bpo-36085: Enable better DLL resolution on Windows (GH-12302)

Doc/library/ctypes.rst

@@ -1322,14 +1322,14 @@ There are several ways to load shared libraries into the Python process.  One
 way is to instantiate one of the following classes:
 
 
-.. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
+.. class:: CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)
 
    Instances of this class represent loaded shared libraries. Functions in these
    libraries use the standard C calling convention, and are assumed to return
    :c:type:`int`.
 
 
-.. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
+.. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)
 
    Windows only: Instances of this class represent loaded shared libraries,
    functions in these libraries use the ``stdcall`` calling convention, and are
@@ -1342,7 +1342,7 @@ way is to instantiate one of the following classes:
       :exc:`WindowsError` used to be raised.
 
 
-.. class:: WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
+.. class:: WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)
 
    Windows only: Instances of this class represent loaded shared libraries,
    functions in these libraries use the ``stdcall`` calling convention, and are
@@ -1394,6 +1394,17 @@ the Windows error code which is managed by the :func:`GetLastError` and
 :func:`ctypes.set_last_error` are used to request and change the ctypes private
 copy of the windows error code.
 
+The *winmode* parameter is used on Windows to specify how the library is loaded
+(since *mode* is ignored). It takes any value that is valid for the Win32 API
+``LoadLibraryEx`` flags parameter. When omitted, the default is to use the flags
+that result in the most secure DLL load to avoiding issues such as DLL
+hijacking. Passing the full path to the DLL is the safest way to ensure the
+correct library and dependencies are loaded.
+
+.. versionchanged:: 3.8
+   Added *winmode* parameter.
+
+
 .. data:: RTLD_GLOBAL
    :noindex:
 

Doc/library/os.rst

@@ -3079,6 +3079,36 @@ to be ignored.
    :func:`signal.signal`.
 
 
+.. function:: add_dll_directory(path)
+
+   Add a path to the DLL search path.
+
+   This search path is used when resolving dependencies for imported
+   extension modules (the module itself is resolved through sys.path),
+   and also by :mod:`ctypes`.
+
+   Remove the directory by calling **close()** on the returned object
+   or using it in a :keyword:`with` statement.
+
+   See the `Microsoft documentation
+   <https://msdn.microsoft.com/44228cf2-6306-466c-8f16-f513cd3ba8b5>`_
+   for more information about how DLLs are loaded.
+
+   .. availability:: Windows.
+
+   .. versionadded:: 3.8
+      Previous versions of CPython would resolve DLLs using the default
+      behavior for the current process. This led to inconsistencies,
+      such as only sometimes searching :envvar:`PATH` or the current
+      working directory, and OS functions such as ``AddDllDirectory``
+      having no effect.
+
+      In 3.8, the two primary ways DLLs are loaded now explicitly
+      override the process-wide behavior to ensure consistency. See the
+      :ref:`porting notes <bpo-36085-whatsnew>` for information on
+      updating libraries.
+
+
 .. function:: execl(path, arg0, arg1, ...)
               execle(path, arg0, arg1, ..., env)
               execlp(file, arg0, arg1, ...)

Doc/whatsnew/3.8.rst

@@ -168,6 +168,16 @@ asyncio
 On Windows, the default event loop is now :class:`~asyncio.ProactorEventLoop`.
 
 
+ctypes
+------
+
+On Windows, :class:`~ctypes.CDLL` and subclasses now accept a *winmode* parameter
+to specify flags for the underlying ``LoadLibraryEx`` call. The default flags are
+set to only load DLL dependencies from trusted locations, including the path
+where the DLL is stored (if a full or partial path is used to load the initial
+DLL) and paths added by :func:`~os.add_dll_directory`.
+
+
 gettext
 -------
 
@@ -238,6 +248,13 @@ Added new function, :func:`math.prod`, as analogous function to :func:`sum`
 that returns the product of a 'start' value (default: 1) times an iterable of
 numbers. (Contributed by Pablo Galindo in :issue:`35606`)
 
+os
+--
+
+Added new function :func:`~os.add_dll_directory` on Windows for providing
+additional search paths for native dependencies when importing extension
+modules or loading DLLs using :mod:`ctypes`.
+
 
 os.path
 -------
@@ -727,6 +744,19 @@ Changes in the Python API
   environment variable and does not use :envvar:`HOME`, which is not normally
   set for regular user accounts.
 
+.. _bpo-36085-whatsnew:
+
+* DLL dependencies for extension modules and DLLs loaded with :mod:`ctypes` on
+  Windows are now resolved more securely. Only the system paths, the directory
+  containing the DLL or PYD file, and directories added with
+  :func:`~os.add_dll_directory` are searched for load-time dependencies.
+  Specifically, :envvar:`PATH` and the current working directory are no longer
+  used, and modifications to these will no longer have any effect on normal DLL
+  resolution. If your application relies on these mechanisms, you should check
+  for :func:`~os.add_dll_directory` and if it exists, use it to add your DLLs
+  directory while loading your library.
+  (See :issue:`36085`.)
+
 
 Changes in the C API
 --------------------

Lib/ctypes/__init__.py

@@ -326,7 +326,8 @@ class CDLL(object):
 
     def __init__(self, name, mode=DEFAULT_MODE, handle=None,
                  use_errno=False,
-                 use_last_error=False):
+                 use_last_error=False,
+                 winmode=None):
         self._name = name
         flags = self._func_flags_
         if use_errno:
@@ -341,6 +342,15 @@ class CDLL(object):
             """
             if name and name.endswith(")") and ".a(" in name:
                 mode |= ( _os.RTLD_MEMBER | _os.RTLD_NOW )
+        if _os.name == "nt":
+            if winmode is not None:
+                mode = winmode
+            else:
+                import nt
+                mode = nt._LOAD_LIBRARY_SEARCH_DEFAULT_DIRS
+                if '/' in name or '\\' in name:
+                    self._name = nt._getfullpathname(self._name)
+                    mode |= nt._LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR
 
         class _FuncPtr(_CFuncPtr):
             _flags_ = flags

Lib/ctypes/test/test_loading.py

@@ -1,6 +1,9 @@
 from ctypes import *
 import os
+import shutil
+import subprocess
 import sys
+import sysconfig
 import unittest
 import test.support
 from ctypes.util import find_library
@@ -112,5 +115,65 @@ class LoaderTest(unittest.TestCase):
         # This is the real test: call the function via 'call_function'
         self.assertEqual(0, call_function(proc, (None,)))
 
+    @unittest.skipUnless(os.name == "nt",
+                         'test specific to Windows')
+    def test_load_dll_with_flags(self):
+        _sqlite3 = test.support.import_module("_sqlite3")
+        src = _sqlite3.__file__
+        if src.lower().endswith("_d.pyd"):
+            ext = "_d.dll"
+        else:
+            ext = ".dll"
+
+        with test.support.temp_dir() as tmp:
+            # We copy two files and load _sqlite3.dll (formerly .pyd),
+            # which has a dependency on sqlite3.dll. Then we test
+            # loading it in subprocesses to avoid it starting in memory
+            # for each test.
+            target = os.path.join(tmp, "_sqlite3.dll")
+            shutil.copy(src, target)
+            shutil.copy(os.path.join(os.path.dirname(src), "sqlite3" + ext),
+                        os.path.join(tmp, "sqlite3" + ext))
+
+            def should_pass(command):
+                with self.subTest(command):
+                    subprocess.check_output(
+                        [sys.executable, "-c",
+                         "from ctypes import *; import nt;" + command],
+                        cwd=tmp
+                    )
+
+            def should_fail(command):
+                with self.subTest(command):
+                    with self.assertRaises(subprocess.CalledProcessError):
+                        subprocess.check_output(
+                            [sys.executable, "-c",
+                             "from ctypes import *; import nt;" + command],
+                            cwd=tmp, stderr=subprocess.STDOUT,
+                        )
+
+            # Default load should not find this in CWD
+            should_fail("WinDLL('_sqlite3.dll')")
+
+            # Relative path (but not just filename) should succeed
+            should_pass("WinDLL('./_sqlite3.dll')")
+
+            # Insecure load flags should succeed
+            should_pass("WinDLL('_sqlite3.dll', winmode=0)")
+
+            # Full path load without DLL_LOAD_DIR shouldn't find dependency
+            should_fail("WinDLL(nt._getfullpathname('_sqlite3.dll'), " +
+                        "winmode=nt._LOAD_LIBRARY_SEARCH_SYSTEM32)")
+
+            # Full path load with DLL_LOAD_DIR should succeed
+            should_pass("WinDLL(nt._getfullpathname('_sqlite3.dll'), " +
+                        "winmode=nt._LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR)")
+
+            # User-specified directory should succeed
+            should_pass("import os; p = os.add_dll_directory(os.getcwd());" +
+                        "WinDLL('_sqlite3.dll'); p.close()")
+
+
+
 if __name__ == "__main__":
     unittest.main()

Lib/os.py

@@ -1070,3 +1070,40 @@ class PathLike(abc.ABC):
     @classmethod
     def __subclasshook__(cls, subclass):
         return hasattr(subclass, '__fspath__')
+
+
+if name == 'nt':
+    class _AddedDllDirectory:
+        def __init__(self, path, cookie, remove_dll_directory):
+            self.path = path
+            self._cookie = cookie
+            self._remove_dll_directory = remove_dll_directory
+        def close(self):
+            self._remove_dll_directory(self._cookie)
+            self.path = None
+        def __enter__(self):
+            return self
+        def __exit__(self, *args):
+            self.close()
+        def __repr__(self):
+            if self.path:
+                return "<AddedDllDirectory({!r})>".format(self.path)
+            return "<AddedDllDirectory()>"
+
+    def add_dll_directory(path):
+        """Add a path to the DLL search path.
+
+        This search path is used when resolving dependencies for imported
+        extension modules (the module itself is resolved through sys.path),
+        and also by ctypes.
+
+        Remove the directory by calling close() on the returned object or
+        using it in a with statement.
+        """
+        import nt
+        cookie = nt._add_dll_directory(path)
+        return _AddedDllDirectory(
+            path,
+            cookie,
+            nt._remove_dll_directory
+        )

Lib/test/test_import/__init__.py

@@ -8,6 +8,8 @@ import os
 import platform
 import py_compile
 import random
+import shutil
+import subprocess
 import stat
 import sys
 import threading
@@ -17,6 +19,7 @@ import unittest.mock as mock
 import textwrap
 import errno
 import contextlib
+import glob
 
 import test.support
 from test.support import (
@@ -460,6 +463,51 @@ class ImportTests(unittest.TestCase):
         finally:
             del sys.path[0]
 
+    @unittest.skipUnless(sys.platform == "win32", "Windows-specific")
+    def test_dll_dependency_import(self):
+        from _winapi import GetModuleFileName
+        dllname = GetModuleFileName(sys.dllhandle)
+        pydname = importlib.util.find_spec("_sqlite3").origin
+        depname = os.path.join(
+            os.path.dirname(pydname),
+            "sqlite3{}.dll".format("_d" if "_d" in pydname else ""))
+
+        with test.support.temp_dir() as tmp:
+            tmp2 = os.path.join(tmp, "DLLs")
+            os.mkdir(tmp2)
+
+            pyexe = os.path.join(tmp, os.path.basename(sys.executable))
+            shutil.copy(sys.executable, pyexe)
+            shutil.copy(dllname, tmp)
+            for f in glob.glob(os.path.join(sys.prefix, "vcruntime*.dll")):
+                shutil.copy(f, tmp)
+
+            shutil.copy(pydname, tmp2)
+
+            env = None
+            env = {k.upper(): os.environ[k] for k in os.environ}
+            env["PYTHONPATH"] = tmp2 + ";" + os.path.dirname(os.__file__)
+
+            # Test 1: import with added DLL directory
+            subprocess.check_call([
+                pyexe, "-Sc", ";".join([
+                    "import os",
+                    "p = os.add_dll_directory({!r})".format(
+                        os.path.dirname(depname)),
+                    "import _sqlite3",
+                    "p.close"
+                ])],
+                stderr=subprocess.STDOUT,
+                env=env,
+                cwd=os.path.dirname(pyexe))
+
+            # Test 2: import with DLL adjacent to PYD
+            shutil.copy(depname, tmp2)
+            subprocess.check_call([pyexe, "-Sc", "import _sqlite3"],
+                                    stderr=subprocess.STDOUT,
+                                    env=env,
+                                    cwd=os.path.dirname(pyexe))
+
 
 @skip_if_dont_write_bytecode
 class FilePermissionTests(unittest.TestCase):

Misc/NEWS.d/next/Windows/2019-03-18-11-44-49.bpo-36085.mLfxfc.rst

@@ -0,0 +1,2 @@
+Enable better DLL resolution on Windows by using safe DLL search paths and
+adding :func:`os.add_dll_directory`.

Modules/_ctypes/callproc.c

@@ -1251,19 +1251,21 @@ static PyObject *format_error(PyObject *self, PyObject *args)
 }
 
 static const char load_library_doc[] =
-"LoadLibrary(name) -> handle\n\
+"LoadLibrary(name, load_flags) -> handle\n\
 \n\
 Load an executable (usually a DLL), and return a handle to it.\n\
 The handle may be used to locate exported functions in this\n\
-module.\n";
+module. load_flags are as defined for LoadLibraryEx in the\n\
+Windows API.\n";
 static PyObject *load_library(PyObject *self, PyObject *args)
 {
     const WCHAR *name;
     PyObject *nameobj;
-    PyObject *ignored;
+    int load_flags = 0;
     HMODULE hMod;
+    DWORD err;
 
-    if (!PyArg_ParseTuple(args, "U|O:LoadLibrary", &nameobj, &ignored))
+    if (!PyArg_ParseTuple(args, "U|i:LoadLibrary", &nameobj, &load_flags))
         return NULL;
 
     name = _PyUnicode_AsUnicode(nameobj);
@@ -1271,11 +1273,22 @@ static PyObject *load_library(PyObject *self, PyObject *args)
         return NULL;
 
     Py_BEGIN_ALLOW_THREADS
-    hMod = LoadLibraryW(name);
+    /* bpo-36085: Limit DLL search directories to avoid pre-loading
+     * attacks and enable use of the AddDllDirectory function.
+     */
+    hMod = LoadLibraryExW(name, NULL, (DWORD)load_flags);
+    err = hMod ? 0 : GetLastError();
     Py_END_ALLOW_THREADS
 
-    if (!hMod)
+    if (err == ERROR_MOD_NOT_FOUND) {
-        return PyErr_SetFromWindowsErr(GetLastError());
+        PyErr_Format(PyExc_FileNotFoundError,
+                     ("Could not find module '%.500S'. Try using "
+                      "the full path with constructor syntax."),
+                     nameobj);
+        return NULL;
+    } else if (err) {
+        return PyErr_SetFromWindowsErr(err);
+    }
 #ifdef _WIN64
     return PyLong_FromVoidPtr(hMod);
 #else
@@ -1291,15 +1304,18 @@ static PyObject *free_library(PyObject *self, PyObject *args)
 {
     void *hMod;
     BOOL result;
+    DWORD err;
     if (!PyArg_ParseTuple(args, "O&:FreeLibrary", &_parse_voidp, &hMod))
         return NULL;
 
     Py_BEGIN_ALLOW_THREADS
     result = FreeLibrary((HMODULE)hMod);
+    err = result ? 0 : GetLastError();
     Py_END_ALLOW_THREADS
 
-    if (!result)
+    if (!result) {
-        return PyErr_SetFromWindowsErr(GetLastError());
+        return PyErr_SetFromWindowsErr(err);
+    }
     Py_RETURN_NONE;
 }
 

Modules/clinic/posixmodule.c.h

@@ -7961,6 +7961,94 @@ exit:
 
 #endif /* defined(HAVE_GETRANDOM_SYSCALL) */
 
+#if defined(MS_WINDOWS)
+
+PyDoc_STRVAR(os__add_dll_directory__doc__,
+"_add_dll_directory($module, /, path)\n"
+"--\n"
+"\n"
+"Add a path to the DLL search path.\n"
+"\n"
+"This search path is used when resolving dependencies for imported\n"
+"extension modules (the module itself is resolved through sys.path),\n"
+"and also by ctypes.\n"
+"\n"
+"Returns an opaque value that may be passed to os.remove_dll_directory\n"
+"to remove this directory from the search path.");
+
+#define OS__ADD_DLL_DIRECTORY_METHODDEF    \
+    {"_add_dll_directory", (PyCFunction)(void(*)(void))os__add_dll_directory, METH_FASTCALL|METH_KEYWORDS, os__add_dll_directory__doc__},
+
+static PyObject *
+os__add_dll_directory_impl(PyObject *module, path_t *path);
+
+static PyObject *
+os__add_dll_directory(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"path", NULL};
+    static _PyArg_Parser _parser = {NULL, _keywords, "_add_dll_directory", 0};
+    PyObject *argsbuf[1];
+    path_t path = PATH_T_INITIALIZE("_add_dll_directory", "path", 0, 0);
+
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 1, 1, 0, argsbuf);
+    if (!args) {
+        goto exit;
+    }
+    if (!path_converter(args[0], &path)) {
+        goto exit;
+    }
+    return_value = os__add_dll_directory_impl(module, &path);
+
+exit:
+    /* Cleanup for path */
+    path_cleanup(&path);
+
+    return return_value;
+}
+
+#endif /* defined(MS_WINDOWS) */
+
+#if defined(MS_WINDOWS)
+
+PyDoc_STRVAR(os__remove_dll_directory__doc__,
+"_remove_dll_directory($module, /, cookie)\n"
+"--\n"
+"\n"
+"Removes a path from the DLL search path.\n"
+"\n"
+"The parameter is an opaque value that was returned from\n"
+"os.add_dll_directory. You can only remove directories that you added\n"
+"yourself.");
+
+#define OS__REMOVE_DLL_DIRECTORY_METHODDEF    \
+    {"_remove_dll_directory", (PyCFunction)(void(*)(void))os__remove_dll_directory, METH_FASTCALL|METH_KEYWORDS, os__remove_dll_directory__doc__},
+
+static PyObject *
+os__remove_dll_directory_impl(PyObject *module, PyObject *cookie);
+
+static PyObject *
+os__remove_dll_directory(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"cookie", NULL};
+    static _PyArg_Parser _parser = {NULL, _keywords, "_remove_dll_directory", 0};
+    PyObject *argsbuf[1];
+    PyObject *cookie;
+
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 1, 1, 0, argsbuf);
+    if (!args) {
+        goto exit;
+    }
+    cookie = args[0];
+    return_value = os__remove_dll_directory_impl(module, cookie);
+
+exit:
+    return return_value;
+}
+
+#endif /* defined(MS_WINDOWS) */
+
 #ifndef OS_TTYNAME_METHODDEF
     #define OS_TTYNAME_METHODDEF
 #endif /* !defined(OS_TTYNAME_METHODDEF) */
@@ -8480,4 +8568,12 @@ exit:
 #ifndef OS_GETRANDOM_METHODDEF
     #define OS_GETRANDOM_METHODDEF
 #endif /* !defined(OS_GETRANDOM_METHODDEF) */
-/*[clinic end generated code: output=1a9c62f5841221ae input=a9049054013a1b77]*/
+
+#ifndef OS__ADD_DLL_DIRECTORY_METHODDEF
+    #define OS__ADD_DLL_DIRECTORY_METHODDEF
+#endif /* !defined(OS__ADD_DLL_DIRECTORY_METHODDEF) */
+
+#ifndef OS__REMOVE_DLL_DIRECTORY_METHODDEF
+    #define OS__REMOVE_DLL_DIRECTORY_METHODDEF
+#endif /* !defined(OS__REMOVE_DLL_DIRECTORY_METHODDEF) */
+/*[clinic end generated code: output=ab36ec0376a422ae input=a9049054013a1b77]*/

Modules/posixmodule.c

@@ -1442,17 +1442,23 @@ win32_error(const char* function, const char* filename)
 }
 
 static PyObject *
-win32_error_object(const char* function, PyObject* filename)
+win32_error_object_err(const char* function, PyObject* filename, DWORD err)
 {
     /* XXX - see win32_error for comments on 'function' */
-    errno = GetLastError();
     if (filename)
         return PyErr_SetExcFromWindowsErrWithFilenameObject(
                     PyExc_OSError,
-                    errno,
+                    err,
                     filename);
     else
-        return PyErr_SetFromWindowsErr(errno);
+        return PyErr_SetFromWindowsErr(err);
+}
+
+static PyObject *
+win32_error_object(const char* function, PyObject* filename)
+{
+    errno = GetLastError();
+    return win32_error_object_err(function, filename, errno);
 }
 
 #endif /* MS_WINDOWS */
@@ -13161,6 +13167,113 @@ error:
 }
 #endif   /* HAVE_GETRANDOM_SYSCALL */
 
+#ifdef MS_WINDOWS
+/* bpo-36085: Helper functions for managing DLL search directories
+ * on win32
+ */
+
+typedef DLL_DIRECTORY_COOKIE (WINAPI *PAddDllDirectory)(PCWSTR newDirectory);
+typedef BOOL (WINAPI *PRemoveDllDirectory)(DLL_DIRECTORY_COOKIE cookie);
+
+/*[clinic input]
+os._add_dll_directory
+
+    path: path_t
+
+Add a path to the DLL search path.
+
+This search path is used when resolving dependencies for imported
+extension modules (the module itself is resolved through sys.path),
+and also by ctypes.
+
+Returns an opaque value that may be passed to os.remove_dll_directory
+to remove this directory from the search path.
+[clinic start generated code]*/
+
+static PyObject *
+os__add_dll_directory_impl(PyObject *module, path_t *path)
+/*[clinic end generated code: output=80b025daebb5d683 input=1de3e6c13a5808c8]*/
+{
+    HMODULE hKernel32;
+    PAddDllDirectory AddDllDirectory;
+    DLL_DIRECTORY_COOKIE cookie = 0;
+    DWORD err = 0;
+
+    /* For Windows 7, we have to load this. As this will be a fairly
+       infrequent operation, just do it each time. Kernel32 is always
+       loaded. */
+    Py_BEGIN_ALLOW_THREADS
+    if (!(hKernel32 = GetModuleHandleW(L"kernel32")) ||
+        !(AddDllDirectory = (PAddDllDirectory)GetProcAddress(
+            hKernel32, "AddDllDirectory")) ||
+        !(cookie = (*AddDllDirectory)(path->wide))) {
+        err = GetLastError();
+    }
+    Py_END_ALLOW_THREADS
+
+    if (err) {
+        return win32_error_object_err("add_dll_directory",
+                                      path->object, err);
+    }
+
+    return PyCapsule_New(cookie, "DLL directory cookie", NULL);
+}
+
+/*[clinic input]
+os._remove_dll_directory
+
+    cookie: object
+
+Removes a path from the DLL search path.
+
+The parameter is an opaque value that was returned from
+os.add_dll_directory. You can only remove directories that you added
+yourself.
+[clinic start generated code]*/
+
+static PyObject *
+os__remove_dll_directory_impl(PyObject *module, PyObject *cookie)
+/*[clinic end generated code: output=594350433ae535bc input=c1d16a7e7d9dc5dc]*/
+{
+    HMODULE hKernel32;
+    PRemoveDllDirectory RemoveDllDirectory;
+    DLL_DIRECTORY_COOKIE cookieValue;
+    DWORD err = 0;
+
+    if (!PyCapsule_IsValid(cookie, "DLL directory cookie")) {
+        PyErr_SetString(PyExc_TypeError,
+            "Provided cookie was not returned from os.add_dll_directory");
+        return NULL;
+    }
+
+    cookieValue = (DLL_DIRECTORY_COOKIE)PyCapsule_GetPointer(
+        cookie, "DLL directory cookie");
+
+    /* For Windows 7, we have to load this. As this will be a fairly
+       infrequent operation, just do it each time. Kernel32 is always
+       loaded. */
+    Py_BEGIN_ALLOW_THREADS
+    if (!(hKernel32 = GetModuleHandleW(L"kernel32")) ||
+        !(RemoveDllDirectory = (PRemoveDllDirectory)GetProcAddress(
+            hKernel32, "RemoveDllDirectory")) ||
+        !(*RemoveDllDirectory)(cookieValue)) {
+        err = GetLastError();
+    }
+    Py_END_ALLOW_THREADS
+
+    if (err) {
+        return win32_error_object_err("remove_dll_directory",
+                                      NULL, err);
+    }
+
+    if (PyCapsule_SetName(cookie, NULL)) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
+#endif
 
 static PyMethodDef posix_methods[] = {
 
@@ -13349,6 +13462,10 @@ static PyMethodDef posix_methods[] = {
     OS_SCANDIR_METHODDEF
     OS_FSPATH_METHODDEF
     OS_GETRANDOM_METHODDEF
+#ifdef MS_WINDOWS
+    OS__ADD_DLL_DIRECTORY_METHODDEF
+    OS__REMOVE_DLL_DIRECTORY_METHODDEF
+#endif
     {NULL,              NULL}            /* Sentinel */
 };
 
@@ -13826,6 +13943,14 @@ all_ins(PyObject *m)
     if (PyModule_AddIntConstant(m, "_COPYFILE_DATA", COPYFILE_DATA)) return -1;
 #endif
 
+#ifdef MS_WINDOWS
+    if (PyModule_AddIntConstant(m, "_LOAD_LIBRARY_SEARCH_DEFAULT_DIRS", LOAD_LIBRARY_SEARCH_DEFAULT_DIRS)) return -1;
+    if (PyModule_AddIntConstant(m, "_LOAD_LIBRARY_SEARCH_APPLICATION_DIR", LOAD_LIBRARY_SEARCH_APPLICATION_DIR)) return -1;
+    if (PyModule_AddIntConstant(m, "_LOAD_LIBRARY_SEARCH_SYSTEM32", LOAD_LIBRARY_SEARCH_SYSTEM32)) return -1;
+    if (PyModule_AddIntConstant(m, "_LOAD_LIBRARY_SEARCH_USER_DIRS", LOAD_LIBRARY_SEARCH_USER_DIRS)) return -1;
+    if (PyModule_AddIntConstant(m, "_LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR", LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR)) return -1;
+#endif
+
     return 0;
 }
 

Python/dynload_win.c

@@ -215,12 +215,14 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
 #if HAVE_SXS
         cookie = _Py_ActivateActCtx();
 #endif
-        /* We use LoadLibraryEx so Windows looks for dependent DLLs
+        /* bpo-36085: We use LoadLibraryEx with restricted search paths
-            in directory of pathname first. */
+           to avoid DLL preloading attacks and enable use of the
-        /* XXX This call doesn't exist in Windows CE */
+           AddDllDirectory function. We add SEARCH_DLL_LOAD_DIR to
+           ensure DLLs adjacent to the PYD are preferred. */
         Py_BEGIN_ALLOW_THREADS
         hDLL = LoadLibraryExW(wpathname, NULL,
-                              LOAD_WITH_ALTERED_SEARCH_PATH);
+                              LOAD_LIBRARY_SEARCH_DEFAULT_DIRS |
+                              LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR);
         Py_END_ALLOW_THREADS
 #if HAVE_SXS
         _Py_DeactivateActCtx(cookie);