diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -1692,6 +1692,10 @@
     PyObject *unicode
     );
 
+/* Checks for NUL characters in a str object. Useful for path sanity checks. */
+PyAPI_FUNC(int) _PyUnicode_HasNULChars(PyObject*);
+
+
 /* --- Methods & Slots ----------------------------------------------------
 
    These are capable of handling Unicode objects and strings on input
diff --git a/Lib/test/test_fileio.py b/Lib/test/test_fileio.py
--- a/Lib/test/test_fileio.py
+++ b/Lib/test/test_fileio.py
@@ -306,6 +306,11 @@
         finally:
             os.unlink(TESTFN)
 
+    def testConstructorHandlesNULChars(self):
+        fn_with_NUL = 'foo\0bar'
+        self.assertRaises(TypeError, _FileIO, fn_with_NUL, 'w')
+        self.assertRaises(TypeError, _FileIO, bytes(fn_with_NUL, 'ascii'), 'w')
+
     def testInvalidFd(self):
         self.assertRaises(ValueError, _FileIO, -10)
         self.assertRaises(OSError, _FileIO, make_bad_fd())
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -363,6 +363,11 @@
             self.assertRaises(exc, fp.seek, 1, self.SEEK_CUR)
             self.assertRaises(exc, fp.seek, -1, self.SEEK_END)
 
+    def test_open_handles_NUL_chars(self):
+        fn_with_NUL = 'foo\0bar'
+        self.assertRaises(TypeError, open, fn_with_NUL, 'w')
+        self.assertRaises(TypeError, open, bytes(fn_with_NUL, 'ascii'), 'w')
+
     def test_raw_file_io(self):
         with self.open(support.TESTFN, "wb", buffering=0) as f:
             self.assertEqual(f.readable(), False)
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -258,6 +258,12 @@
 
 #ifdef MS_WINDOWS
     if (PyUnicode_Check(nameobj)) {
+        int rv = _PyUnicode_HasNULChars(nameobj);
+        if (rv) {
+            if (rv != -1)
+                PyErr_SetString(PyExc_TypeError, "embedded NUL character");
+            return -1;
+        }
         widename = PyUnicode_AsUnicode(nameobj);
         if (widename == NULL)
             return -1;
@@ -265,28 +271,10 @@
 #endif
     if (fd < 0)
     {
-        if (PyBytes_Check(nameobj) || PyByteArray_Check(nameobj)) {
-            Py_ssize_t namelen;
-            if (PyObject_AsCharBuffer(nameobj, &name, &namelen) < 0)
-                return -1;
+        if (!PyUnicode_FSConverter(nameobj, &stringobj)) {
+            return -1;
         }
-        else {
-            PyObject *u = PyUnicode_FromObject(nameobj);
-
-            if (u == NULL)
-                return -1;
-
-            stringobj = PyUnicode_EncodeFSDefault(u);
-            Py_DECREF(u);
-            if (stringobj == NULL)
-                return -1;
-            if (!PyBytes_Check(stringobj)) {
-                PyErr_SetString(PyExc_TypeError,
-                                "encoder failed to return bytes");
-                goto error;
-            }
-            name = PyBytes_AS_STRING(stringobj);
-        }
+        name = PyBytes_AS_STRING(stringobj);
     }
 
     s = mode;
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3575,6 +3575,21 @@
 
 
 int
+_PyUnicode_HasNULChars(PyObject* s)
+{
+    static PyObject *nul;
+    int rv;
+
+    if (nul == NULL)
+         nul = PyUnicode_FromStringAndSize("\0", 1);
+    if (nul == NULL)
+        return -1;
+    rv = PyUnicode_Contains(s, nul);
+    return rv;
+}
+
+
+int
 PyUnicode_FSConverter(PyObject* arg, void* addr)
 {
     PyObject *output = NULL;