# CVE-2026-126357: Hono: `mount()` and `app.mount()` can forward a path that bypasses middleware scoped inside the mount prefix

> Unauthenticated clients can reach protected inner routes in Hono apps using mount() by bypassing scoped middleware.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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 (`mount()`, `app.mount()`)
- **Fix available:** Yes
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-4m76-xfj5-7975

## What is CVE-2026-126357?

CVE-2026-126357 is a medium-severity authentication 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. Prior to 4.13.16, mount() and app.mount() can remove a dynamic mount prefix using the route-pattern length rather than the matched-prefix length, allowing a crafted path to be forwarded as a protected inner route while the outer application routes the original path. Since 4.12.21, rebuilding the forwarded URL from the decoded pathname can also convert a decoded backslash to a slash and collapse parent-directory segments, again causing the mounted application to receive a protected path. An unauthenticated client can use either applicable path transformation to reach a mounted route without authentication or authorization middleware scoped to a subpath inside the mount prefix. This issue is fixed in version 4.13.16.

Weakness: CWE-288 (Authentication Bypass Using an Alternate Path or Channel)

## Which versions are affected by CVE-2026-126357?

1 open source package ship an affected version, including hono.

An affected version is only exploitable if your software calls mount() or app.mount(). Many projects that include it are not exposed.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `hono` | npm | All versions | 4.13.16 |

## How do I fix CVE-2026-126357?

Upgrade hono to 4.13.16 or later.

## Is CVE-2026-126357 being exploited?

There is no known exploitation of CVE-2026-126357 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126357 and fix it if it is affected.

What it is: CVE-2026-126357 is a medium-severity authentication 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-2026-126357 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls mount() or app.mount(). Many projects that include it are not exposed.

Affected packages and fixed versions:
- hono (npm): affected all versions; 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 mount() or app.mount() 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-2026-126357/ (Markdown: https://insights.sbomify.com/CVE-2026-126357/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126357
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126357
- OSV: https://osv.dev/vulnerability/CVE-2026-126357

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126357/
Summaries are generated and may contain mistakes; the official description above is the record.