diff -r 3d7000549eb1 Lib/shutil.py
--- a/Lib/shutil.py	Wed Jan 23 03:01:23 2013 -0800
+++ b/Lib/shutil.py	Wed Jan 23 16:02:12 2013 +0200
@@ -1084,7 +1084,11 @@
             return cmd
         return None
 
-    path = (path or os.environ.get("PATH", os.defpath)).split(os.pathsep)
+    if path is None:
+        path = os.environ.get("PATH", os.defpath)
+    if not path:
+        return None
+    path = path.split(os.pathsep)
 
     if sys.platform == "win32":
         # The current directory takes precedence on Windows.
diff -r 3d7000549eb1 Lib/test/test_shutil.py
--- a/Lib/test/test_shutil.py	Wed Jan 23 03:01:23 2013 -0800
+++ b/Lib/test/test_shutil.py	Wed Jan 23 16:02:12 2013 +0200
@@ -1352,6 +1352,20 @@
         rv = shutil.which(self.temp_file.name[:-4], path=self.dir)
         self.assertEqual(rv, self.temp_file.name[:-4] + ".EXE")
 
+    def test_environ_path(self):
+        with support.EnvironmentVarGuard() as env:
+            env['PATH'] = self.dir
+            rv = shutil.which(self.file)
+            self.assertEqual(rv, self.temp_file.name)
+
+    def test_empty_path(self):
+        base_dir = os.path.dirname(self.dir)
+        with support.temp_cwd(path=self.dir), \
+             support.EnvironmentVarGuard() as env:
+            env['PATH'] = self.dir
+            rv = shutil.which(self.file, path='')
+            self.assertIsNone(rv)
+
 
 class TestMove(unittest.TestCase):
 
diff -r 3d7000549eb1 Misc/NEWS
--- a/Misc/NEWS	Wed Jan 23 03:01:23 2013 -0800
+++ b/Misc/NEWS	Wed Jan 23 16:02:12 2013 +0200
@@ -220,6 +220,9 @@
 Library
 -------
 
+- Issue #17012: shutil.which() no longer fallbacks to the PATH environment
+  variable if empty path argument is specified.
+
 - Issue #12411: Fix to cgi.parse_multipart to correctly use bytes boundaries
   and bytes data. Patch by Jonas Wagner.