PocketMine-MP LogDoS via Malformed Login Packet

A denial-of-service vulnerability affects PocketMine-MP servers due to insufficient validation of the LoginPacket. An attacker can exploit this by sending a specially crafted packet containing a large or complex JSON structure within the clientData JWT body's unknown properties. The PocketMine-MP server, versions prior to 5.4.1, processes this malformed data, generating excessively long log messages. This excessive logging consumes significant CPU resources and memory, and can ultimately lead to a crash due to the server attempting to serialize the large, offending data structure for logging. The issue stems from the JsonMapper instance being configured to warn instead of reject unexpected properties, which, while intended to accommodate changes from Microsoft, creates an exploitable vulnerability.

Attack Chain

1. An attacker crafts a custom Minecraft client.
2. The client generates a LoginPacket to initiate a connection with the PocketMine-MP server.
3. Within the LoginPacket, the clientData field contains a JWT body.
4. The attacker injects a custom, unexpected JSON property (e.g., "invalid_key") into the JWT body of the clientData.
5. The value of the "invalid_key" is set to a large or complex object, such as a deeply nested array or an array with millions of elements.
6. The server receives the LoginPacket and processes the clientData JWT.
7. The server's JsonMapper encounters the unexpected "invalid_key" property.
8. The server's warnUndefinedJsonPropertyHandler attempts to serialize and log the large object value, leading to excessive resource consumption, and eventually an Out-of-Memory crash.

Impact

Successful exploitation of this vulnerability results in a denial-of-service condition on the PocketMine-MP server. The excessive logging can rapidly consume disk space and CPU resources, degrading server performance for legitimate users. In severe cases, the server may crash entirely due to memory exhaustion. This vulnerability affects PocketMine-MP servers exposed to public networks where unauthorized actors can send malicious login packets. The number of potential victims is any server running a vulnerable PocketMine-MP version (< 5.4.1) accessible to the public internet.

Recommendation

• Upgrade PocketMine-MP to version 5.4.1 or later to apply the patch that removes the vulnerable var_export and limits the logged property name length (reference: Patches section).
• Implement a plugin that handles the DataPacketReceiveEvent to inspect LoginPacket data and use JsonMapper to reject packets with unusual properties. This will prevent the processing of malicious packets (reference: Workarounds section).
• Monitor server logs for unusually long messages or repeated warnings about undefined JSON properties within clientData. This can be an indicator of exploitation attempts.
• Deploy the provided Sigma rule to detect connection attempts with unusually large client data in the login packet.