CVE-2026-126366

HIGH 7.5

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

CVSS Score

7.5

EPSS Score

0.0%

EPSS Percentile

0th

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.

CWE	CWE-770 CWE-1325
Vendor	msgpack
Product	msgpack-ruby
Published	Nov 12, 2026

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CVSS v3 Breakdown

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Affected Versions

msgpack / msgpack-ruby

< 1.8.7

References

NVD ↗ CVE.org ↗ EPSS Data ↗

github.com: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj github.com: https://github.com/msgpack/msgpack-ruby/commit/ca0efc1256c687b428c46dd68ce000ff515d53f2 github.com: https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.6 github.com: https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.7