CVE-2021-21368

What it is

msgpack5 is a msgpack v5 implementation for node.js and the browser. In msgpack5 before versions 3.6.1, 4.5.1, and 5.2.1 there is a "Prototype Poisoning" vulnerability. When msgpack5 decodes a map containing a key "__proto__", it assigns the decoded value to __proto__. Object.prototype.__proto__ is an accessor property for the receiver's prototype. If the value corresponding to the key __proto__ decodes to an object or null, msgpack5 sets the decoded object's prototype to that value. An attacker who can submit crafted MessagePack data to a service can use this to produce values that appear to be of other types; may have unexpected prototype properties and methods (for example length, numeric properties, and push et al if __proto__'s value decodes to an Array); and/or may throw unexpected exceptions when used (for example if the __proto__ value decodes to a Map or Date). Other unexpected behavior might be produced for other types. There is no effect on the global prototype. This "prototype poisoning" is sort of a very limited inversion of a prototype pollution attack. Only the decoded value's prototype is affected, and it can only be set to msgpack5 values (though if the victim makes use of custom codecs, anything could be a msgpack5 value). We have not found a way to escalate this to true prototype pollution (absent other bugs in the consumer's code). This has been fixed in msgpack5 version 3.6.1, 4.5.1, and 5.2.1. See the referenced GitHub Security Advisory for an example and more details.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account, but needs a user to act. High risk of data tampering, service disruption.

Exploit odds

1.6% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-03-12 17:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:35 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.6% in the next 30 days (EPSS).
• Severity: 6.7 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-03-12 17:15 UTC

Last modified

2026-06-17 03:35 UTC

Exploit probability

1.6% · EPSS, next 30 days

Weakness

CWE-915

Record status

Modified

Affected

• msgpack5 Project msgpack5 < 3.6.1
• msgpack5 Project msgpack5 < 4.5.1
• msgpack5 Project msgpack5 < 5.2.1

Attack profile CVSS 3.1 · 6.7

Attack vector

Network

Complexity

High

Privileges needed

Low

User interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

High

Availability

High

Is it in your code?

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Read https://cve-list.com/agent/cve/CVE-2021-21368.md and follow it. Check whether this project is affected by CVE-2021-21368. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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