Executive brief
fast-uri is a JavaScript library used to parse and normalize URLs in web applications. The library fails to properly validate IPv6 addresses in URLs, silently converting malformed IPv6 syntax into different valid addresses. This allows attackers to craft URLs that bypass security checks and redirect applications to internal or private network addresses, potentially enabling server-side request forgery attacks against internal services, databases, or cloud metadata endpoints.
Technical details
The vulnerability is an improper input validation flaw (CWE-20) in the URL parsing logic that handles RFC 3986 bracketed IPv6 literals. When fast-uri encounters a malformed IPv6 literal with invalid trailing text (e.g., `[::not-valid]` or `[fc00::not-hex]`), it silently truncates the invalid portion and normalizes it to a valid IPv6 address (e.g., `[::]` or `[fc00::]`) without reporting an error. This is an instance of server-side request forgery (CWE-918) when applications trust the normalized host for security decisions. An attacker can exploit this by supplying a crafted IPv6 URL that normalizes to a private address like loopback (::1), causing outbound requests, redirects, or host-policy checks to target unintended internal services. The vulnerability is network-accessible with no authentication or user interaction required. Patches are available in versions 2.4.5, 3.1.6, and 4.1.3, which reject malformed IPv6 literals with a host error instead of silently normalizing them.
Affected products
- fastify fast-uri 2.3.1 to 2.4.4, 3.0.0 to 3.1.5, 4.0.0 to 4.1.2
Timeline
- 2026-08-23: disclosed: Initial disclosure by Matteo Collina
- 2026-08-23: patched: Patches released: v2.4.5, v3.1.6, v4.1.3
- 2026-09-02: advisory: GitHub Advisory published