CVE-2026-126356 is a medium-severity (CVSS 5.3) vulnerability in hono. A fix is available for hono — see the affected versions and patch details below.
Hono: `validator('form')` skips validation for `multipart/form-data` requests with extra parameters
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
hononpmDescription
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 applications that use validator('form') with multipart/form-data requests and whose validation callback accepts an empty object.
Workarounds
Reject an empty object in the validator('form') callback, or validate the result of await c.req.parseBody() in the handler.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | hono | ≥ 4.5.1&&< 4.13.16 | 4.13.16npm install hono@4.13.16 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hono, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update hono to 4.13.16 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-126356 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-126356 in your dependencies?
Find it across npm, including transitive dependencies.