PocketMine-MP vulnerable to server crash with certain invalid JSON payloads in `LoginPacket` due to vulnerable dependency
Discription

### Impact
An attacker could crash the server by sending malformed JWT JSON in `LoginPacket` due to a security vulnerability in [`netresearch/jsonmapper`](https://github.com/cweiske/JsonMapper), due to improper checking for mapping JSON arrays and objects onto scalar model properties such as strings.

### Patches
The problem was fixed in a fork of JsonMapper in dktapps/JsonMapper@a31902a31f5b6fdb832f57c0e3a3f16a3b41c012. PocketMine-MP releases 4.20.5 and 4.21.1 have been released with the fix.

### Workarounds
– Users of PocketMine-MP source installations may manually install the patched version of JsonMapper by backporting commit pmmp/PocketMine-MP@09668a37d66c6023685a948b7550c918620e98f2.
– A plugin may also be able to workaround this issue by using `DataPacketReceiveEvent` to attempt detection of suspicious payloads. An `ErrorException` will be thrown in the crash case, which can be caught by plugins.

### References
cweiske/jsonmapper#210Read More

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