CVE-2026-21452Public exploitNetwork attack vector

Service crash via unchecked EXT32 payload allocation

Published Jan 2, 2026 · Updated Jan 2, 2026

Denial of service in MessagePack for Java before 0.9.11 allows remote attackers to crash JVM applications via crafted EXT32 data. MessageUnpacker.readPayload(int), reached through ExtensionValue.getData(), allocates a byte array from the declared payload length before verifying that the payload exists. An application must deserialize attacker-controlled MessagePack content and materialize the extension data; a tiny valid input can exhaust heap and terminate its process.

CVSS severity7.5
High
EPSS probability0.61%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in MessagePack for Java before 0.9.11 allows remote attackers to crash JVM applications via crafted EXT32 data. MessageUnpacker.readPayload(int), reached through ExtensionValue.getData(), allocates a byte array from the declared payload length before verifying that the payload exists. An application must deserialize attacker-controlled MessagePack content and materialize the extension data; a tiny valid input can exhaust heap and terminate its process.

The record

CVE
CVE-2026-21452
Published
Jan 2, 2026
Updated
Jan 2, 2026
Vendor
MessagePack-CSharp
Product
MessagePack for Java
Classifications
CWE-400, CWE-789, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 2, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 2, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.9.11

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

MessagePack-CSharp · MessagePack for Javaversion=< 0.9.11

Published CVSS scores

7.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-789Memory Allocation with Excessive Size Value
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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