Python-Multipart has Arbitrary File Write via Non-Default Configuration

@@ -375,7 +375,9 @@ def __init__(self, file_name: bytes | None, field_name: bytes | None = None, con
 
         # Split the extension from the filename.
         if file_name is not None:
-            base, ext = os.path.splitext(file_name)
+            # Extract just the basename to avoid directory traversal
+            basename = os.path.basename(file_name)
+            base, ext = os.path.splitext(basename)
             self._file_base = base
             self._ext = ext
 

@@ -0,0 +1,26 @@
+from pathlib import Path
+
+from python_multipart.multipart import File
+
+
+def test_upload_dir_with_leading_slash_in_filename(tmp_path: Path):
+    upload_dir = tmp_path / "upload"
+    upload_dir.mkdir()
+
+    # When the file_name provided has a leading slash, we should only use the basename.
+    # This is to avoid directory traversal.
+    to_upload = tmp_path / "foo.txt"
+
+    file = File(
+        bytes(to_upload),
+        config={
+            "UPLOAD_DIR": bytes(upload_dir),
+            "UPLOAD_KEEP_FILENAME": True,
+            "UPLOAD_KEEP_EXTENSIONS": True,
+            "MAX_MEMORY_FILE_SIZE": 10,
+        },
+    )
+    file.write(b"123456789012")
+    assert not file.in_memory
+    assert Path(upload_dir / "foo.txt").exists()
+    assert Path(upload_dir / "foo.txt").read_bytes() == b"123456789012"