What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-03 · updated 2026-09-04
• Weakness: CWE-436

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.

Affected products

• fast-uri: 2.4.5, 3.1.6, 4.1.3

References

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

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