CVE-2026-21452: MessagePack for Java is a serializer implementation for Java. A denial-of-service vulnerability exists in versions prior to 0.9.11 when deserializing ...

Description

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, user interaction, or elevated privileges and can be exploited remotely in real-world environments such as model registries, inference services, CI/CD pipelines, and cloud-based model hosting platforms that accept or fetch .msgpack artifacts. Because the malicious file is extremely small yet valid, it can bypass basic validation and scanning mechanisms, resulting in complete service unavailability and potential cascading failures in production systems. Version 0.9.11 fixes the vulnerability.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

0.55%

41.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Msgpack	Messagepack	0.9.10

References

https://github.com/msgpack/msgpack-java/commit/daa2ea6b2f11f500e22c70a22f689f7a9debdeaePatch
https://github.com/msgpack/msgpack-java/releases/tag/v0.9.11Release Notes
https://github.com/msgpack/msgpack-java/security/advisories/GHSA-cw39-r4h6-8j3xExploit, Third Party Advisory

Timeline

Published: Jan 2, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-21452?

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, user interaction, or elevated privileges and can be exploited remotely in real-world environments such as model registries, inference services, CI/CD pipelines, and cloud-based model hosting platforms that accept or fetch .msgpack artifacts. Because the malicious file is extremely small yet valid, it can bypass basic validation and scanning mechanisms, resulting in complete service unavailability and potential cascading failures in production systems. Version 0.9.11 fixes the vulnerability.

How severe is CVE-2026-21452?

CVE-2026-21452 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 0.55% probability of exploitation in the next 30 days.

How do I fix CVE-2026-21452?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.