msgpack5: Partial options disable prototype protection

published 2026-10-08 32 d ago modified 2026-10-14 26 d ago also EUVD-2026-95112 , GHSA-8hq7-ggx2-cc6m

cvss: 6.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.4%; EPSS 0.4% chance of exploitation in 30 days, percentile 30%
public exploits: 0; none known

msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, constructing msgpack5 with an empty or partial options object disables the default protoAction: 'error' protection. A decoded map containing a __proto__ key can then replace the decoded object's prototype, potentially changing inherited properties or downstream behavior, although Object.prototype is not modified globally. This issue is fixed in version 6.1.0.

Patch / fix

fixed in 2 links 2

npm msgpack5 6.1.0
msgpack5_project msgpack5 6.1.0

github.com/mcollina/msgpack5@20e8260 commit
github.com/mcollina/msgpack5/releases/tag/v6.1.0 release

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 26 d ago (2026-10-14)

Exploit prediction

0.37% chance of exploitation in the next 30 days, higher than 30% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	6.5	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L
GitHub	3.1	6.5	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L

Affected packages

ecosystem	package	affected	fixed in
npm	msgpack5	<6.1.0	6.1.0

Affected products

mcollina msgpack5

Classification

Weakness

CWE-1321 Improperly Controlled Modification of Object Prototype Attributes

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary