fix: bound recursive form serialization

Reject excessively nested multipart values before recursion can exhaust the process stack. This also restores the code-quality gate broken by the unbounded walkers.

litellm/litellm_core_utils/llm_request_utils.py

@@ -2,6 +2,7 @@ from collections.abc import Mapping
 from typing import Final
 
 import litellm
+from litellm.constants import DEFAULT_MAX_RECURSE_DEPTH
 
 
 def _form_field_value(value: object) -> str:
@@ -12,13 +13,17 @@ def _form_field_value(value: object) -> str:
     return str(value)
 
 
-def _flatten_form_field(key: str, value: object) -> tuple[tuple[str, str], ...]:
+def _flatten_form_field(key: str, value: object, depth: int = 0) -> tuple[tuple[str, str], ...]:
+    if depth > DEFAULT_MAX_RECURSE_DEPTH:
+        raise ValueError("Form field value exceeds maximum nesting depth")
     if isinstance(value, Mapping):
         return tuple(
-            item for subkey, subvalue in value.items() for item in _flatten_form_field(f"{key}[{subkey}]", subvalue)
+            item
+            for subkey, subvalue in value.items()
+            for item in _flatten_form_field(f"{key}[{subkey}]", subvalue, depth + 1)
         )
     if isinstance(value, (list, tuple)):
-        return tuple(item for entry in value for item in _flatten_form_field(f"{key}[]", entry))
+        return tuple(item for entry in value for item in _flatten_form_field(f"{key}[]", entry, depth + 1))
     if value is None:
         return ()
     serialized: Final = _form_field_value(value)
@@ -31,18 +36,20 @@ def _is_form_scalar(value: object) -> bool:
     return value is not None and not isinstance(value, (Mapping, list, tuple))
 
 
-def _flatten_form_data_field(key: str, value: object) -> tuple[tuple[str, str | tuple[str, ...]], ...]:
+def _flatten_form_data_field(key: str, value: object, depth: int = 0) -> tuple[tuple[str, str | tuple[str, ...]], ...]:
+    if depth > DEFAULT_MAX_RECURSE_DEPTH:
+        raise ValueError("Form field value exceeds maximum nesting depth")
     if isinstance(value, Mapping):
         return tuple(
             item
             for subkey, subvalue in value.items()
-            for item in _flatten_form_data_field(f"{key}[{subkey}]", subvalue)
+            for item in _flatten_form_data_field(f"{key}[{subkey}]", subvalue, depth + 1)
         )
     if isinstance(value, (list, tuple)):
         if all(_is_form_scalar(entry) for entry in value):
             serialized_fields: Final = tuple(field for entry in value if (field := _form_field_value(entry)))
             return ((key, serialized_fields),) if serialized_fields else ()
-        return tuple(item for entry in value for item in _flatten_form_data_field(f"{key}[]", entry))
+        return tuple(item for entry in value for item in _flatten_form_data_field(f"{key}[]", entry, depth + 1))
     if value is None:
         return ()
     serialized: Final = _form_field_value(value)

tests/code_coverage_tests/recursive_detector.py

@@ -60,8 +60,8 @@ IGNORE_FUNCTIONS = [
     "json_string_leaves",  # max depth set (MAX_STRUCTURED_CONTENT_SCAN_DEPTH); fails closed by raising at the cap so nothing goes unscanned.
     "with_json_string_leaves",  # transitively bounded: only runs on a tree json_string_leaves already walked under the cap.
     "json_unrewritable_labels",  # max depth set (MAX_STRUCTURED_CONTENT_SCAN_DEPTH); returns the None sentinel at the cap so the caller blocks.
-    "_flatten_form_field",  # bounded by the nesting depth of the already-parsed request body (a finite JSON tree, no cycles possible).
-    "_flatten_form_data_field",  # bounded by the nesting depth of the already-parsed request body (a finite JSON tree, no cycles possible).
+    "_flatten_form_field",  # bounded by DEFAULT_MAX_RECURSE_DEPTH; raises at the cap.
+    "_flatten_form_data_field",  # bounded by DEFAULT_MAX_RECURSE_DEPTH; raises at the cap.
 ]
 
 

tests/test_litellm/litellm_core_utils/test_llm_request_utils.py

@@ -1,4 +1,5 @@
 import httpx
+import pytest
 
 from litellm.litellm_core_utils.llm_request_utils import (
     flatten_form_field_values,
@@ -97,3 +98,13 @@ def test_flatten_form_field_values_scalar_list_survives_update_into_multipart():
 
     assert names.count("loras") == 2
     assert names.count("model") == 1
+
+
+def test_form_field_serializers_reject_cyclic_values():
+    cyclic: dict[str, object] = {}
+    cyclic["self"] = cyclic
+
+    with pytest.raises(ValueError, match="maximum nesting depth"):
+        flatten_form_field_values({"metadata": cyclic})
+    with pytest.raises(ValueError, match="maximum nesting depth"):
+        serialize_multipart_form_fields({"metadata": cyclic})