CVE-2026-107301: msgpack5: Partial options disable prototype protection

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-08 · updated 2026-10-14
Weakness: CWE-1321

Description

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.

Affected products

mcollina msgpack5: before 6.1.0

References

https://github.com/mcollina/msgpack5/commit/20e82600ac9462e679c8a45e5723315f21e2c774
https://github.com/mcollina/msgpack5/releases/tag/v6.1.0
https://github.com/mcollina/msgpack5/security/advisories/GHSA-8hq7-ggx2-cc6m

Get alerted when CVE-2026-107301 or the next vulnerability affects your stack