chore(proxy): surface form-parse failures instead of caching empty body

Starlette's ``request.form()`` raises ``MultiPartException`` /
``ValueError`` / ``AssertionError`` on malformed multipart input
(missing boundary, malformed chunk encoding, etc.). The outer
``except Exception: return {}`` swallowed every form-parse failure
and cached an empty parsed body — auth-time pre-reads saw ``{}`` and
skipped every banned-param check while a later raw-body re-read in
the handler still saw the original payload. Same TOCTOU shape as the
substring-match bypass: the auth gate and the handler don't agree on
what the body is.

Wrap ``request.form()`` in a narrow ``try`` that converts any parse
failure to a 400 ``ProxyException``. The outer broad ``except`` is
retained for unrelated unexpected errors but no longer covers
form-parse-side bypass shapes.

Adds a regression test parametrised over the exception classes
Starlette can raise from ``request.form()``.

litellm/proxy/common_utils/http_parsing_utils.py

@@ -66,7 +66,23 @@ async def _read_request_body(request: Optional[Request]) -> Dict:
         content_type = _request_headers.get("content-type", "")
 
         if _is_form_content_type(content_type):
-            parsed_body = dict(await request.form())
+            try:
+                form_data = await request.form()
+            except Exception as e:
+                # ``request.form()`` raises on malformed multipart (missing
+                # boundary, malformed chunk encoding, …). Surface as 400 so
+                # the auth-time pre-read does not silently cache ``{}`` while
+                # a later raw-body re-read sees the original payload —
+                # banned-param checks must see the same body the handler
+                # acts on.
+                verbose_proxy_logger.error(f"Invalid form payload: {e}")
+                raise ProxyException(
+                    message=f"Invalid form payload: {e}",
+                    type="invalid_request_error",
+                    param="request_body",
+                    code=status.HTTP_400_BAD_REQUEST,
+                )
+            parsed_body = dict(form_data)
             if "metadata" in parsed_body and isinstance(parsed_body["metadata"], str):
                 parsed_body["metadata"] = json.loads(parsed_body["metadata"])
         else:

tests/test_litellm/proxy/common_utils/test_http_parsing_utils.py

@@ -930,6 +930,34 @@ class TestReadRequestBodyNonCanonicalContentType:
         mock_request.form.assert_awaited_once()
 
 
+class TestReadRequestBodyFormParseFailure:
+    """
+    A failed ``request.form()`` parse (e.g. multipart with missing boundary)
+    must surface as a 400, not silently return ``{}`` — otherwise the
+    auth-time pre-read sees an empty body while a later raw-body re-read
+    sees the original payload, defeating every banned-param check.
+    """
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "raised_exception",
+        [
+            ValueError("Missing boundary in multipart."),
+            AssertionError("malformed chunk"),
+            RuntimeError("form parser exploded"),
+        ],
+    )
+    async def test_form_parse_failure_raises_400(self, raised_exception):
+        mock_request = MagicMock()
+        mock_request.form = AsyncMock(side_effect=raised_exception)
+        mock_request.headers = {"content-type": "multipart/form-data"}
+        mock_request.scope = {}
+
+        with pytest.raises(ProxyException) as exc_info:
+            await _read_request_body(mock_request)
+        assert str(exc_info.value.code) == "400"
+
+
 class TestIsJsonContentType:
     @pytest.mark.parametrize(
         "content_type",