Hono: app.mount() strips mount prefix using undecoded path, causing incorrect routing for percent-encoded paths

### 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.

Source published

4 Jun 2026 18:01:00 UTC

Source modified

10 Sep 2026 03:51:07 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Publisher rating · MODERATE