CVE-2026-126356 Hono: `validator('form')` skips validation for `multipart/form-data` requests with extra parameters
Unauthenticated clients can bypass Hono form validator allow-lists or required checks using multipart requests with extra parameters.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Harmful if only installed
- No
- Needs a specific call
- Yes
- Fix
- Available
What is CVE-2026-126356?
CVE-2026-126356 is a medium-severity security feature bypass vulnerability in honojs hono, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-126356?
1 open source package ship an affected version, including hono.
An affected version is only exploitable if your software calls validator('form'). Many projects that include it are not exposed.
| Package | Affected | Fixed in |
|---|---|---|
hono npm | From 4.5.1 | 4.13.16 |
How do I fix CVE-2026-126356?
Upgrade hono to 4.13.16 or later.
Upgrade to the fixed version, and do not rely on a single control for protection. More about Security feature bypass.
Is CVE-2026-126356 being exploited?
There is no known exploitation of CVE-2026-126356 in the wild and no public exploit code.
Sources
Also known as GHSA-4h7h-rv7c-7m2v.