{"id":"CVE-2026-110306","aliases":["GHSA-w74x-v6f9-j3xg"],"url":"https://o3.security/vulnerability/CVE-2026-110306","summary":"fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding","details":"### Impact\n\n`fast-uri` percent-decodes the scheme component with the legacy global `unescape()` before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but `fast-uri` does.\n\nFor example, `normalize('%6Aavascript:alert(1)')` returns `javascript:alert(1)`, `resolve('https://a.example/', '%6Aavascript:alert(1)')` returns `javascript:alert(1)`, and `parse('%6Aavascript:alert(1)').scheme` returns `javascript`. The `%uXXXX` form (`%u006Aavascript:`) produces the same result, and `equal('%68ttp://a/', 'http://a/')` returns `true`.\n\nAn application that screens dangerous schemes on the raw input and then stores or renders `normalize(input)` or `resolve(base, input)` turns an inert string into an executable `javascript:`, `data:`, or `vbscript:` URL, a stored or reflected cross-site scripting vector in the consuming application. `equal()` also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists.\n\n### Patches\n\nUpgrade to `fast-uri` `2.4.8`, `3.1.9`, or `4.2.3`.\n\n### Workarounds\n\nReject input whose scheme part (before the first `:`, and before any `/`, `?`, or `#`) contains `%`. Alternatively, check `parse(x).scheme` against an allowlist after normalization and use that same parsed result rather than the raw string.","published":"2026-10-20T07:03:47.312Z","modified":"2026-10-24T02:30:31.601627307Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"npm","name":"fast-uri","fixedVersion":"2.4.8"},{"ecosystem":"npm","name":"fast-uri","fixedVersion":"3.1.9"},{"ecosystem":"npm","name":"fast-uri","fixedVersion":"4.2.3"}],"fix":{"url":"https://github.com/fastify/fast-uri/commit/39933a1f1422986f14e2d844f552238feb40f5d4","label":"fastify/fast-uri@39933a1"},"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/110xxx/CVE-2026-110306.json"},{"type":"ADVISORY","url":"https://github.com/fastify/fast-uri/security/advisories/GHSA-w74x-v6f9-j3xg"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-110306"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/commit/39933a1f1422986f14e2d844f552238feb40f5d4"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/commit/597f254c86d151568511eeb9a1df761fd12520b3"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/commit/e60e716f41d1d73717e2d803d20b68d2c6d20a0f"},{"type":"PACKAGE","url":"https://github.com/fastify/fast-uri"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/releases/tag/v2.4.8"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/releases/tag/v3.1.9"},{"type":"WEB","url":"https://github.com/fastify/fast-uri/releases/tag/v4.2.3"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-24T02:30:31.601627307Z"}}