diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -122,6 +122,8 @@ class BuiltinTest(unittest.TestCase):
         self.assertRaises(TypeError, __import__, 1, 2, 3, 4)
         self.assertRaises(ValueError, __import__, '')
         self.assertRaises(TypeError, __import__, 'sys', name='sys')
+        # embedded null character
+        self.assertRaises(TypeError, __import__, 'a\x00b')
 
     def test_abs(self):
         # int
@@ -892,7 +894,11 @@ class BuiltinTest(unittest.TestCase):
             self.assertEqual(fp.read(1000), 'YYY'*100)
         finally:
             fp.close()
-        unlink(TESTFN)
+            unlink(TESTFN)
+
+        # embedded null character
+        self.assertRaises(TypeError, open, b'a\x00b')
+        self.assertRaises(TypeError, open, 'a\x00b')
 
     def test_ord(self):
         self.assertEqual(ord(' '), 32)
diff --git a/Lib/test/test_grp.py b/Lib/test/test_grp.py
--- a/Lib/test/test_grp.py
+++ b/Lib/test/test_grp.py
@@ -48,6 +48,9 @@ class GroupDatabaseTestCase(unittest.Tes
         self.assertRaises(TypeError, grp.getgrgid)
         self.assertRaises(TypeError, grp.getgrnam)
         self.assertRaises(TypeError, grp.getgrall, 42)
+        # reject embedded null bytes and characters
+        self.assertRaises(TypeError, grp.getgrnam, b'a\x00b')
+        self.assertRaises(TypeError, grp.getgrnam, 'a\x00b')
 
         # try to get some errors
         bynames = {}
diff --git a/Lib/test/test_imp.py b/Lib/test/test_imp.py
--- a/Lib/test/test_imp.py
+++ b/Lib/test/test_imp.py
@@ -181,6 +181,10 @@ class ImportTests(unittest.TestCase):
         self.assertRaises(SyntaxError,
                           imp.find_module, "badsyntax_pep3120", [path])
 
+    def test_load_source(self):
+        self.assertRaisesRegex(TypeError, 'embedded NUL character',
+                               imp.load_source, __name__, __file__ + "\0")
+
 
 class ReloadTests(unittest.TestCase):
 
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -361,10 +361,17 @@ PyCurses_ConvertToString(PyCursesWindowO
 {
     if (PyUnicode_Check(obj)) {
 #ifdef HAVE_NCURSESW
+        Py_ssize_t wlen;
         assert (wstr != NULL);
-        *wstr = PyUnicode_AsWideCharString(obj, NULL);
+
+        *wstr = PyUnicode_AsWideCharString(obj, &wlen);
         if (*wstr == NULL)
             return 0;
+        if (wlen != wcslen(wstr)) {
+            PyErr_SetString(PyExc_TypeError,
+                            "embedded null character");
+            return 0;
+        }
         return 2;
 #else
         assert (wstr == NULL);
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -256,15 +256,23 @@ fileio_init(PyObject *oself, PyObject *a
 
 #ifdef MS_WINDOWS
     if (PyUnicode_Check(nameobj)) {
-        widename = PyUnicode_AsUnicode(nameobj);
+        Py_ssize_t namelen;
+
+        widename = PyUnicode_AsUnicodeAndSize(nameobj, &namelen);
         if (widename == NULL)
             return -1;
+
+        if (wcslen(widename) != namelen) {
+            PyErr_SetString(PyExc_TypeError, "embedded null character");
+            return -1;
+        }
     } else
 #endif
     if (fd < 0)
     {
+        Py_ssize_t namelen;
+
         if (PyBytes_Check(nameobj) || PyByteArray_Check(nameobj)) {
-            Py_ssize_t namelen;
             if (PyObject_AsCharBuffer(nameobj, &name, &namelen) < 0)
                 return -1;
         }
@@ -284,6 +292,12 @@ fileio_init(PyObject *oself, PyObject *a
                 goto error;
             }
             name = PyBytes_AS_STRING(stringobj);
+            namelen = PyBytes_GET_SIZE(stringobj);
+        }
+
+        if (strlen(name) != namelen) {
+            PyErr_SetString(PyExc_TypeError, "embedded null character");
+            goto error;
         }
     }
 
diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -205,16 +205,27 @@ PyLocale_strcoll(PyObject* self, PyObjec
 {
     PyObject *os1, *os2, *result = NULL;
     wchar_t *ws1 = NULL, *ws2 = NULL;
+    Py_ssize_t wlen;
 
     if (!PyArg_ParseTuple(args, "UU:strcoll", &os1, &os2))
         return NULL;
     /* Convert the unicode strings to wchar[]. */
-    ws1 = PyUnicode_AsWideCharString(os1, NULL);
+    ws1 = PyUnicode_AsWideCharString(os1, &wlen);
     if (ws1 == NULL)
         goto done;
+    if (wlen != wcslen(ws1)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "embedded null character");
+        goto done;
+    }
     ws2 = PyUnicode_AsWideCharString(os2, NULL);
     if (ws2 == NULL)
         goto done;
+    if (wlen != wcslen(ws2)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "embedded null character");
+        goto done;
+    }
     /* Collate the strings. */
     result = PyLong_FromLong(wcscoll(ws1, ws2));
   done:
@@ -246,6 +257,11 @@ PyLocale_strxfrm(PyObject* self, PyObjec
     s = PyUnicode_AsWideCharString(str, &n1);
     if (s == NULL)
         goto exit;
+    if (n1 != wcslen(s)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "embedded null character");
+        goto exit;
+    }
 
     /* assume no change in size, first */
     n1 = n1 + 1;
diff --git a/Modules/grpmodule.c b/Modules/grpmodule.c
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -112,6 +112,7 @@ grp_getgrnam(PyObject *self, PyObject *a
         return NULL;
     if ((bytes = PyUnicode_EncodeFSDefault(arg)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bytes, &name, NULL) == -1)
         goto out;
 
diff --git a/Modules/nismodule.c b/Modules/nismodule.c
--- a/Modules/nismodule.c
+++ b/Modules/nismodule.c
@@ -173,6 +173,7 @@ nis_match (PyObject *self, PyObject *arg
         return NULL;
     if ((bkey = PyUnicode_EncodeFSDefault(ukey)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bkey, &key, &keylen) == -1) {
         Py_DECREF(bkey);
         return NULL;
diff --git a/Modules/pwdmodule.c b/Modules/pwdmodule.c
--- a/Modules/pwdmodule.c
+++ b/Modules/pwdmodule.c
@@ -133,6 +133,7 @@ pwd_getpwnam(PyObject *self, PyObject *a
         return NULL;
     if ((bytes = PyUnicode_EncodeFSDefault(arg)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bytes, &name, NULL) == -1)
         goto out;
     if ((p = getpwnam(name)) == NULL) {
diff --git a/Modules/spwdmodule.c b/Modules/spwdmodule.c
--- a/Modules/spwdmodule.c
+++ b/Modules/spwdmodule.c
@@ -119,6 +119,7 @@ static PyObject* spwd_getspnam(PyObject 
         return NULL;
     if ((bytes = PyUnicode_EncodeFSDefault(arg)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bytes, &name, NULL) == -1)
         goto out;
     if ((p = getspnam(name)) == NULL) {
diff --git a/Modules/zipimport.c b/Modules/zipimport.c
--- a/Modules/zipimport.c
+++ b/Modules/zipimport.c
@@ -1094,11 +1094,18 @@ static PyObject *
 compile_source(PyObject *pathname, PyObject *source)
 {
     PyObject *code, *fixed_source, *pathbytes;
+    char *cpathname;
 
     pathbytes = PyUnicode_EncodeFSDefault(pathname);
     if (pathbytes == NULL)
         return NULL;
 
+    /* check for embedded null bytes */
+    if (PyBytes_AsStringAndSize(pathbytes, &cpathname, NULL) < 0) {
+        Py_DECREF(pathbytes);
+        return NULL;
+    }
+
     fixed_source = normalize_line_endings(source);
     if (fixed_source == NULL) {
         Py_DECREF(pathbytes);
@@ -1106,7 +1113,7 @@ compile_source(PyObject *pathname, PyObj
     }
 
     code = Py_CompileString(PyBytes_AsString(fixed_source),
-                            PyBytes_AsString(pathbytes),
+                            cpathname,
                             Py_file_input);
     Py_DECREF(pathbytes);
     Py_DECREF(fixed_source);
diff --git a/Python/fileutils.c b/Python/fileutils.c
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -250,20 +250,36 @@ _Py_stat(PyObject *path, struct stat *st
     int err;
     struct _stat wstatbuf;
     wchar_t *wpath;
+    Py_ssize_t pathlen;
 
-    wpath = PyUnicode_AsUnicode(path);
+    wpath = PyUnicode_AsUnicodeAndSize(path, &pathlen);
     if (wpath == NULL)
         return -2;
+    if (wcslen(wpath) != pathlen) {
+        PyErr_SetString(PyExc_TypeError, "embedded null character");
+        return -2;
+    }
+
     err = _wstat(wpath, &wstatbuf);
     if (!err)
         statbuf->st_mode = wstatbuf.st_mode;
     return err;
 #else
     int ret;
-    PyObject *bytes = PyUnicode_EncodeFSDefault(path);
+    PyObject *bytes;
+    char *cpath;
+
+    bytes = PyUnicode_EncodeFSDefault(path);
     if (bytes == NULL)
         return -2;
-    ret = stat(PyBytes_AS_STRING(bytes), statbuf);
+
+    /* check for embedded null bytes */
+    if (PyBytes_AsStringAndSize(bytes, &cpath, NULL) == -1) {
+        Py_DECREF(bytes);
+        return -2;
+    }
+
+    ret = stat(cpath, statbuf);
     Py_DECREF(bytes);
     return ret;
 #endif
@@ -309,10 +325,15 @@ _Py_fopen(PyObject *path, const char *mo
     wchar_t *wpath;
     wchar_t wmode[10];
     int usize;
+    Py_ssize_t pathlen;
 
-    wpath = PyUnicode_AsUnicode(path);
+    wpath = PyUnicode_AsUnicodeAndSize(path, &pathlen);
     if (wpath == NULL)
         return NULL;
+    if (wcslen(wpath) != pathlen) {
+        PyErr_SetString(PyExc_TypeError, "embedded null character");
+        return NULL;
+    }
 
     usize = MultiByteToWideChar(CP_ACP, 0, mode, -1, wmode, sizeof(wmode));
     if (usize == 0)
@@ -321,10 +342,20 @@ _Py_fopen(PyObject *path, const char *mo
     return _wfopen(wpath, wmode);
 #else
     FILE *f;
-    PyObject *bytes = PyUnicode_EncodeFSDefault(path);
+    PyObject *bytes;
+    char *cpath;
+
+    bytes = PyUnicode_EncodeFSDefault(path);
     if (bytes == NULL)
         return NULL;
-    f = fopen(PyBytes_AS_STRING(bytes), mode);
+
+    /* check for embedded null bytes */
+    if (PyBytes_AsStringAndSize(bytes, &cpath, NULL) == -1) {
+        Py_DECREF(bytes);
+        return NULL;
+    }
+
+    f = fopen(cpath, mode);
     Py_DECREF(bytes);
     return f;
 #endif
diff --git a/Python/import.c b/Python/import.c
--- a/Python/import.c
+++ b/Python/import.c
@@ -4159,6 +4159,7 @@ NullImporter_init(NullImporter *self, Py
     PyObject *pathobj;
     DWORD rv;
     wchar_t *path;
+    Py_ssize_t len;
 
     if (!_PyArg_NoKeywords("NullImporter()", kwds))
         return -1;
@@ -4172,9 +4173,16 @@ NullImporter_init(NullImporter *self, Py
         return -1;
     }
 
-    path = PyUnicode_AsWideCharString(pathobj, NULL);
+    path = PyUnicode_AsWideCharString(pathobj, &len);
     if (path == NULL)
         return -1;
+    if (len != wcslen(path)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "embedded null character");
+        PyMem_Free(path);
+        return -1;
+    }
+
     /* see issue1293 and issue3677:
      * stat() on Windows doesn't recognise paths like
      * "e:\\shared\\" and "\\\\whiterab-c2znlh\\shared" as dirs.