CVE-2026-126366
msgpack-java JRuby decoder allocates from declared header counts with no cap: 5-byte message forces multi-GiB OOM
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.
INFO
Published Date :
Nov. 12, 2026, 10:41 p.m.
Last Modified :
Nov. 12, 2026, 10:41 p.m.
Remotely Exploit :
Yes !
Source :
GitHub_M
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | HIGH | MITRE-CVE |
Solution
- Update the msgpack gem to version 1.8.7.
- Ensure untrusted input is validated before decoding.
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