Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M

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

Medium 6.5

In plain English

A medium-severity prototype pollution in msgpack5 from mcollina. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 31% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.4%top 69.4% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	msgpack5	6.1.0	GHSA-8hq7-ggx2-cc6m

Affected products (as reported)

Vendor	Product	Versions
mcollina	msgpack5	< 6.1.0

Weakness: CWE-1321 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L

References

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