# 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.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **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 (`validator('form')`)
- **Fix available:** Yes
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-4h7h-rv7c-7m2v

## 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.

Weakness: CWE-20 (Improper Input Validation), CWE-1471 (Insufficiently Controlled Parser Differential)

## 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 | Ecosystem | 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.

## Is CVE-2026-126356 being exploited?

There is no known exploitation of CVE-2026-126356 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126356 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-126356 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls validator('form'). Many projects that include it are not exposed.

Affected packages and fixed versions:
- hono (npm): affected from 4.5.1; fixed in 4.13.16

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.
   If an affected version is present, also search the code for calls to validator('form') and report whether any exist.
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-2026-126356/ (Markdown: https://insights.sbomify.com/CVE-2026-126356/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126356
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126356
- OSV: https://osv.dev/vulnerability/CVE-2026-126356

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126356/
Summaries are generated and may contain mistakes; the official description above is the record.