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 a…

Affected: npm: msgpack5

### Impact

The issue is as follows: when `msgpack5` decodes a map containing a key `"__proto__"`, it assigns the decoded value to `__proto__`. As you are no doubt aware, `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.

An example:

```js const msgpack5 = require('msgpack5')();

const payload = {}; Object.defineProperty(payload, '__proto__', { value: new Map().set(1, 2), enumerable: true });

const encoded = msgpack5.encode(payload); console.log(encoded); // <Buffer 81 a9 5f 5f 70 72 6f 74 6f 5f 5f 81 01 02>

const decoded = msgpack5.decode(encoded);

// decoded's prototype has been overwritten console.log(Object.getPrototypeOf(decoded)); // Map(1) { 1 => 2 } console.log(decoded.get); // [Function: get]

// decoded appears to most common typechecks to be a Map consol…