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GHSA-7p8r-x3mc-p8w7

fast-uri vulnerable to host confusion via backslash authority introducer

Source: GitHub Advisory Database (reviewed) High CVE-2026-18446

Packages

EcosystemPackageAffectedFixed in
npm fast-uri < 2.4.4 2.4.4
npm fast-uri >= 3.0.0, < 3.1.5 3.1.5
npm fast-uri >= 4.0.0, < 4.1.2 4.1.2

Details

As written in the advisory (Markdown source, shown as plain text).

### Impact `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. ### Patches Upgrade to `fast-uri` v4.1.2, v3.1.5, v2.4.4. ### Workarounds None. Upgrade to the patched version.

References

External links from the advisory. They lead to third-party sites.