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

Advisory record · `GHSA-cw39-r4h6-8j3x`

- **Identifiers:** `GHSA-cw39-r4h6-8j3x` · `CVE-2026-21452`
- **State:** Confirmed
- **Severity:** CVSS 7.5 High
- **Critical:** No
- **Weakness (CWE):** [CWE-400](https://cwe.mitre.org/data/definitions/400.html) · [CWE-789](https://cwe.mitre.org/data/definitions/789.html)
- **Tags:** [Denial of service](https://agentprime.io/advisories/tag/dos.md)
- **Published:** 5 Jan 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 5 Jan 2026, 14:59 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `org.msgpack:msgpack-core` (Maven): affected `<0.9.11`; fixed in `0.9.11`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> MessagePack for Java (org.msgpack:msgpack-core) versions before 0.9.11 are vulnerable to a denial of service when deserializing a crafted .msgpack file containing an EXT32 object with an excessively large declared payload length. An attacker who can supply such a file, for example to a service that loads untrusted model files, can cause the JVM to allocate unbounded memory and crash or become unavailable. Upgrade to msgpack-core 0.9.11, which allocates memory gradually for large inputs. No workaround is given in the source.

Written 2 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.5 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0060, the 49th percentile, on 12 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:17 UTC: First seen by agentprime
- 31 Oct 2026, 14:17 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-cw39-r4h6-8j3x)

## Sources

- [GitHub Advisory Database: GHSA-cw39-r4h6-8j3x](https://github.com/advisories/GHSA-cw39-r4h6-8j3x)
- [OSV: GHSA-cw39-r4h6-8j3x](https://osv.dev/vulnerability/GHSA-cw39-r4h6-8j3x)
- <https://github.com/msgpack/msgpack-java/security/advisories/GHSA-cw39-r4h6-8j3x>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-21452>
- <https://github.com/msgpack/msgpack-java/commit/daa2ea6b2f11f500e22c70a22f689f7a9debdeae>
- <https://github.com/msgpack/msgpack-java>
- <https://github.com/msgpack/msgpack-java/releases/tag/v0.9.11>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-cw39-r4h6-8j3x>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-cw39-r4h6-8j3x/>. Everything for agents: <https://agentprime.io/llms.txt>.