Is there a public exploit for CVE-2026-126366?

No public exploit for CVE-2026-126366 was found in Exploit-DB, Metasploit, Nuclei templates or public proof-of-concept repositories. That is an absence of evidence rather than proof that none exists.

How severe is CVE-2026-126366?

CVE-2026-126366 scores CVSS 7.5 (v3.1), rated high. EPSS puts the chance of exploitation in the next 30 days at 0%.

What does CVE-2026-126366 affect?

CVE-2026-126366 affects msgpack msgpack.

7.5 High

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

CVE-2026-126366

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 SHRTMAX 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 byte[size]. Below the failure threshold there is a silent band: one streaming unpacker fed only an array32 header with count 2×10⁷ (5 wire bytes, zero elements) grows used heap +78 MiB immediately, no exception (each swallows the underflow), and the footprint…

Assessment

CVSS: 7.5 (v3.1)
Severity: High
EPSS: 0% chance of exploitation in the next 30 days
Known exploited: Not listed in CISA KEV
Exploit availability: Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
Published: 12 November 2026
Last updated: 12 November 2026
Affected: msgpack · msgpack-ruby · msgpack
Weakness: CWE-1325, CWE-770
Assigner: GitHub_M

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

At the source

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