CVE-2026-110306

fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding

Workarounds

Reject 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.

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.

For 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.

An 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.

CVE-2026-110306 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.4.8, 3.1.9, 4.2.3); upgrading removes the vulnerable code path.

Kodem intelligence

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Upgrade to fast-uri 2.4.8, 3.1.9, or 4.2.3.

1. What is CVE-2026-110306? CVE-2026-110306 is a medium-severity security vulnerability in fast-uri (npm), affecting versions < 2.4.8. It is fixed in 2.4.8, 3.1.9, 4.2.3.
2. How severe is CVE-2026-110306? CVE-2026-110306 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-uri are affected by CVE-2026-110306? fast-uri (npm) versions < 2.4.8 is affected.
4. Is there a fix for CVE-2026-110306? Yes. CVE-2026-110306 is fixed in 2.4.8, 3.1.9, 4.2.3. Upgrade to this version or later.
5. Is CVE-2026-110306 exploitable, and should I be worried? Whether CVE-2026-110306 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-110306 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-110306?
   • Upgrade fast-uri to 2.4.8 or later
   • Upgrade fast-uri to 3.1.9 or later
   • Upgrade fast-uri to 4.2.3 or later

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