{"id":"CVE-2026-47676","aliases":["GHSA-2gcr-mfcq-wcc3"],"url":"https://o3.security/vulnerability/CVE-2026-47676","summary":"Hono: app.mount() strips mount prefix using undecoded path, causing incorrect routing for percent-encoded paths","details":"### Summary\n\n`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.\n\n### Details\n\nWhen `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.\n\nAs 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.\n\nThis 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.\n\n### Impact\n\nA mounted sub-application may receive an incorrectly stripped path, causing requests to be routed to unintended handlers within the sub-application.\n\nThis may lead to:\n\n- Middleware or route handlers in the sub-application being bypassed or incorrectly matched due to the malformed path\n- Requests reaching sub-application routes that the developer did not intend to be accessible via the mounted path\n\nThis issue affects applications that use `app.mount()` where the request URL may contain percent-encoded characters in the mount prefix or subsequent path segments.","published":"2026-05-28T15:26:01.672Z","modified":"2026-10-16T02:48:01.980774441Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"},"epss":{"score":0.0026,"percentile":0.17874,"asOf":"2026-08-20"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"hono","fixedVersion":"4.12.21"}],"fix":{"url":"https://github.com/honojs/hono/commit/6cbb025ff87fca1a3d00d0ccca0eaf3a6385c3f1","label":"honojs/hono@6cbb025"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/47xxx/CVE-2026-47676.json"},{"type":"ADVISORY","url":"https://github.com/honojs/hono/security/advisories/GHSA-2gcr-mfcq-wcc3"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-47676"},{"type":"WEB","url":"https://github.com/honojs/hono/commit/6cbb025ff87fca1a3d00d0ccca0eaf3a6385c3f1"},{"type":"PACKAGE","url":"https://github.com/honojs/hono"},{"type":"WEB","url":"https://github.com/honojs/hono/releases/tag/v4.12.21"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-16T02:48:01.980774441Z"}}