# CVE-2026-126366: msgpack-java JRuby decoder allocates from declared header counts with no cap: 5-byte message forces multi-GiB OOM

> Attackers sending short MessagePack headers can force huge allocations and exhaust the JVM heap in apps decoding untrusted input with msgpack-java.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Harmful if only installed:** No
- **Needs a specific call:** Yes (`MessagePack.unpack`, `MessagePack::Unpacker`)
- **Fix available:** Yes
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-73v3-wf97-vpgj

## What is CVE-2026-126366?

CVE-2026-126366 is a high-severity denial of service vulnerability in msgpack-ruby, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

MessagePack for Ruby is an implementation of the MessagePack binary serialization format. Prior to 1.8.7, the Java-platform msgpack gem for JRuby allocated arrays and byte buffers directly from attacker-controlled array32, str32, and bin32 declared sizes before confirming that corresponding elements or bytes were available. A short MessagePack header could therefore trigger a very large allocation or sustained heap growth when an application decoded untrusted input through MessagePack.unpack or MessagePack::Unpacker, potentially exhausting the shared JVM heap. map32 and declared counts at or above 2^31 did not use the affected allocation behavior. This issue is fixed in version 1.8.7.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-1325 (Improperly Controlled Sequential Memory Allocation)

## Which versions are affected by CVE-2026-126366?

1 open source package ship an affected version, including msgpack.

An affected version is only exploitable if your software calls MessagePack.unpack or MessagePack::Unpacker. Many projects that include it are not exposed.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `msgpack` | RubyGems | All versions | 1.8.7 |

## How do I fix CVE-2026-126366?

Upgrade msgpack to 1.8.7 or later.

## Is CVE-2026-126366 being exploited?

There is no known exploitation of CVE-2026-126366 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126366 and fix it if it is affected.

What it is: CVE-2026-126366 is a high-severity denial of service vulnerability in msgpack-ruby, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-126366 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls MessagePack.unpack or MessagePack::Unpacker. Many projects that include it are not exposed.

Affected packages and fixed versions:
- msgpack (RubyGems): affected all versions; fixed in 1.8.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
   If an affected version is present, also search the code for calls to MessagePack.unpack or MessagePack::Unpacker and report whether any exist.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-126366/ (Markdown: https://insights.sbomify.com/CVE-2026-126366/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126366
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126366
- OSV: https://osv.dev/vulnerability/CVE-2026-126366

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126366/
Summaries are generated and may contain mistakes; the official description above is the record.