Valibot helps validate data using a schema. Versions prior to 1.4.2 can throw a TypeError inside its flatten() helper when validation issues contain attacker-controlled object keys such as toString, valueOf, or hasOwnProperty. The issue is reachable through normal record() validation. record() intentionally filters __proto__, prototype, and constructor, but it still accepts other own keys that collide with inherited Object.prototype properties. If the record key schema or value schema rejects such an entry, Valibot creates an issue path containing that key. Passing the resulting issues to Valibot's documented flatten() helper causes flatErrors.nested[dotPath] to resolve to the inherited method instead of an own error array, and the helper calls .push(...) on that function. This is not a global prototype pollution issue. The impact is availability/error handling: applications that validate user-controlled objects with record() and flatten validation errors for API responses can crash the request path with a TypeError instead of returning structured validation errors. This issue has been fixed in version 1.4.2.

Affected: npm: valibot

## Summary

`valibot` 1.4.1 can throw a `TypeError` inside its `flatten()` helper when validation issues contain attacker-controlled object keys such as `toString`, `valueOf`, or `hasOwnProperty`.

The issue is reachable through normal `record()` validation. `record()` intentionally filters `__proto__`, `prototype`, and `constructor`, but it still accepts other own keys that collide with inherited `Object.prototype` properties. If the record key schema or value schema rejects such an entry, Valibot creates an issue path containing that key. Passing the resulting issues to Valibot's documented `flatten()` helper causes `flatErrors.nested[dotPath]` to resolve to the inherited method instead of an own error array, and the helper calls `.push(...)` on that function.

This is not a global prototype pollution issue. The impact is availability/error handling: applications that validate user-controlled objects with `record()` and flatten validation errors for API responses can crash the request path with a `TypeError` instead of returning structured validation errors.

## Affected package

- Ecosystem: npm - Package: `valibot` - Affected version verified: `1.4.1` - Fixed version: none known - Repository: `open-circle/valibot` - Current main ref tested by source review: `9bb6617`

## Root cause

`record()` uses `_isValidObjectKey()` before validating record entries. The helper blocks the three classic prototype pollution keys:

```ts key !== '__proto__' && key !== 'prototype' && key…