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High, CVSS 7.5amended

msgpack-ruby JRuby decoder makes uncapped header-count allocations; a 5-byte message forces multi-GiB OOM

CVE-2026-126366GHSA-73v3-wf97-vpgj

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (msgpack 1.8.7), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems msgpack

    Affected
    < 1.8.7
    Fixed in
    1.8.7

Sources

Severity and exploitation
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-2026-126366: 0.53% estimated exploitation activity in the next 30 days; percentile 45.21 (FIRST EPSS, 2026-11-14). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The JRuby implementation of msgpack (`msgpack` gem, `java` platform — pure Java decoder under `ext/java/`) sizes containers directly from the element count written in the message header, with no cap at all: no equivalent of the C unpacker's `SHRT_MAX` clamp (since 1.7.3) and none of the buffered-bytes cap that commit c8d42d8 added for CRuby in 1.8.6. A single 5-byte message decides how much memory a JVM process tries to allocate: * `array32` header declaring `0x7FFFFFFF` elements → `new IRubyObject[0x7FFFFFFF]` (~8 GiB of reference slots) attempted immediately → `java.lang.OutOfMemoryError: Requested array size exceeds VM limit`. * Counts *below* the VM's max-array-size but above available heap (e.g. 5×10⁸ with a 512 MiB heap) produce a real allocation attempt — `OutOfMemoryError: Java heap space` — the JVM genuinely tries to take the memory before failing. * `str32`/`bin32` headers declaring `0x7FFFFFFF` bytes do the same via `readBytes(size)` → `new…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-73v3-wf97-vpgj

  2. Revision 1 · created ·

    Published from ghsa record GHSA-73v3-wf97-vpgj

Agent brief

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# Patch Next advisory brief: CVE-2026-126366

Advisory: msgpack-ruby JRuby decoder makes uncapped header-count allocations; a 5-byte message forces multi-GiB OOM
IDs: CVE-2026-126366, GHSA-73v3-wf97-vpgj
Record: https://patchnext.com/advisories/cve-2026-126366 (revision 2, source checked 13 Nov 2026, 06:40 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: msgpack 1.8.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems msgpack: affected < 1.8.7; fixed in 1.8.7

## Suggested inspection (read-only)
- RubyGems msgpack: 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 msgpack.
- 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-73v3-wf97-vpgj (GitHub-reviewed advisory GHSA-73v3-wf97-vpgj lists affected ranges: RubyGems msgpack >=0 <=1.8.6)
- Verified: https://osv.dev/vulnerability/GHSA-73v3-wf97-vpgj (OSV record GHSA-73v3-wf97-vpgj lists affected ranges: RubyGems msgpack >=0 <1.8.7 (fixed))

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