MessagePack for Java Vulnerable to Remote DoS via Malicious EXT Payload Allocation

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-21452: 0.60% estimated exploitation activity in the next 30 days; percentile 48.88 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Affected Components: 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 Download dependency: curl -LO https://repo1.maven.org/maven2/org/msgpack/msgpack-core/0.9.8/msgpack-core-0.9.8.jar Java Reproducer Compile Run (with limited heap) Observed Result: Run: A…

Published by the source

5 Jan 2026, 14:59 UTC

Last modified by the source

5 Jan 2026, 14:59 UTC

Source last checked

26 Oct 2026, 16:43 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-cw39-r4h6-8j3x

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# Patch Next advisory brief: CVE-2026-21452

Advisory: MessagePack for Java Vulnerable to Remote DoS via Malicious EXT Payload Allocation
IDs: CVE-2026-21452, GHSA-cw39-r4h6-8j3x
Record: https://patchnext.com/advisories/cve-2026-21452 (revision 1, source checked 26 Oct 2026, 16:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.msgpack:msgpack-core 0.9.11. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.msgpack:msgpack-core: affected < 0.9.11; fixed in 0.9.11

## Suggested inspection (read-only)
- Maven org.msgpack:msgpack-core: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.msgpack:msgpack-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-cw39-r4h6-8j3x (GitHub-reviewed advisory GHSA-cw39-r4h6-8j3x lists affected ranges: Maven org.msgpack:msgpack-core >=0 <0.9.11)

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