msgpack5 CVE-2026-107301

Exploitation requires that the application constructs the msgpack5 instance with an empty ({}) or partial options object that omits protoAction - this specific usage pattern silently disables the default protoAction:'error' protection - AND that the application then decodes attacker-controlled msgpack input containing a map with a __proto__ key. Key LIMITING factors: the impact is confined to the decoded object's own prototype and does NOT pollute Object.prototype globally; applications that explicitly set protoAction:'error' (or upgrade to 6.1.0+) are not affected; and the attacker needs a reachable code path that feeds untrusted bytes into the decoder. No authentication is required per PR:N, and no user interaction per UI:N.

This is a genuine but moderate-severity prototype pollution flaw (CWE-1321), not a top-tier priority. The provided CVSS 3.1 vector (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L, base 6.5) indicates a network-reachable, unauthenticated issue of high attack complexity - the complexity rating aligns with the key constraint that exploitation only matters when the application constructs msgpack5 with an empty or partial options object AND then decodes attacker-controlled data. Critically, both the description and the GHSA advisory confirm the pollution is scoped to the decoded object's prototype and does NOT modify Object.prototype globally, which materially lowers real-world blast radius; for that reason the vector's I:H rating appears generous relative to the described impact (see assessed vector below). No EPSS score, SSVC decision data, KEV listing, or public POC was supplied in the intelligence - so exploitation status is 'no public exploit identified at time of analysis.' There is no signal of active exploitation. Net: a legitimate supply-chain/dependency hygiene item to patch, but absent active-exploitation or automatability signals it does not warrant emergency prioritization over actively-exploited vulnerabilities.

An application uses msgpack5 (constructed with no options or a partial options object, which silently disables the default protoAction:'error' guard) to decode msgpack data received from an untrusted source. An attacker sends a crafted msgpack map containing a __proto__ key, which replaces the decoded object's prototype and alters its inherited properties, potentially corrupting downstream logic that reads those properties. No public exploit code was identified in the provided data.