[1.9.x] Fixed #26325 -- Made MultiPartParser ignore filenames that normalize to an empty string.

Backport of 4b129ac81f from master

django/http/multipartparser.py

@@ -181,10 +181,11 @@ class MultiPartParser(object):
                 elif item_type == FILE:
                     # This is a file, use the handler...
                     file_name = disposition.get('filename')
+                    if file_name:
+                        file_name = force_text(file_name, encoding, errors='replace')
+                        file_name = self.IE_sanitize(unescape_entities(file_name))
                     if not file_name:
                         continue
-                    file_name = force_text(file_name, encoding, errors='replace')
-                    file_name = self.IE_sanitize(unescape_entities(file_name))
 
                     content_type, content_type_extra = meta_data.get('content-type', ('', {}))
                     content_type = content_type.strip()

docs/releases/1.8.12.txt

@@ -9,4 +9,6 @@ Django 1.8.12 fixes several bugs in 1.8.11.
 Bugfixes
 ========
 
-* ...
+* Made ``MultiPartParser`` ignore filenames that normalize to an empty string
+  to fix crash in ``MemoryFileUploadHandler`` on specially crafted user input
+  (:ticket:`26325`).

docs/releases/1.9.5.txt

@@ -9,4 +9,6 @@ Django 1.9.5 fixes several bugs in 1.9.4.
 Bugfixes
 ========
 
-* ...
+* Made ``MultiPartParser`` ignore filenames that normalize to an empty string
+  to fix crash in ``MemoryFileUploadHandler`` on specially crafted user input
+  (:ticket:`26325`).

tests/file_uploads/tests.py

@@ -179,6 +179,41 @@ class FileUploadTests(TestCase):
         response = self.client.request(**r)
         self.assertEqual(response.status_code, 200)
 
+    def test_blank_filenames(self):
+        """
+        Receiving file upload when filename is blank (before and after
+        sanitization) should be okay.
+        """
+        # The second value is normalized to an empty name by
+        # MultiPartParser.IE_sanitize()
+        filenames = ['', 'C:\\Windows\\']
+
+        payload = client.FakePayload()
+        for i, name in enumerate(filenames):
+            payload.write('\r\n'.join([
+                '--' + client.BOUNDARY,
+                'Content-Disposition: form-data; name="file%s"; filename="%s"' % (i, name),
+                'Content-Type: application/octet-stream',
+                '',
+                'You got pwnd.\r\n'
+            ]))
+        payload.write('\r\n--' + client.BOUNDARY + '--\r\n')
+
+        r = {
+            'CONTENT_LENGTH': len(payload),
+            'CONTENT_TYPE': client.MULTIPART_CONTENT,
+            'PATH_INFO': '/echo/',
+            'REQUEST_METHOD': 'POST',
+            'wsgi.input': payload,
+        }
+        response = self.client.request(**r)
+        self.assertEqual(response.status_code, 200)
+
+        # Empty filenames should be ignored
+        received = json.loads(response.content.decode('utf-8'))
+        for i, name in enumerate(filenames):
+            self.assertIsNone(received.get('file%s' % i))
+
     def test_dangerous_file_names(self):
         """Uploaded file names should be sanitized before ever reaching the view."""
         # This test simulates possible directory traversal attacks by a