CVE-2026-84394

fast-uri accepts a host that contains an unbalanced or misplaced authority bracket without reporting an error. A host that starts with an opening bracket but does not end with a closing bracket is neither validated as an IP literal nor canonicalized as a domain name, so parse() returns it as the host with error undefined, while Nodes URL and the HTTP clients built on it resolve the same string to a different host. An application that reads the parsed host to make a host decision, such as 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. This affects fast-uri versions 2.4.5, 3.1.6, and 4.1.3, and is fixed in 2.4.6, 3.1.7, and 4.1.4, where parse() reports a malformed host for any host that contains a bracket but is not a valid IPv6 literal.

Weakness

Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B’s state.

Affected Software

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/105.html
• https://cwe.mitre.org/data/definitions/273.html
• https://cwe.mitre.org/data/definitions/34.html

References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/fastify/fast-uri/security/advisories/GHSA-58mr-gqgx-xq4g