CVE-2026-84394

fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority

Workarounds

If upgrading is not immediately possible, reject any URL whose host contains a [ or ] that is not a well-formed IPv6 literal before making a host decision. Clients that fail closed on credential-bearing URLs, such as Node's global fetch(), are not affected by the reported vector.

fast-uri accepts a host that contains an unbalanced or misplaced authority bracket ([ or ]) without reporting an error. A host that starts with [ but does not end with ], such as [@127.0.0.1, is neither validated as an IP literal nor canonicalized as a domain name, so parse() returns it as the host with error undefined, while Node's URL (and http.get, axios, got, and other clients built on it) resolve the same string to 127.0.0.1. An application that reads parse().host to make a host decision (an SSRF denylist, a redirect allowlist, or proxy routing) and then passes the original URL to an HTTP client evaluates its policy against a string that is not the host the request reaches. The same host is carried through normalize(), equal(), and resolve().

CVE-2026-84394 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 (2.4.6, 3.1.7, 4.1.4); upgrading removes the vulnerable code path.

Kodem intelligence

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This vulnerability has been patched in fast-uri 4.1.4, 3.1.7, and 2.4.6. parse() now reports URI host is malformed. for any host that contains a bracket but is not a valid [IPv6] literal. All users should upgrade.

1. What is CVE-2026-84394? CVE-2026-84394 is a high-severity security vulnerability in fast-uri (npm), affecting versions = 2.4.5. It is fixed in 2.4.6, 3.1.7, 4.1.4.
2. How severe is CVE-2026-84394? CVE-2026-84394 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-2026-84394? fast-uri (npm) versions = 2.4.5 is affected.
4. Is there a fix for CVE-2026-84394? Yes. CVE-2026-84394 is fixed in 2.4.6, 3.1.7, 4.1.4. Upgrade to this version or later.
5. Is CVE-2026-84394 exploitable, and should I be worried? Whether CVE-2026-84394 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-2026-84394 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-2026-84394?
   • Upgrade fast-uri to 2.4.6 or later
   • Upgrade fast-uri to 3.1.7 or later
   • Upgrade fast-uri to 4.1.4 or later

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