open-circle valibot up to 1.4.1 flatten prototype pollution

Summaryinfo

A vulnerability classified as critical was found in open-circle valibot up to 1.4.1. The impacted element is the function flatten. Such manipulation leads to prototype pollution. This vulnerability is documented as CVE-2026-59952. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in open-circle valibot up to 1.4.1. It has been rated as critical. Affected by this issue is the function flatten. The manipulation with an unknown input leads to a prototype pollution vulnerability. Using CWE to declare the problem leads to CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Impacted is confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-59952 since 07/07/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1059.

Upgrading to version 1.4.2 eliminates this vulnerability.

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Productinfo

Vendor

• open-circle

Name

• valibot

Version

• 1.4.0
• 1.4.1

Website

• Product: https://github.com/open-circle/valibot/

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