CVE-2026-75931

fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references

Workarounds

Resolve scheme-relative references against a base that carries a scheme before performing any host-policy or origin check.

fast-uri canonicalizes a host to its ASCII form only when the input carries an explicit scheme. When resolve() resolves a scheme-relative reference (//host/) against a scheme-bearing base, it still emits the host verbatim even though the effective scheme is known, so re-parsing the resolved URI yields a different host than the one resolve() returned. An application that resolves an untrusted reference with fast-uri and then checks or routes on the resulting host can make a policy decision on one host and reach another. This is an incomplete-fix variant of CVE-2026-13676, whose IDN canonicalization was applied only to the scheme-bearing form.

CVE-2026-75931 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 (2.4.5, 3.1.6, 4.1.3); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. resolve() now canonicalizes the host once the effective scheme is known, and fails closed if a raw non-ASCII host cannot be converted.

1. What is CVE-2026-75931? CVE-2026-75931 is a high-severity security vulnerability in fast-uri (npm), affecting versions >= 2.4.2, < 2.4.5. It is fixed in 2.4.5, 3.1.6, 4.1.3.
2. How severe is CVE-2026-75931? CVE-2026-75931 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-2026-75931? fast-uri (npm) versions >= 2.4.2, < 2.4.5 is affected.
4. Is there a fix for CVE-2026-75931? Yes. CVE-2026-75931 is fixed in 2.4.5, 3.1.6, 4.1.3. Upgrade to this version or later.
5. Is CVE-2026-75931 exploitable, and should I be worried? Whether CVE-2026-75931 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-2026-75931 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-2026-75931?
   • Upgrade fast-uri to 2.4.5 or later
   • Upgrade fast-uri to 3.1.6 or later
   • Upgrade fast-uri to 4.1.3 or later

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