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

Why it is Amber

• CVSS 7.5 high
• Remotely exploitable without logging in
• On your watchlist: npm

Affected packages

References

• https://github.com/advisories/GHSA-58mr-gqgx-xq4g
• https://github.com/fastify/fast-uri/security/advisories/GHSA-58mr-gqgx-xq4g
• https://nvd.nist.gov/vuln/detail/CVE-2026-84394
• https://github.com/fastify/fast-uri/pull/214
• https://github.com/fastify/fast-uri/commit/e00815236bc94107e44ef1b2f5318a06bac225c0
• https://cna.openjsf.org/security-advisories.html
• https://github.com/fastify/fast-uri/releases/tag/v2.4.6
• https://github.com/fastify/fast-uri/releases/tag/v3.1.7
• https://github.com/fastify/fast-uri/releases/tag/v4.1.4
• https://github.com/advisories/GHSA-58mr-gqgx-xq4g

Original description

### Impact `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()`. ### Patches 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. ### 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.