CVE-2026-47676: Hono Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2gcr-mfcq-wcc3",
  "modified" : "2026-06-04T18:01:00Z",
  "published" : "2026-06-04T18:01:00Z",
  "aliases" : [ "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "hono"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.12.21"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-444", "CWE-693" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-04T18:01:00Z",
    "nvd_published_at" : "2026-05-28T17:16:32Z"
  }
}