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.

Affected: npm: hono

### Summary

`validator('form')` only recognizes a `multipart/form-data` `Content-Type` that carries nothing but a single `boundary` parameter. A request with another legal parameter, such as `charset`, or with extra whitespace after the `;` is not recognized as a form. The validation callback then receives an empty object, while `c.req.parseBody()` still parses the body as a form.

### Details

The check that decides whether a request body is a form was written as a strict pattern over the whole `Content-Type` value. Any parameter list that does not look exactly like `; boundary=...` fails the check. When it fails, the middleware does not read the body and passes an empty object to the validation callback. If the callback accepts an empty object, the request proceeds to the handler, which can still read all submitted fields through `c.req.parseBody()` or `c.req.formData()`.

`application/x-www-form-urlencoded` is not affected. Only the `multipart/form-data` branch has the strict pattern.

This issue arises when an application relies on `validator('form')` to reject or restrict fields and the validation callback does not fail on an empty object.

### Impact

An unauthenticated client can send form fields that the `form` validator never sees, by adding a legal parameter to the `Content-Type` header.

This may lead to:

- Allow-list or required-field checks in the validator being skipped - Fields the application intended to reject reaching the handler

This issue affects appli…