+		return err
+	}
+	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || hasReparsePoint(path) {
+		return fmt.Errorf("%w: %s is not a regular private file", ErrUnsafeDataDirectory, path)
+	}
+	return applyPrivateStateACL(path)
+}
+
+func ensurePrivateDirectory(path string) error {
+	info, err := os.Lstat(path)
+	if os.IsNotExist(err) {
+		if err := os.MkdirAll(path, 0o700); err != nil {
+			return err
+		}
+		info, err = os.Lstat(path)
+	}
+	if err != nil {
+		return err
+	}
+	if !info.IsDir() || info.Mode()&os.ModeSymlink != 0 || hasReparsePoint(path) {
+		return fmt.Errorf("%w: %s is not a real private directory", ErrUnsafeDataDirectory, path)
+	}
+	return applyPrivateStateACL(path)
+}
+
+func securePrivateFile(path string) error { return applyPrivateStateACL(path) }
+
+func hasReparsePoint(path string) bool {
+	value, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return true
+	}
+	attributes, err := windows.GetFileAttributes(value)
+	return err != nil || attributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0
+}
+
+func applyPrivateStateACL(path string) error {
+	descriptor, err := windows.SecurityDescriptorFromString(privateStateSDDL)
+	if err != nil {
+		return err
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil {
+		return err
+	}
+	return windows.SetNamedSecurityInfo(path, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION, nil, nil, dacl, nil)
+}
diff --git a/internal/client/spool/spool.go b/internal/client/spool/spool.go
index 11db4a7..33b21b1 100644
--- a/internal/client/spool/spool.go
+++ b/internal/client/spool/spool.go
@@ -9,7 +9,6 @@ import (
 	"errors"
 	"fmt"
 	"net/url"
-	"os"
 	"path/filepath"
 	"strconv"
 	"sync"
@@ -145,45 +144,4 @@ func (store *Store) Close() error {
 	return result
 }
 
-func ensurePrivateFile(path string) error {
-	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
-	if err == nil {
-		return file.Close()
-	}
-	if !errors.Is(err, os.ErrExist) {
-		return err
-	}
-	info, err := os.Lstat(path)
-	if err != nil {
-		return err
-	}
-	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 {
-		return fmt.Errorf("%w: %s is not a regular file", ErrUnsafeDataDirectory, path)
-	}
-	if info.Mode().Perm()&0o077 != 0 {
-		return fmt.Errorf("%w: %s permissions %04o expose private state", ErrUnsafeDataDirectory, path, info.Mode().Perm())
-	}
-	return nil
-}
-
-func ensurePrivateDirectory(path string) error {
-	info, err := os.Lstat(path)
-	if os.IsNotExist(err) {
-		if err := os.MkdirAll(path, 0o700); err != nil {
-			return err
-		}
-		info, err = os.Lstat(path)
-	}
-	if err != nil {
-		return err
-	}
-	if info.Mode()&os.ModeSymlink != 0 || !info.IsDir() {
-		return fmt.Errorf("%w: %s is not a real directory", ErrUnsafeDataDirectory, path)
-	}
-	if info.Mode().Perm()&0o077 != 0 {
-		return fmt.Errorf("%w: %s permissions %04o expose private state", ErrUnsafeDataDirectory, path, info.Mode().Perm())
-	}
-	return nil
-}
-
 func immutableDigest(payload []byte) [32]byte { return sha256.Sum256(payload) }