Executive brief
SvelteKit is a popular framework for building web applications with Svelte. When using remote form functions with file inputs, an attacker can supply a malicious file input name (e.g., containing "__proto__") to manipulate JavaScript object prototypes, potentially removing methods and disrupting application functionality. This requires user interaction and affects applications using the experimental remote function feature.
Technical details
This vulnerability is a prototype pollution flaw (CWE-1321) in SvelteKit's remote form function handler. When processing file input deletion in forms, the code fails to safely handle user-controlled file input names that contain prototype-polluting paths like "__proto__" or "constructor.prototype". An attacker can craft a form submission with a specially-named file input to inject properties into Object.prototype, potentially removing methods from built-in objects. The attack requires network access and user interaction (form submission), and affects SvelteKit versions <= 2.69.0. The issue is patched in versions 2.69.1 and 3.0.0-next.7 by using safe property deletion mechanisms already employed elsewhere in the codebase.
Affected products
- Svelte SvelteKit <=2.69.0
Timeline
- 2026-07-24: disclosed
- 2026-07-24: patched: Patched in version 2.69.1 and 3.0.0-next.7