CVE Vulnerabilities


Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Published: Mar 12, 2021 | Modified: Oct 24, 2022
CVSS 3.x
CVSS 2.x

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 receivers prototype. If the value corresponding to the key proto decodes to an object or null, msgpack5 sets the decoded objects 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 values 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 consumers 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.


The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.

Affected Software

Name Vendor Start Version End Version
Msgpack5 Msgpack5_project 5.0.0 *
Msgpack5 Msgpack5_project 4.0.0 *
Msgpack5 Msgpack5_project * *

Extended Description

By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the product depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf). This weakness is usually exploited by using a special attribute of objects called proto, constructor or prototype. Such attributes give access to the object prototype. This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.

Potential Mitigations