CVE-2026-21452 MessagePack for Java Vulnerable to Remote DoS via Malicious EXT Payload Allocation
Remote attackers can cause denial of service in Java applications by submitting malicious MessagePack files that trigger unbounded memory allocation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-21452?
CVE-2026-21452 is a high-severity denial of service vulnerability in msgpack-java, published on 2 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-21452?
2 open source packages ship an affected version, including org.msgpack:msgpack-core. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
org.msgpack:msgpack-core Maven | All versions | 0.9.11 |
org.msgpack:msgpack-core Maven | All versions | 0.9.11 |
Products include Msgpack Messagepack.
How do I fix CVE-2026-21452?
Upgrade org.msgpack:msgpack-core to 0.9.11 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-21452 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21452 exists.
Other vulnerabilities in msgpack-java
Sources
Also known as CLEANSTART-2026-ED59299, CLEANSTART-2026-MQ41699, DEBIAN-CVE-2026-21452, GHSA-cw39-r4h6-8j3x, UBUNTU-CVE-2026-21452.