CVE-2026-13676

fast-uri vulnerable to host confusion via failed IDN canonicalization

Workarounds

None. Upgrade to the patched version.

fast-uri versions >= 2.3.1, <= 4.0.0 fail to canonicalize Unicode/IDN hostnames for HTTP-family URLs. The IDN conversion path calls URL.domainToASCII(...) on the global WHATWG URL constructor, where that helper does not exist. The resulting TypeError is silently routed into parsed.error, but parse(), normalize(), and equal() all return with the host left in its original Unicode form.

For example, http://127。0。0。1/ is treated by fast-uri as host 127。0。0。1, while Node's WHATWG URL parser and fetch() canonicalize the same input to 127.0.0.1.

Applications that use fast-uri to enforce host-based policy (denylists, loopback 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.

CVE-2026-13676 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 (4.0.1, 3.1.3, 2.4.2); upgrading removes the vulnerable code path.

Kodem intelligence

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Upgrade to fast-uri v4.0.1, v3.1.3, or v2.4.2

1. What is CVE-2026-13676? CVE-2026-13676 is a high-severity security vulnerability in fast-uri (npm), affecting versions >= 4.0.0, < 4.0.1. It is fixed in 4.0.1, 3.1.3, 2.4.2.
2. How severe is CVE-2026-13676? CVE-2026-13676 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-2026-13676? fast-uri (npm) versions >= 4.0.0, < 4.0.1 is affected.
4. Is there a fix for CVE-2026-13676? Yes. CVE-2026-13676 is fixed in 4.0.1, 3.1.3, 2.4.2. Upgrade to this version or later.
5. Is CVE-2026-13676 exploitable, and should I be worried? Whether CVE-2026-13676 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-2026-13676 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-2026-13676?
   • Upgrade fast-uri to 4.0.1 or later
   • Upgrade fast-uri to 3.1.3 or later
   • Upgrade fast-uri to 2.4.2 or later

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