CVE-2026-16221

fast-uri vulnerable to host confusion via literal backslash authority delimiter

Workarounds

None. Upgrade to the patched version.

fast-uri v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) normalizes \ to / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input string.

For example, http://evil.com\@allowed.com is treated by fast-uri as host allowed.com with userinfo evil.com\, while Node's WHATWG URL parser and fetch() see host evil.com with path /@allowed.com.

Applications 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, including cloud metadata endpoints, loopback, or internal hosts.

CVE-2026-16221 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. 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 (2.4.3, 3.1.4, 4.1.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to fast-uri v4.1.1, v3.1.4, or v2.4.3.

1. What is CVE-2026-16221? CVE-2026-16221 is a high-severity security vulnerability in fast-uri (npm), affecting versions >= 2.3.1, <= 2.4.2. It is fixed in 2.4.3, 3.1.4, 4.1.1.
2. How severe is CVE-2026-16221? CVE-2026-16221 has a CVSS score of 7.5 (High). 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-2026-16221? fast-uri (npm) versions >= 2.3.1, <= 2.4.2 is affected.
4. Is there a fix for CVE-2026-16221? Yes. CVE-2026-16221 is fixed in 2.4.3, 3.1.4, 4.1.1. Upgrade to this version or later.
5. Is CVE-2026-16221 exploitable, and should I be worried? Whether CVE-2026-16221 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-2026-16221 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-2026-16221?
   • Upgrade fast-uri to 2.4.3 or later
   • Upgrade fast-uri to 3.1.4 or later
   • Upgrade fast-uri to 4.1.1 or later

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