npm · fast-uri
fast-uri vulnerable to host confusion via backslash authority introducer
fast-uri v4.1.1 and earlier require a literal // to recognize a URI authority, so a reference that uses \\, /\, or \/ as the authority introducer (in place of //, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) instead treats \ as interchangeable with / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input.
For example, fast-uri resolves \\evil.com/path against base https://allowed.com/ to https://allowed.com/%5C%5Cevil.com/path (confined to the trusted host), while Node's WHATWG URL resolves the same reference to https://evil.com/path.
Applications that use fast-uri to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination.
Upgrade to fast-uri v4.1.2, v3.1.5, v2.4.4.
None. Upgrade to the patched version.
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