CVE-2026-126366 msgpack-java JRuby decoder allocates from declared header counts with no cap: 5-byte message forces multi-GiB OOM
Attackers sending short MessagePack headers can force huge allocations and exhaust the JVM heap in apps decoding untrusted input with msgpack-java.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Harmful if only installed
- No
- Needs a specific call
- Yes
- Fix
- Available
What is CVE-2026-126366?
CVE-2026-126366 is a high-severity denial of service vulnerability in msgpack-ruby, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-126366?
1 open source package ship an affected version, including msgpack.
An affected version is only exploitable if your software calls MessagePack.unpack or MessagePack::Unpacker. Many projects that include it are not exposed.
| Package | Affected | Fixed in |
|---|---|---|
msgpack RubyGems | All versions | 1.8.7 |
How do I fix CVE-2026-126366?
Upgrade msgpack to 1.8.7 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-126366 being exploited?
There is no known exploitation of CVE-2026-126366 in the wild and no public exploit code.
Sources
Also known as GHSA-73v3-wf97-vpgj.