GHSA-H6RJ-3M53-887H

PoC

1. Connect to the server using a custom client.

2. Send a Minecraft LoginPacket containing an unexpected JSON property (e.g., invalid_key) within the ClientData.

3. Set the value of invalid_key to a highly recursive or massive object structure (e.g., an array containing millions of elements or deeply nested arrays).

4. The server hits the warnUndefinedJsonPropertyHandler, which attempts to var_export the malicious object, leading to an Out-of-Memory crash.

A := make([]interface{}, 1)
	ptr := &A
	for i := 0; i < 500; i++ {
		next := make([]interface{}, 1000)
		(*ptr)[0] = next
		ptr = &next
	}
	data := make([]int, 2000000)
	for i := 0; i < 100; i++ {
		data[i] = i
	}
	(*ptr)[0] = data
	d.PlayFabID = A

Workarounds

Plugins can handle DataPacketReceiveEvent to capture LoginPacket, and pre-process the clientData JWT to ensure it doesn't have any unusual properties in it. This can be achieved using JsonMapper (see the original affected code below) and setting the bExceptionOnUndefinedProperty flag to true. A JsonMapper_Exception will be thrown if the JWT is problematic.

However, it's important to caveat that this approach may cause login failures if any unexpected properties appear out of the blue in future versions (which has happened in the past).

Attackers can put large and/or complex structures as a value to an unknown property in the clientData JWT body in the Minecraft LoginPacket, causing the server to generate very long log messages.
Additionally, the property name is logged without any length limitations or sanitization, which can also be abused for LogDoS.

This may be used to spam the log/console, waste CPU time serializing the offending structure, and potentially to crash the server entirely.

This happens because the JsonMapper instance used to process the JWT body is configured to warn on unexpected properties instead of rejecting them outright. While this behaviour increases flexibility for random changes introduced by Microsoft, it also creates vulnerabilities if not handled carefully.

This vulnerability affects PocketMine-MP servers exposed to a public network where unknown actors may have access.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

GHSA-H6RJ-3M53-887H has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.41.1); upgrading removes the vulnerable code path.

Kodem intelligence

The issue was addressed in https://github.com/pmmp/PocketMine-MP/commit/87d1c0cea09d972fd4c2fafb84dac2ecab7649f0 by removing the relevant var_export and limiting the length of the logged property name to 80 characters.

1. What is GHSA-H6RJ-3M53-887H? GHSA-H6RJ-3M53-887H is a high-severity uncontrolled resource consumption vulnerability in pocketmine/pocketmine-mp (composer), affecting versions < 5.41.1. It is fixed in 5.41.1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is GHSA-H6RJ-3M53-887H? GHSA-H6RJ-3M53-887H has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of pocketmine/pocketmine-mp are affected by GHSA-H6RJ-3M53-887H? pocketmine/pocketmine-mp (composer) versions < 5.41.1 is affected.
4. Is there a fix for GHSA-H6RJ-3M53-887H? Yes. GHSA-H6RJ-3M53-887H is fixed in 5.41.1. Upgrade to this version or later.
5. Is GHSA-H6RJ-3M53-887H exploitable, and should I be worried? Whether GHSA-H6RJ-3M53-887H is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-H6RJ-3M53-887H is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-H6RJ-3M53-887H? Upgrade pocketmine/pocketmine-mp to 5.41.1 or later.