GHSA-2gcr-mfcq-wcc3 Hono: app.mount() strips mount prefix using undecoded path, causing incorrect routing for percent-encoded paths

• npm hono: < 4.12.21 → fixed in 4.12.21

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.