PT-2026-46843 · Npm · Hono

Published 2026-06-04 · Updated 2026-06-04

OSV

CVSS v3.1

5.3

Medium

Base vector

AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Summary

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.

Impact

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.

Fix

HTTP Request/Response Smuggling

Protection Mechanism Failure

Weakness Enumeration

CWE-444

CWE-693

Related Identifiers

GHSA-2GCR-MFCQ-WCC3

Affected Products

Hono

References · 6

• https://nvd.nist.gov/vuln/detail/CVE-2026-47676 · Security Note
• https://osv.dev/vulnerability/GHSA-2gcr-mfcq-wcc3 · Vendor Advisory
• https://github.com/honojs/hono/security/advisories/GHSA-2gcr-mfcq-wcc3 · Note
• https://github.com/honojs/hono · Note
• https://github.com/honojs/hono/commit/6cbb025ff87fca1a3d00d0ccca0eaf3a6385c3f1 · Note
• https://github.com/honojs/hono/releases/tag/v4.12.21 · Note