GMS-2023-1796: PocketMine-MP vulnerable to server crash using badly formatted sign NBT in BlockActorDataPacket

Summary

A player sending a packet can cause the server to crash by providing incorrect sign data in NBT in BlockActorDataPacket.

Details

This vulnerability was discovered using the BlockActorDataPacket, but other packets may also be affected. The player would seem to just need to send an NBT with an incorrect type to throw this error.

[Server thread/CRITICAL]: pocketminebt\UnexpectedTagTypeException: "Expected a tag of type pocketminebt\tag\CompoundTag, got pocketminebt\tag\ByteTag" (EXCEPTION) in "pmsrc/vendor/pocketmine/nbt/src/tag/CompoundTag" at line 107
--- Stack trace ---
 #0 pmsrc/src/network/mcpe/handler/InGamePacketHandler(751): pocketminebt\tag\CompoundTag->getCompoundTag(string[9] FrontText)
 #1 pmsrc/vendor/pocketmine/bedrock-protocol/src/BlockActorDataPacket(50): pocketmineetwork\mcpe\handler\InGamePacketHandler->handleBlockActorData(object pocketmineetwork\mcpe\protocol\BlockActorDataPacket#220241)
 #2 pmsrc/src/network/mcpe/NetworkSession(433): pocketmineetwork\mcpe\protocol\BlockActorDataPacket->handle(object pocketmineetwork\mcpe\handler\InGamePacketHandler#190572)

PoC

Use a bot or proxy to send a packet when editing a sign. This packet should contain an NBT with incorrect types but correct architecture.

Impact

This makes it possible to shutdown a server for someone who knows how to operate it. As this was discovered in 4.22.1, everyone with at least this version is affected.

Patches

This bug was fixed by 0c250a2ef09627b48aa52302f6cc7e1f2afb70ea in the 4.22.3 and 5.2.1 releases.

Workarounds

A plugin may be able to handle DataPacketReceiveEvent for BlockActorDataPacket, and verify that the FrontText tag is a TAG_Compound.