CVE-2026-59952 in valibot
Summary
by MITRE • 07/30/2026
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.
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Analysis
by VulDB Data Team • 07/30/2026
The vulnerability in Valibot versions prior to 1.4.2 represents a critical error handling flaw that can lead to application crashes through improper manipulation of object property access during validation processes. This issue specifically targets the flatten() helper function within the validation library, which is designed to transform nested validation errors into a flattened structure for easier consumption by applications. The vulnerability arises from the library's handling of attacker-controlled object keys that collide with inherited properties from Object.prototype, creating a scenario where legitimate validation error paths inadvertently reference built-in methods instead of error arrays.
The technical flaw manifests when record() validation processes user-controlled data containing object keys that match inherited properties such as toString, valueOf, or hasOwnProperty. While the record() function does filter out dangerous keys like _proto_, prototype, and constructor, it fails to prevent other legitimate own properties that happen to collide with Object.prototype methods. When validation rejects entries with these conflicting keys, the resulting error paths contain references to the actual inherited methods rather than error containers. The flatten() helper then attempts to call .push() on these method references, causing a TypeError when the JavaScript engine tries to execute the push operation on a function object instead of an array.
This vulnerability operates within the context of application availability and error handling rather than representing a traditional prototype pollution attack that affects global object properties. The impact is particularly severe for web applications that process user-controlled data through Valibot's validation system, especially in API endpoints where validation errors are flattened for response formatting. When an attacker crafts input containing conflicting object keys, the validation flow can be disrupted to the point of complete request failure, preventing proper error responses from being returned to clients and effectively causing a denial of service condition.
The flaw demonstrates a classic example of improper input handling combined with inadequate protection against prototype pollution vectors in validation libraries. According to CWE classification, this vulnerability relates to CWE-471: "Use of Non-Standard Functionality in a Context that Does Not Expect It" and CWE-706: "Use of Incorrectly-Resolved Name or Reference." The issue also connects to ATT&CK techniques involving application misconfiguration and denial of service through error manipulation. Security practitioners should note that this vulnerability specifically affects applications using record() validation with user-provided data and subsequent flattening of validation errors, making it particularly relevant for API security and input validation frameworks.
The fix implemented in Valibot version 1.4.2 addresses the core issue by ensuring that the flatten() helper properly distinguishes between legitimate error arrays and inherited object methods during path resolution. This prevents the accidental invocation of .push() on function objects while maintaining the expected behavior for valid error handling scenarios. Organizations should prioritize updating to version 1.4.2 or later, particularly those running applications that process untrusted input through Valibot's validation system, as this vulnerability can be exploited to cause complete request failures and disrupt service availability. The resolution demonstrates the importance of careful consideration when designing validation helper functions that must operate across potentially conflicting object property namespaces.