diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 0e44808050220f..f0ececc6a4a87a 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -764,23 +764,27 @@ PyAPI_FUNC(Py_UCS4*) PyUnicode_AsUCS4(
    exception set. */
 PyAPI_FUNC(Py_UCS4*) PyUnicode_AsUCS4Copy(PyObject *unicode);
 
+#ifndef Py_LIMITED_API
 /* Return a read-only pointer to the Unicode object's internal
    Py_UNICODE buffer.
    If the wchar_t/Py_UNICODE representation is not yet available, this
    function will calculate it. */
 
-#ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_UNICODE *) PyUnicode_AsUnicode(
     PyObject *unicode           /* Unicode object */
     );
-#endif
+
+/* Similar to PyUnicode_AsUnicode(), but raises a ValueError if the string
+   contains null characters. */
+PyAPI_FUNC(const Py_UNICODE *) _PyUnicode_AsUnicode(
+    PyObject *unicode           /* Unicode object */
+    );
 
 /* Return a read-only pointer to the Unicode object's internal
    Py_UNICODE buffer and save the length at size.
    If the wchar_t/Py_UNICODE representation is not yet available, this
    function will calculate it. */
 
-#ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_UNICODE *) PyUnicode_AsUnicodeAndSize(
     PyObject *unicode,          /* Unicode object */
     Py_ssize_t *size            /* location where to save the length */
diff --git a/Lib/ctypes/test/test_loading.py b/Lib/ctypes/test/test_loading.py
index 28468c1cd3d57b..3de457f4c3223e 100644
--- a/Lib/ctypes/test/test_loading.py
+++ b/Lib/ctypes/test/test_loading.py
@@ -64,6 +64,8 @@ def test_load_library(self):
             windll["kernel32"].GetModuleHandleW
             windll.LoadLibrary("kernel32").GetModuleHandleW
             WinDLL("kernel32").GetModuleHandleW
+            # embedded null character
+            self.assertRaises(ValueError, windll.LoadLibrary, "kernel32\0")
         elif os.name == "ce":
             windll.coredll.GetModuleHandleW
             windll["coredll"].GetModuleHandleW
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 0b033408a8c4a2..18e80c72918114 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -151,6 +151,8 @@ def test_import(self):
         self.assertRaises(TypeError, __import__, 1, 2, 3, 4)
         self.assertRaises(ValueError, __import__, '')
         self.assertRaises(TypeError, __import__, 'sys', name='sys')
+        # embedded null character
+        self.assertRaises((ImportError, ValueError), __import__, 'string\x00')
 
     def test_abs(self):
         # int
@@ -1002,6 +1004,10 @@ def test_open(self):
             self.assertEqual(fp.read(300), 'XXX'*100)
             self.assertEqual(fp.read(1000), 'YYY'*100)
 
+        # embedded null bytes and characters
+        self.assertRaises(ValueError, open, 'a\x00b')
+        self.assertRaises(ValueError, open, b'a\x00b')
+
     def test_open_default_encoding(self):
         old_environ = dict(os.environ)
         try:
diff --git a/Lib/test/test_curses.py b/Lib/test/test_curses.py
index 3d8c50bcb6c6ae..0d0b160d7645a5 100644
--- a/Lib/test/test_curses.py
+++ b/Lib/test/test_curses.py
@@ -81,7 +81,7 @@ def test_window_funcs(self):
         win2 = curses.newwin(15,15, 5,5)
 
         for meth in [stdscr.addch, stdscr.addstr]:
-            for args in [('a'), ('a', curses.A_BOLD),
+            for args in [('a',), ('a', curses.A_BOLD),
                          (4,4, 'a'), (5,5, 'a', curses.A_BOLD)]:
                 with self.subTest(meth=meth.__qualname__, args=args):
                     meth(*args)
@@ -194,6 +194,15 @@ def test_window_funcs(self):
         self.assertRaises(ValueError, stdscr.instr, -2)
         self.assertRaises(ValueError, stdscr.instr, 2, 3, -2)
 
+    def test_embedded_null_chars(self):
+        # reject embedded null bytes and characters
+        stdscr = self.stdscr
+        for arg in ['a', b'a']:
+            with self.subTest(arg=arg):
+                self.assertRaises(ValueError, stdscr.addstr, 'a\0')
+                self.assertRaises(ValueError, stdscr.addnstr, 'a\0', 1)
+                self.assertRaises(ValueError, stdscr.insstr, 'a\0')
+                self.assertRaises(ValueError, stdscr.insnstr, 'a\0', 1)
 
     def test_module_funcs(self):
         "Test module-level functions"
diff --git a/Lib/test/test_grp.py b/Lib/test/test_grp.py
index 272b08615dcc22..c218ea32551e56 100644
--- a/Lib/test/test_grp.py
+++ b/Lib/test/test_grp.py
@@ -50,6 +50,8 @@ def test_errors(self):
         self.assertRaises(TypeError, grp.getgrgid)
         self.assertRaises(TypeError, grp.getgrnam)
         self.assertRaises(TypeError, grp.getgrall, 42)
+        # embedded null character
+        self.assertRaises(ValueError, grp.getgrnam, 'a\x00b')
 
         # try to get some errors
         bynames = {}
diff --git a/Lib/test/test_imp.py b/Lib/test/test_imp.py
index ee9ee1ad8c3bcc..433de34c264551 100644
--- a/Lib/test/test_imp.py
+++ b/Lib/test/test_imp.py
@@ -315,6 +315,10 @@ def test_multiple_calls_to_get_data(self):
         loader.get_data(imp.__file__)  # File should be closed
         loader.get_data(imp.__file__)  # Will need to create a newly opened file
 
+    def test_load_source(self):
+        with self.assertRaisesRegex(ValueError, 'embedded null'):
+            imp.load_source(__name__, __file__ + "\0")
+
 
 class ReloadTests(unittest.TestCase):
 
diff --git a/Lib/test/test_locale.py b/Lib/test/test_locale.py
index fae2c3dabb370d..8d40a94e3eb178 100644
--- a/Lib/test/test_locale.py
+++ b/Lib/test/test_locale.py
@@ -339,9 +339,14 @@ def test_strcoll(self):
         self.assertLess(locale.strcoll('a', 'b'), 0)
         self.assertEqual(locale.strcoll('a', 'a'), 0)
         self.assertGreater(locale.strcoll('b', 'a'), 0)
+        # embedded null character
+        self.assertRaises(ValueError, locale.strcoll, 'a\0', 'a')
+        self.assertRaises(ValueError, locale.strcoll, 'a', 'a\0')
 
     def test_strxfrm(self):
         self.assertLess(locale.strxfrm('a'), locale.strxfrm('b'))
+        # embedded null character
+        self.assertRaises(ValueError, locale.strxfrm, 'a\0')
 
 
 class TestEnUSCollation(BaseLocalizedTest, TestCollation):
diff --git a/Lib/test/test_time.py b/Lib/test/test_time.py
index 76b894eece46e0..04962b223e88a4 100644
--- a/Lib/test/test_time.py
+++ b/Lib/test/test_time.py
@@ -114,6 +114,10 @@ def test_strftime(self):
             except ValueError:
                 self.fail('conversion specifier: %r failed.' % format)
 
+        self.assertRaises(TypeError, time.strftime, b'%S', tt)
+        # embedded null character
+        self.assertRaises(ValueError, time.strftime, '%S\0', tt)
+
     def _bounds_checking(self, func):
         # Make sure that strftime() checks the bounds of the various parts
         # of the time tuple (0 is valid for *all* values).
diff --git a/Lib/test/test_winsound.py b/Lib/test/test_winsound.py
index 4a8ab7de582c10..caac3c3c18c922 100644
--- a/Lib/test/test_winsound.py
+++ b/Lib/test/test_winsound.py
@@ -87,6 +87,10 @@ def test_errors(self):
             winsound.PlaySound,
             "none", winsound.SND_ASYNC | winsound.SND_MEMORY
         )
+        self.assertRaises(TypeError, winsound.PlaySound, b"bad", 0)
+        self.assertRaises(TypeError, winsound.PlaySound, 1, 0)
+        # embedded null character
+        self.assertRaises(ValueError, winsound.PlaySound, 'bad\0', 0)
 
     def test_aliases(self):
         aliases = [
diff --git a/Modules/_ctypes/callproc.c b/Modules/_ctypes/callproc.c
index 873df8b7bdb88c..70e416b950b0ec 100644
--- a/Modules/_ctypes/callproc.c
+++ b/Modules/_ctypes/callproc.c
@@ -1235,14 +1235,15 @@ The handle may be used to locate exported functions in this\n\
 module.\n";
 static PyObject *load_library(PyObject *self, PyObject *args)
 {
-    WCHAR *name;
+    const WCHAR *name;
     PyObject *nameobj;
     PyObject *ignored;
     HMODULE hMod;
-    if (!PyArg_ParseTuple(args, "O|O:LoadLibrary", &nameobj, &ignored))
+
+    if (!PyArg_ParseTuple(args, "U|O:LoadLibrary", &nameobj, &ignored))
         return NULL;
 
-    name = PyUnicode_AsUnicode(nameobj);
+    name = _PyUnicode_AsUnicode(nameobj);
     if (!name)
         return NULL;
 
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index 26d27317d03f2c..0cf17dec76cf16 100644
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -342,6 +342,7 @@ static int
 PyCurses_ConvertToString(PyCursesWindowObject *win, PyObject *obj,
                          PyObject **bytes, wchar_t **wstr)
 {
+    char *str;
     if (PyUnicode_Check(obj)) {
 #ifdef HAVE_NCURSESW
         assert (wstr != NULL);
@@ -354,12 +355,20 @@ PyCurses_ConvertToString(PyCursesWindowObject *win, PyObject *obj,
         *bytes = PyUnicode_AsEncodedString(obj, win->encoding, NULL);
         if (*bytes == NULL)
             return 0;
+        /* check for embedded null bytes */
+        if (PyBytes_AsStringAndSize(*bytes, &str, NULL) < 0) {
+            return 0;
+        }
         return 1;
 #endif
     }
     else if (PyBytes_Check(obj)) {
         Py_INCREF(obj);
         *bytes = obj;
+        /* check for embedded null bytes */
+        if (PyBytes_AsStringAndSize(*bytes, &str, NULL) < 0) {
+            return 0;
+        }
         return 1;
     }
 
diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index 4ce71debcf8e4a..39e0c54463bef8 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -252,6 +252,11 @@ PyLocale_strxfrm(PyObject* self, PyObject* args)
     s = PyUnicode_AsWideCharString(str, &n1);
     if (s == NULL)
         goto exit;
+    if (wcslen(s) != (size_t)n1) {
+        PyErr_SetString(PyExc_ValueError,
+                        "embedded null character");
+        goto exit;
+    }
 
     /* assume no change in size, first */
     n1 = n1 + 1;
diff --git a/Modules/grpmodule.c b/Modules/grpmodule.c
index 3a134a0333e71a..277fce4d286b0a 100644
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -140,6 +140,7 @@ grp_getgrnam_impl(PyObject *module, PyObject *name)
 
     if ((bytes = PyUnicode_EncodeFSDefault(name)) == NULL)
         return NULL;
+    /* check for embedded null bytes */
     if (PyBytes_AsStringAndSize(bytes, &name_chars, NULL) == -1)
         goto out;
 
diff --git a/Modules/nismodule.c b/Modules/nismodule.c
index 64eb5dbc3d0299..3942acb2c7170c 100644
--- a/Modules/nismodule.c
+++ b/Modules/nismodule.c
@@ -173,6 +173,7 @@ nis_match (PyObject *self, PyObject *args, PyObject *kwdict)
         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/posixmodule.c b/Modules/posixmodule.c
index 8d32ddf131ce72..3dcebf496f3e0b 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3821,9 +3821,9 @@ os__getfinalpathname_impl(PyObject *module, PyObject *path)
     wchar_t *target_path;
     int result_length;
     PyObject *result;
-    wchar_t *path_wchar;
+    const wchar_t *path_wchar;
 
-    path_wchar = PyUnicode_AsUnicode(path);
+    path_wchar = _PyUnicode_AsUnicode(path);
     if (path_wchar == NULL)
         return NULL;
 
@@ -7164,7 +7164,7 @@ posix_readlink(PyObject *self, PyObject *args, PyObject *kwargs)
 static PyObject *
 win_readlink(PyObject *self, PyObject *args, PyObject *kwargs)
 {
-    wchar_t *path;
+    const wchar_t *path;
     DWORD n_bytes_returned;
     DWORD io_result;
     PyObject *po, *result;
@@ -7183,7 +7183,7 @@ win_readlink(PyObject *self, PyObject *args, PyObject *kwargs)
                           ))
         return NULL;
 
-    path = PyUnicode_AsUnicode(po);
+    path = _PyUnicode_AsUnicode(po);
     if (path == NULL)
         return NULL;
 
@@ -9075,7 +9075,8 @@ static PyObject *
 os_putenv_impl(PyObject *module, PyObject *name, PyObject *value)
 /*[clinic end generated code: output=d29a567d6b2327d2 input=ba586581c2e6105f]*/
 {
-    wchar_t *env;
+    const wchar_t *env;
+    Py_ssize_t size;
 
     /* Search from index 1 because on Windows starting '=' is allowed for
        defining hidden environment variables. */
@@ -9089,16 +9090,21 @@ os_putenv_impl(PyObject *module, PyObject *name, PyObject *value)
     if (unicode == NULL) {
         return NULL;
     }
-    if (_MAX_ENV < PyUnicode_GET_LENGTH(unicode)) {
+
+    env = PyUnicode_AsUnicodeAndSize(unicode, &size);
+    if (env == NULL)
+        goto error;
+    if (size > _MAX_ENV) {
         PyErr_Format(PyExc_ValueError,
                      "the environment variable is longer than %u characters",
                      _MAX_ENV);
         goto error;
     }
-
-    env = PyUnicode_AsUnicode(unicode);
-    if (env == NULL)
+    if (wcslen(env) != (size_t)size) {
+        PyErr_SetString(PyExc_ValueError, "embedded null character");
         goto error;
+    }
+
     if (_wputenv(env)) {
         posix_error();
         goto error;
diff --git a/Modules/pwdmodule.c b/Modules/pwdmodule.c
index 7416cf71d59a6f..1b0d499a4ba62f 100644
--- a/Modules/pwdmodule.c
+++ b/Modules/pwdmodule.c
@@ -150,6 +150,7 @@ pwd_getpwnam_impl(PyObject *module, PyObject *arg)
 
     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
index 4b9f3cd9b711e1..065f79cbf2008e 100644
--- a/Modules/spwdmodule.c
+++ b/Modules/spwdmodule.c
@@ -134,6 +134,7 @@ spwd_getspnam_impl(PyObject *module, PyObject *arg)
 
     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/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 64375da602d11f..66cb4afbe0d374 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3894,6 +3894,20 @@ PyUnicode_AsUnicode(PyObject *unicode)
     return PyUnicode_AsUnicodeAndSize(unicode, NULL);
 }
 
+const Py_UNICODE *
+_PyUnicode_AsUnicode(PyObject *unicode)
+{
+    Py_ssize_t size;
+    const Py_UNICODE *wstr;
+
+    wstr = PyUnicode_AsUnicodeAndSize(unicode, &size);
+    if (wstr && wcslen(wstr) != (size_t)size) {
+        PyErr_SetString(PyExc_ValueError, "embedded null character");
+        return NULL;
+    }
+    return wstr;
+}
+
 
 Py_ssize_t
 PyUnicode_GetSize(PyObject *unicode)
diff --git a/PC/_msi.c b/PC/_msi.c
index 9e7e36da7164df..967b7fa2ee3731 100644
--- a/PC/_msi.c
+++ b/PC/_msi.c
@@ -600,8 +600,12 @@ summary_setproperty(msiobj* si, PyObject *args)
         return NULL;
 
     if (PyUnicode_Check(data)) {
+        const WCHAR *value = _PyUnicode_AsUnicode(data);
+        if (value == NULL) {
+            return NULL;
+        }
         status = MsiSummaryInfoSetPropertyW(si->h, field, VT_LPSTR,
-            0, NULL, PyUnicode_AsUnicode(data));
+            0, NULL, value);
     } else if (PyLong_CheckExact(data)) {
         long value = PyLong_AsLong(data);
         if (value == -1 && PyErr_Occurred()) {
diff --git a/Python/dynload_win.c b/Python/dynload_win.c
index f2c796e94d2e61..3f533d1756cf20 100644
--- a/Python/dynload_win.c
+++ b/Python/dynload_win.c
@@ -192,13 +192,13 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
 {
     dl_funcptr p;
     char funcname[258], *import_python;
-    wchar_t *wpathname;
+    const wchar_t *wpathname;
 
 #ifndef _DEBUG
     _Py_CheckPython3();
 #endif
 
-    wpathname = PyUnicode_AsUnicode(pathname);
+    wpathname = _PyUnicode_AsUnicode(pathname);
     if (wpathname == NULL)
         return NULL;
 
diff --git a/Python/fileutils.c b/Python/fileutils.c
index 23eed71321853d..5072118c9c9e65 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -710,21 +710,32 @@ _Py_stat(PyObject *path, struct stat *statbuf)
 #ifdef MS_WINDOWS
     int err;
     struct _stat wstatbuf;
-    wchar_t *wpath;
+    const wchar_t *wpath;
 
-    wpath = PyUnicode_AsUnicode(path);
+    wpath = _PyUnicode_AsUnicode(path);
     if (wpath == NULL)
         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
@@ -1083,7 +1094,7 @@ _Py_fopen_obj(PyObject *path, const char *mode)
     FILE *f;
     int async_err = 0;
 #ifdef MS_WINDOWS
-    wchar_t *wpath;
+    const wchar_t *wpath;
     wchar_t wmode[10];
     int usize;
 
@@ -1097,7 +1108,7 @@ _Py_fopen_obj(PyObject *path, const char *mode)
                      Py_TYPE(path));
         return NULL;
     }
-    wpath = PyUnicode_AsUnicode(path);
+    wpath = _PyUnicode_AsUnicode(path);
     if (wpath == NULL)
         return NULL;