CVE-2026-126357: Hono mount() path traversal bypasses middleware scoping

CVE-2026-126357 · Severity: medium · CVSS 5.3 · Published 2026-11-13

Hono is a lightweight web framework used to build APIs and web applications. A flaw in how the framework handles nested application mounting allows unauthenticated attackers to bypass authentication or authorization checks by crafting requests with specially encoded path segments, potentially reaching protected administrative routes.

Technologies: Hono, hono (npm). Vendors: Hono, npm.

Quick answers

Is CVE-2026-126357 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-126357 being exploited in the wild.
How severe is CVE-2026-126357?
Medium severity, with a CVSS score of 5.3, published 13 November 2026.
Which products and versions are affected?
hono (npm) < 4.13.16, SEMVER:introduced=0,fixed=4.13.16.
Is there a fix for CVE-2026-126357?
Yes. hono (npm): upgrade to 4.13.16 or later. Until then: Register authentication and authorization middleware inside the mounted application rather than relying on outer-application middleware scoped to the mount prefix. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-126357 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-126357?
Check whether you run Hono or hono (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • hono (npm): upgrade to 4.13.16 or later
    npm install hono@4.13.16

Register authentication and authorization middleware inside the mounted application rather than relying on outer-application middleware scoped to the mount prefix.

Technical details

The vulnerability exists in Hono's mount() and app.mount() functions, which strip a prefix from the request path before forwarding to a mounted application. Two implementation errors allow path bypass: first, dynamic route prefixes with parameters are removed by pattern length rather than actual matched segment length; second, since version 4.12.21, re-normalization of decoded paths converts encoded backslashes to forward slashes and collapses parent directory references. An attacker can exploit this to reach mounted application routes that outer-application middleware (such as authentication on `/app/admin/*`) would normally protect.

Affected products

  • Hono Hono

Identifiers

  • CVE-2026-126357
  • GHSA-4m76-xfj5-7975

Timeline

  • 2026-11-12: disclosed

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