CVE-2026-84394: fast-uri accepts a host that contains an unbalanced or
CVE-2026-84394 analysis covering impact, affected products, versions, remediation, and cross-checked defender guidance.
CVE-2026-84394 is a vulnerability published by NVD on 9/3/2026.
What happened
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.
Affected products and versions
- openjsf fast-uri 2.4.5
- openjsf fast-uri 3.1.6
- openjsf fast-uri 4.1.3
Severity and weakness
CVSS: HIGH 7.5. Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
Weaknesses: CWE-436.
Known exploitation
No CISA KEV entry is currently attached to the NVD record.
What defenders should do
- Identify whether the affected product and version range exists in production.
- Review vendor advisories and release notes before change windows.
- Patch, upgrade, disable the vulnerable component, or apply vendor mitigations.
- Verify the running version after deployment, not only the package inventory.
Official and supporting references
- https://cna.openjsf.org/security-advisories.html
- https://github.com/fastify/fast-uri/security/advisories/GHSA-58mr-gqgx-xq4g