Use trust_x_header and test it.

python_multipart/multipart.py

@@ -1798,7 +1798,10 @@ def create_form_parser(
     content_type = content_type.decode("latin-1")
 
     # File names are optional.
-    file_name = headers.get("X-File-Name")
+    if trust_x_headers:
+        file_name = headers.get("X-File-Name")
+    else:
+        file_name = None
 
     # Instantiate a form parser.
     form_parser = FormParser(content_type, on_field, on_file, boundary=boundary, file_name=file_name, config=config)

tests/test_multipart.py

@@ -1368,6 +1368,7 @@ def on_header_begin() -> None:
         self.assertEqual(calls, 3)
 
 
+@parametrize_class
 class TestHelperFunctions(unittest.TestCase):
     def test_create_form_parser(self) -> None:
         r = create_form_parser({"Content-Type": b"application/octet-stream"}, None, None)
@@ -1390,6 +1391,24 @@ def test_parse_form(self) -> None:
         # 15 - i.e. all data is written.
         self.assertEqual(on_file.call_args[0][0].size, 15)
 
+    @parametrize("trust_x_headers", [True, False])
+    def test_parse_form_trust_x_false(self, trust_x_headers: bool) -> None:
+        on_field = Mock()
+        on_file = Mock()
+
+        headers = {"Content-Type": b"application/octet-stream", "X-File-Name": b"foo.txt"}
+        parser = create_form_parser(headers, on_field, on_file, trust_x_headers=trust_x_headers)
+        parser.write(b"123456789012345")
+        parser.finalize()
+
+        assert on_file.call_count == 1
+
+        # The first argument (a File Object) name should come from the X header only if allowed.
+        if trust_x_headers:
+            self.assertEqual(on_file.call_args[0][0].file_name, b"foo.txt")
+        else:
+            self.assertEqual(on_file.call_args[0][0].file_name, None)
+
     def test_parse_form_content_length(self) -> None:
         files: list[FileProtocol] = []