CVE-2026-47676

app.mount() strips the mount prefix from the incoming request path using the raw URL pathname, while route matching is performed against the percent-decoded path. This inconsistency causes the prefix to be stripped at the wrong position when the path contains percent-encoded multi-byte characters, resulting in the mounted sub-application receiving an incorrect path.

Details

When app.mount(prefix, subApp) is called, Hono calculates the number of characters to strip based on the decoded mount prefix length, but then applies that slice to the raw URL pathname. When the URL contains percent-encoded characters that expand to fewer characters when decoded (such as encoded non-ASCII characters), the two representations have different lengths, so the prefix is stripped at the wrong byte offset.

As a result, the sub-application receives a path that does not correspond to the intended sub-path, it may receive a partial or garbled path instead of the expected value after the mount prefix is removed.

This issue arises when an application uses app.mount() with paths that contain percent-encoded characters, particularly when the mount prefix itself or the request path contains encoded non-ASCII characters.

A mounted sub-application may receive an incorrectly stripped path, causing requests to be routed to unintended handlers within the sub-application.

This may lead to:

• Middleware or route handlers in the sub-application being bypassed or incorrectly matched due to the malformed path
• Requests reaching sub-application routes that the developer did not intend to be accessible via the mounted path

This issue affects applications that use app.mount() where the request URL may contain percent-encoded characters in the mount prefix or subsequent path segments.

CVE-2026-47676 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.12.21); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-47676? CVE-2026-47676 is a medium-severity security vulnerability in hono (npm), affecting versions < 4.12.21. It is fixed in 4.12.21.
2. How severe is CVE-2026-47676? CVE-2026-47676 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of hono are affected by CVE-2026-47676? hono (npm) versions < 4.12.21 is affected.
4. Is there a fix for CVE-2026-47676? Yes. CVE-2026-47676 is fixed in 4.12.21. Upgrade to this version or later.
5. Is CVE-2026-47676 exploitable, and should I be worried? Whether CVE-2026-47676 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-47676 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-47676? Upgrade hono to 4.12.21 or later.