Impact An attacker could crash PocketMine-MP by sending malformed JSON in "LoginPacket". This happened due to the particular handling of NULL types in the json mapper which accepts NULL type values in typed arrays which PocketMine-MP did not expect. Code processing arrays in the JSON data could then crash due to unexpected "NULL" elements. Patches This problem was fixed in 5.3.1 and 4.23.1 by updating JsonMapper to include the following commit: pmmp/netresearch-jsonmapper@4f90e8dab1c9df331fad7d3d89823404e882668c An upstream patch for this issue was proposed via https://github.com/cweiske/jsonmapper/pull/211; however, as of 2024-05-15, the patch has not been accepted upstream due to debate about how to deal with the behavior. For now, a fork of JsonMapper is used by PocketMine-MP to workaround the issue. Workarounds A plugin may handle "DataPacketReceiveEvent" for "LoginPacket" and check that none of the input arrays contain "NULL" where it's not expected, but this is rather cumbersome. References Proposed upstream patch for a behavior change: https://github.com/cweiske/jsonmapper/pull/211