CVE-2023-54392 in PocketMine-MP
Summary
by MITRE • 09/10/2026
PocketMine-MP versions >= 4.20.0 before 4.22.3 (and before 5.2.1 in the 5.x branch) fail to validate NBT tag types in BlockActorDataPacket. A player can crash the server by sending a packet containing sign NBT data with an incorrect tag type, triggering an unhandled UnexpectedTagTypeException that terminates the server process.
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Analysis
by VulDB Data Team • 09/10/2026
The vulnerability identified in PocketMine-MP versions ranging from 4.20.0 up to but not including 4.22.3, as well as version 5.2.1 and earlier in the 5.x branch, represents a critical server-side crash vector rooted in insufficient input validation within the network packet handling subsystem. Specifically, the flaw resides in the processing of BlockActorDataPacket structures, which are responsible for conveying state changes regarding block entities such as signs, chests, or furnaces to connected clients and vice versa. The core technical deficiency is the failure to strictly validate the Network Binary Tag (NBT) tag types embedded within these packets before attempting to deserialize or process them. NBT is a structured data format used extensively in Minecraft-based servers to store complex hierarchical data, where each piece of information is associated with a specific type identifier indicating its nature, such as string, integer, list, or compound. When the server receives a BlockActorDataPacket containing sign-related NBT data, it expects the tag types to conform to predefined schemas that align with valid game state representations.
The operational impact of this vulnerability is severe due to its potential for remote denial-of-service attacks without authentication requirements in many default configurations. An attacker can exploit this flaw by crafting a malicious network packet that includes sign NBT data where one or more tags possess an incorrect or unexpected type identifier. Upon receiving this malformed payload, the server's parsing logic attempts to interpret the tag based on its assumed context rather than verifying its validity against expected constraints. This mismatch triggers an unhandled UnexpectedTagTypeException within the Java Virtual Machine environment running PocketMine-MP. Because this exception is not caught by a local error handler or wrapped in a try-catch block that gracefully degrades performance, it propagates up the call stack and causes the server process to terminate abruptly. Consequently, all connected players are disconnected, and the game world state may be lost if automatic backups have not been configured, leading to significant downtime for multiplayer communities relying on this software infrastructure.
From a threat modeling perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the application fails to verify that user-supplied data conforms to expected formats and types before processing. Furthermore, it maps directly to MITRE ATT&CK technique T1499 Endpoint Denial of Service under the Impact category, specifically reflecting methods such as Resource Exhaustion: Crash/Exit/Reboot or Application Crashing via malformed input packets. The exploitability is high because network protocols in Minecraft-based servers are often exposed to untrusted clients during normal gameplay operations, and many server administrators do not implement strict packet filtering at the firewall level for local area networks where trusted players connect. This allows a malicious actor with basic networking knowledge to craft simple raw TCP or UDP packets that trigger the exception remotely over the internet if port forwarding is enabled.
Mitigation strategies must prioritize immediate patching of the PocketMine-MP software to version 4.22.3 or later, and for those on the newer architecture, version 5.2.1 or higher, where this validation logic has been corrected. In environments where updating is not immediately feasible, administrators should implement network-level controls such as firewall rules that restrict incoming connections only from known IP addresses, thereby limiting exposure to untrusted sources. Additionally, deploying a reverse proxy with deep packet inspection capabilities can help filter out malformed NBT structures before they reach the application layer. Long-term resilience requires integrating automated fuzzing into development pipelines to detect similar input validation gaps in other packet handlers and enforcing strict type checking during deserialization processes across all network-facing components of the server software.