{"id":"CVE-2026-18446","aliases":["GHSA-7p8r-x3mc-p8w7"],"url":"https://o3.security/vulnerability/CVE-2026-18446","summary":"fast-uri vulnerable to host confusion via backslash authority introducer","details":"### Impact\n\n`fast-uri` v4.1.1 and earlier require a literal `//` to recognize a URI authority, so a reference that uses `\\\\`, `/\\`, or `\\/` as the authority introducer (in place of `//`, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) instead treats `\\` as interchangeable with `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input.\n\nFor example, `fast-uri` resolves `\\\\evil.com/path` against base `https://allowed.com/` to `https://allowed.com/%5C%5Cevil.com/path` (confined to the trusted host), while Node's WHATWG URL resolves the same reference to `https://evil.com/path`.\n\nApplications that use `fast-uri` to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or `fetch()` consumers see a policy/use desync and can be steered to an unintended destination.\n\n### Patches\n\nUpgrade to `fast-uri` v4.1.2, v3.1.5, v2.4.4.\n\n### Workarounds\n\nNone. Upgrade to the patched version.","published":"2026-07-31T14:37:01.584Z","modified":"2026-10-28T02:30:22.833156684Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"},"epss":{"score":0.00221,"percentile":0.12872,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"fast-uri","fixedVersion":"2.4.4"},{"ecosystem":"npm","name":"fast-uri","fixedVersion":"3.1.5"},{"ecosystem":"npm","name":"fast-uri","fixedVersion":"4.1.2"}],"fix":{"url":"https://github.com/fastify/fast-uri/commit/f3c6c905f47831007490f466c5945012e905cc52","label":"fastify/fast-uri@f3c6c90"},"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/18xxx/CVE-2026-18446.json"},{"type":"ADVISORY","url":"https://github.com/fastify/fast-uri/security/advisories/GHSA-7p8r-x3mc-p8w7"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-18446"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/commit/f3c6c905f47831007490f466c5945012e905cc52"},{"type":"PACKAGE","url":"https://github.com/fastify/fast-uri"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/releases/tag/v4.1.2"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:22.833156684Z"}}