CVE-2025-57820

Description

## 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.

How to fix CVE-2025-57820

To remediate CVE-2025-57820, upgrade the affected package to a fixed version below.

• —upgrade to 5.3.2 or later

Is CVE-2025-57820 being exploited?

Low — EPSS is 0.2%, meaning exploitation activity has not been observed at scale.

Affected packages (1)

• from 0, < 5.3.2

CVSS scores

References (4)

• ADVISORYnvd.nist.gov/vuln/detail/CVE-2025-57820
• PATCHgithub.com/sveltejs/devalue
• WEBgithub.com/sveltejs/devalue/commit/0623a47c9555b639c03ff1baea82951b2d9d1132
• WEBgithub.com/sveltejs/devalue/security/advisories/GHSA-vj54-72f3-p5jv