# CVE-2026-126356: Hono: `validator('form')` skips validation for `multipart/form-data` requests with extra parameters

Status: published. Published: 2026-11-12. Updated: 2026-11-12.

Hono is a Web application framework that provides support for any JavaScript runtime. From 4.5.1 until 4.13.16, validator('form') uses a strict whole-header pattern that fails to recognize multipart/form-data when its Content-Type includes another legal parameter or extra whitespace after a semicolon. The validation callback receives an empty object even though c.req.parseBody() or c.req.formData() can still parse and expose all submitted fields to the handler. An unauthenticated client can therefore bypass allow-list or required-field checks when the application relies on the form validator and its callback accepts an empty object. application/x-www-form-urlencoded requests are not affected. This issue is fixed in version 4.13.16.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- honojs / hono: >= 4.5.1, < 4.13.16 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 3.1: 5.3 (medium); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-20, CWE-1471

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/honojs/hono/security/advisories/GHSA-4h7h-rv7c-7m2v (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/honojs/hono/commit/d88fab372415277a9e540e895650943dd08eef4a (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/honojs/hono/releases/tag/v4.13.16 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-126356; retrieved 2026-11-12; source updated 2026-11-12

Revision: 66d836cf3a2371429bde4b5d507ff9a18317983561f08986725cf22ca14454da

Canonical: https://y2.dev/cve/CVE-2026-126356

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