🔒️ Add `strict_content_type` checking for JSON requests

fastapi/applications.py

@@ -840,6 +840,29 @@ class FastAPI(Starlette):
                 """
             ),
         ] = None,
+        strict_content_type: Annotated[
+            bool,
+            Doc(
+                """
+                Enable strict checking for request Content-Type headers.
+
+                When `True` (the default), requests with a body that do not include
+                a `Content-Type` header will **not** be parsed as JSON.
+
+                This prevents potential cross-site request forgery (CSRF) attacks
+                that exploit the browser's ability to send requests without a
+                Content-Type header, bypassing CORS preflight checks. In particular
+                applicable for apps that need to be run locally (in localhost).
+
+                When `False`, requests without a `Content-Type` header will have
+                their body parsed as JSON, which maintains compatibility with
+                certain clients that don't send `Content-Type` headers.
+
+                Read more about it in the
+                [FastAPI docs for Strict Content-Type](https://fastapi.tiangolo.com/advanced/strict-content-type/).
+                """
+            ),
+        ] = True,
         **extra: Annotated[
             Any,
             Doc(
@@ -974,6 +997,7 @@ class FastAPI(Starlette):
             include_in_schema=include_in_schema,
             responses=responses,
             generate_unique_id_function=generate_unique_id_function,
+            strict_content_type=strict_content_type,
         )
         self.exception_handlers: dict[
             Any, Callable[[Request, Any], Response | Awaitable[Response]]

fastapi/routing.py

@@ -329,6 +329,7 @@ def get_request_handler(
     response_model_exclude_none: bool = False,
     dependency_overrides_provider: Any | None = None,
     embed_body_fields: bool = False,
+    strict_content_type: bool = True,
 ) -> Callable[[Request], Coroutine[Any, Any, Response]]:
     assert dependant.call is not None, "dependant.call must be a function"
     is_coroutine = dependant.is_coroutine_callable
@@ -370,7 +371,8 @@ def get_request_handler(
                         json_body: Any = Undefined
                         content_type_value = request.headers.get("content-type")
                         if not content_type_value:
-                            json_body = await request.json()
+                            if not strict_content_type:
+                                json_body = await request.json()
                         else:
                             message = email.message.Message()
                             message["content-type"] = content_type_value
@@ -599,6 +601,7 @@ class APIRoute(routing.Route):
         openapi_extra: dict[str, Any] | None = None,
         generate_unique_id_function: Callable[["APIRoute"], str]
         | DefaultPlaceholder = Default(generate_unique_id),
+        strict_content_type: bool = True,
     ) -> None:
         self.path = path
         self.endpoint = endpoint
@@ -625,6 +628,7 @@ class APIRoute(routing.Route):
         self.callbacks = callbacks
         self.openapi_extra = openapi_extra
         self.generate_unique_id_function = generate_unique_id_function
+        self.strict_content_type = strict_content_type
         self.tags = tags or []
         self.responses = responses or {}
         self.name = get_name(endpoint) if name is None else name
@@ -713,6 +717,7 @@ class APIRoute(routing.Route):
             response_model_exclude_none=self.response_model_exclude_none,
             dependency_overrides_provider=self.dependency_overrides_provider,
             embed_body_fields=self._embed_body_fields,
+            strict_content_type=self.strict_content_type,
         )
 
     def matches(self, scope: Scope) -> tuple[Match, Scope]:
@@ -963,6 +968,29 @@ class APIRouter(routing.Router):
                 """
             ),
         ] = Default(generate_unique_id),
+        strict_content_type: Annotated[
+            bool,
+            Doc(
+                """
+                Enable strict checking for request Content-Type headers.
+
+                When `True` (the default), requests with a body that do not include
+                a `Content-Type` header will **not** be parsed as JSON.
+
+                This prevents potential cross-site request forgery (CSRF) attacks
+                that exploit the browser's ability to send requests without a
+                Content-Type header, bypassing CORS preflight checks. In particular
+                applicable for apps that need to be run locally (in localhost).
+
+                When `False`, requests without a `Content-Type` header will have
+                their body parsed as JSON, which maintains compatibility with
+                certain clients that don't send `Content-Type` headers.
+
+                Read more about it in the
+                [FastAPI docs for Strict Content-Type](https://fastapi.tiangolo.com/advanced/strict-content-type/).
+                """
+            ),
+        ] = True,
     ) -> None:
         # Determine the lifespan context to use
         if lifespan is None:
@@ -1009,6 +1037,7 @@ class APIRouter(routing.Router):
         self.route_class = route_class
         self.default_response_class = default_response_class
         self.generate_unique_id_function = generate_unique_id_function
+        self.strict_content_type = strict_content_type
 
     def route(
         self,
@@ -1059,6 +1088,7 @@ class APIRouter(routing.Router):
         openapi_extra: dict[str, Any] | None = None,
         generate_unique_id_function: Callable[[APIRoute], str]
         | DefaultPlaceholder = Default(generate_unique_id),
+        strict_content_type: bool | None = None,
     ) -> None:
         route_class = route_class_override or self.route_class
         responses = responses or {}
@@ -1105,6 +1135,9 @@ class APIRouter(routing.Router):
             callbacks=current_callbacks,
             openapi_extra=openapi_extra,
             generate_unique_id_function=current_generate_unique_id,
+            strict_content_type=strict_content_type
+            if strict_content_type is not None
+            else self.strict_content_type,
         )
         self.routes.append(route)
 
@@ -1480,6 +1513,7 @@ class APIRouter(routing.Router):
                     callbacks=current_callbacks,
                     openapi_extra=route.openapi_extra,
                     generate_unique_id_function=current_generate_unique_id,
+                    strict_content_type=route.strict_content_type,
                 )
             elif isinstance(route, routing.Route):
                 methods = list(route.methods or [])