CVE-2026-84394: fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority

Vulnerability summary

CVE-2026-84394 is a high (CVSS 7.5) Interpretation Conflict vulnerability in fast-uri. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 32% of scored vulnerabilities.

Timeline

1. 02-09-2026
   PublishedVulnerability first disclosed.
2. 03-09-2026
   Last modifiedVulnerability information updated.

Description

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 Node's 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.

• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.