Affected: msgpack-java

MessagePack for Java is a serializer implementation for Java. A denial-of-service vulnerability exists in versions prior to 0.9.11 when deserializing .msgpack files containing EXT32 objects with attacker-controlled payload lengths. While MessagePack-Java parses extension headers lazily, it later trusts the declared EXT payload length when materializing the extension data. When ExtensionValue.getData() is invoked, the library attempts to allocate a byte array of the declared length without enforcing any upper bound. A malicious .msgpack file of only a few bytes can therefore trigger unbounded heap allocation, resulting in JVM heap exhaustion, process termination, or service unavailability. This vulnerability is triggered during model loading / deserialization, making it a model format vulnerability suitable for remote exploitation. The vulnerability enables a remote denial-of-service attack against applications that deserialize untrusted .msgpack model files using MessagePack for Java. A specially crafted but syntactically valid .msgpack file containing an EXT32 object with an attacker-controlled, excessively large payload length can trigger unbounded memory allocation during deserialization. When the model file is loaded, the library trusts the declared length metadata and attempts to allocate a byte array of that size, leading to rapid heap exhaustion, excessive garbage collection, or immediate JVM termination with an OutOfMemoryError. The attack requires no malformed bytes…

Affected: Maven: org.msgpack:msgpack-core

### Summary Affected Components: ``` org.msgpack.core.MessageUnpacker.readPayload() org.msgpack.core.MessageUnpacker.unpackValue() org.msgpack.value.ExtensionValue.getData() ``` A denial-of-service vulnerability exists in MessagePack for Java when deserializing .msgpack files containing EXT32 objects with attacker-controlled payload lengths. While MessagePack-Java parses extension headers lazily, it later trusts the declared EXT payload length when materializing the extension data. When ExtensionValue.getData() is invoked, the library attempts to allocate a byte array of the declared length without enforcing any upper bound. A malicious .msgpack file of only a few bytes can therefore trigger unbounded heap allocation, resulting in JVM heap exhaustion, process termination, or service unavailability. This vulnerability is triggered during model loading / deserialization, making it a model format vulnerability suitable for remote exploitation.

### PoC ``` import msgpack import struct import os

OUTPUT_DIR = "bombs" os.makedirs(OUTPUT_DIR, exist_ok=True)

# EXT format: fixext / ext8 / ext16 / ext32 # ext32 allows attacker-controlled length (uint32)

length = 1 step = 10_000_000

while True: try: # EXT32: 0xC9 | length (4 bytes) | type (1 byte) header = b'\xC9' + struct.pack(">I", length) + b'\x01' payload = b'A' # actual data tiny

data = header + payload

fname = f"{OUTPUT_DIR}/ext_length_{length}.msgpack" with open(fname,…