# CVE-2026-59952: Valibot: record() issue paths can make flatten() throw for inherited Object property names

> Attackers can crash applications by triggering a TypeError in the flatten helper when validating objects with specific inherited property names.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 30 Jul 2026
- **Updated:** 30 Jul 2026
- **Also known as:** CLSA-2026-1792473247, GHSA-5qjj-4xww-7phc, MINI-cw78-33gv-44cf

## What is CVE-2026-59952?

CVE-2026-59952 is a medium-severity denial of service vulnerability in open-circle valibot, published on 30 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-755 (Improper Handling of Exceptional Conditions)

## Which versions are affected by CVE-2026-59952?

2 open source packages ship an affected version, including valibot.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `valibot` | npm | All versions | 1.4.2 |
| `valibot` | npm | All versions | 1.4.2 |

## How do I fix CVE-2026-59952?

Upgrade valibot to 1.4.2 or later.

## Is CVE-2026-59952 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59952 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59952 and fix it if it is affected.

What it is: CVE-2026-59952 is a medium-severity denial of service vulnerability in open-circle valibot, published on 30 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59952 exists.

Affected packages and fixed versions:
- valibot (npm): affected all versions; fixed in 1.4.2
- valibot (npm): affected all versions; fixed in 1.4.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-59952/ (Markdown: https://insights.sbomify.com/CVE-2026-59952/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59952
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59952
- OSV: https://osv.dev/vulnerability/CVE-2026-59952

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59952/
Summaries are generated and may contain mistakes; the official description above is the record.