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

Status: published. Published: 2026-11-12. Updated: 2026-11-12.

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.

## Remediation

- fix: msgpack (RubyGems): OSV reports a fixed version 1.8.7. Check the advisory for supported release branches. (source: OSV GHSA-73v3-wf97-vpgj) — https://github.com/advisories/GHSA-73v3-wf97-vpgj

## Affected products and versions

- msgpack / msgpack-ruby: < 1.8.7 (affected). Source: CNA: GitHub_M
-  / msgpack (msgpack): 0.0.1 (affected, RubyGems); 0.2.0 (affected, RubyGems); 0.2.1 (affected, RubyGems); 0.2.2 (affected, RubyGems); 0.3.0 (affected, RubyGems); 0.3.1 (affected, RubyGems); 0.3.2 (affected, RubyGems); 0.3.3 (affected, RubyGems); 0.3.4 (affected, RubyGems); 0.3.5 (affected, RubyGems); 0.3.6 (affected, RubyGems); 0.3.7 (affected, RubyGems); 0.3.8 (affected, RubyGems); 0.3.9 (affected, RubyGems); 0.4.0 (affected, RubyGems); 0.4.1 (affected, RubyGems); 0.4.2 (affected, RubyGems); 0.4.3 (affected, RubyGems); 0.4.4 (affected, RubyGems); 0.4.5 (affected, RubyGems); 0.4.6 (affected, RubyGems); 0.4.7 (affected, RubyGems); 0.5.0 (affected, RubyGems); 0.5.1 (affected, RubyGems); 0.5.10 (affected, RubyGems); 0.5.11 (affected, RubyGems); 0.5.12 (affected, RubyGems); 0.5.2 (affected, RubyGems); 0.5.3 (affected, RubyGems); 0.5.4 (affected, RubyGems); 0.5.5 (affected, RubyGems); 0.5.6 (affected, RubyGems); 0.5.7 (affected, RubyGems); 0.5.8 (affected, RubyGems); 0.5.9 (affected, RubyGems); 0.6.0 (affected, RubyGems); 0.6.0pre1 (affected, RubyGems); 0.6.1 (affected, RubyGems); 0.6.2 (affected, RubyGems); 0.7.0 (affected, RubyGems); 0.7.0dev1 (affected, RubyGems); 0.7.1 (affected, RubyGems); 0.7.2 (affected, RubyGems); 0.7.2dev1 (affected, RubyGems); 0.7.3 (affected, RubyGems); 0.7.4 (affected, RubyGems); 0.7.5 (affected, RubyGems); 0.7.6 (affected, RubyGems); 1.0.0 (affected, RubyGems); 1.0.1 (affected, RubyGems); 1.0.2 (affected, RubyGems); 1.0.3 (affected, RubyGems); 1.1.0 (affected, RubyGems); 1.2.0 (affected, RubyGems); 1.2.1 (affected, RubyGems); 1.2.10 (affected, RubyGems); 1.2.2 (affected, RubyGems); 1.2.3 (affected, RubyGems); 1.2.4 (affected, RubyGems); 1.2.5 (affected, RubyGems); 1.2.6 (affected, RubyGems); 1.2.7 (affected, RubyGems); 1.2.8 (affected, RubyGems); 1.2.9 (affected, RubyGems); 1.3.0 (affected, RubyGems); 1.3.1 (affected, RubyGems); 1.3.2 (affected, RubyGems); 1.3.3 (affected, RubyGems); 1.4.0 (affected, RubyGems); 1.4.0.pre1 (affected, RubyGems); 1.4.1 (affected, RubyGems); 1.4.2 (affected, RubyGems); 1.4.3 (affected, RubyGems); 1.4.4 (affected, RubyGems); 1.4.4.pre1 (affected, RubyGems); 1.4.5 (affected, RubyGems); 1.5.0 (affected, RubyGems); 1.5.1 (affected, RubyGems); 1.5.2 (affected, RubyGems); 1.5.3 (affected, RubyGems); 1.5.4 (affected, RubyGems); 1.5.5 (affected, RubyGems); 1.5.6 (affected, RubyGems); 1.6.0 (affected, RubyGems); 1.6.1 (affected, RubyGems); 1.7.0 (affected, RubyGems); 1.7.1 (affected, RubyGems); 1.7.2 (affected, RubyGems); 1.7.3 (affected, RubyGems); 1.7.4 (affected, RubyGems); 1.7.5 (affected, RubyGems); 1.8.0 (affected, RubyGems); 1.8.1 (affected, RubyGems); 1.8.2 (affected, RubyGems); 1.8.3 (affected, RubyGems); 1.8.4 (affected, RubyGems); 1.8.5 (affected, RubyGems); 1.8.6 (affected, RubyGems); Ecosystem version events: introduced 0; fixed 1.8.7. Source: OSV GHSA-73v3-wf97-vpgj

## Severity and prioritization

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-770, CWE-1325

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/msgpack/msgpack-ruby/commit/ca0efc1256c687b428c46dd68ce000ff515d53f2 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.6 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.7 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/advisories/GHSA-73v3-wf97-vpgj (OSV / GitHub Security Advisories; advisory)

- https://osv.dev/vulnerability/GHSA-73v3-wf97-vpgj (OSV / GitHub Security Advisories; osv)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-126366; retrieved 2026-11-12; source updated 2026-11-12

- OSV / GitHub Security Advisories: https://osv.dev/vulnerability/GHSA-73v3-wf97-vpgj; retrieved 2026-11-12; source updated 2026-11-12

Revision: dab2eb9d8df97637253ca35df075b3cdc7f1443f2c2e560856f9fb086c18dbd9

Canonical: https://y2.dev/cve/CVE-2026-126366

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