Impact

PocketMine-MP caps maximum chat message length at 512 Unicode characters, or about 2048 bytes. No more than 2 chat messages may be sent per tick. However, due to legacy reasons, incoming chat message blobs are split by \n, and each part is treated as a separate message, the length of each part is individually checked. The length of the whole message is not checked.

This leads to an exploitable performance issue, in which a malicious client may send a chat packet of several megabytes containing nothing but \n newline characters. The server will parse this into a very large array and spend a long time (several milliseconds) iterating over it for no reason.

Furthermore, due to the lack of sufficient rate limit checks before parsing messages, malicious clients may bombard the server with many thousands of these malicious messages, causing lockups for a significant amount of time (seconds or minutes).

Patches

This bug was addressed in df33e17 by:

• checking the length of the incoming message as a whole before parsing it - it may not be larger than messageCounter * maxChatMessageSize (messageCounter is decremented once for every message sent)
• limiting the maximum number of times a message may be split on newlines before giving up and discarding the message (maximum 3 parts; anything after the first 2 parts is discarded)

Workarounds

Handle DataPacketReceiveEvent and check for these excessive newlines in incoming TextPacket.

For more information

If you have any questions or comments about this advisory:

• Email us at team@pmmp.io