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

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

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.

How do I fix CVE-2026-126357?

Upgrade hono to 4.13.16 or later.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-126357 being exploited?

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

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-4m76-xfj5-7975.