Impact

fast-uri's mailto scheme parser compares each query field name to the reserved names (to, subject, body) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as %74o (percent-encoded to) is not recognized as a recipient at parse time (parse().to shows only the legitimate recipient) but materializes as a literal to= field after serialize(), and reparsing then treats it as a recipient. The same technique smuggles subject and body through %73ubject and %62ody.

An application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on parse().to, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra to= sees nothing, because the injected field appears only after fast-uri serializes.

Patches

Upgrade to fast-uri 4.1.5.

Workarounds

Percent-decode and compare mailto field names case-insensitively before trusting parse().to, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.

Impact

fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference (//host) there is no scheme, so the host canonicalization that repairs this on a scheme-bearing URL does not run. As a result parse, normalize, and equal disagree on the same host: parse("//%41.com").host returns "A.com" while parse("//a.com").host and parse("//A.com").host return "a.com", and equal("//%41.com", "//a.com") is false even though equal("//A.com", "//a.com") is true. An application that makes a case-sensitive host decision on fast-uri output for a scheme-relative reference, for example a host allowlist or denylist that compares parse(url).host or uses fast-uri.equal, can be steered past the check with a percent-encoded uppercase octet. Because a hostname is case-insensitive in DNS and HTTP routing, the evading spelling reaches the same host the check meant to gate, so the effect is check evasion rather than reaching a different registrable host.

Patches

Upgrade to fast-uri 4.1.5, 3.1.8, or 2.4.7.

Workarounds

Compare hosts case-insensitively (lowercase the parsed host before any allowlist or denylist decision), or avoid making case-sensitive host decisions on scheme-relative input until upgrading.

Impact

fast-uri accepts a host that contains an unbalanced or misplaced authority bracket ([ or ]) without reporting an error. A host that starts with [ but does not end with ], such as [@127.0.0.1, 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 http.get, axios, got, and other clients built on it) resolve the same string to 127.0.0.1. An application that reads parse().host to make a host decision (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().

Patches

This vulnerability has been patched in fast-uri 4.1.4, 3.1.7, and 2.4.6. parse() now reports URI host is malformed. for any host that contains a bracket but is not a valid [IPv6] literal. All users should upgrade.

Workarounds

If upgrading is not immediately possible, reject any URL whose host contains a [ or ] that is not a well-formed IPv6 literal before making a host decision. Clients that fail closed on credential-bearing URLs, such as Node's global fetch(), are not affected by the reported vector.

Impact

fast-uri does not validate the complete RFC 3986 grammar for bracketed IPv6 literals, so a malformed literal with invalid trailing text is silently truncated to a different valid IPv6 address with no error reported. For example, normalize('http://[::not-valid]/private') returns http://[::]/private, and [fc00::not-hex] and [fe80::not-hex] collapse to [fc00::] and [fe80::]. An application that normalizes an untrusted URL before an outbound request, redirect, or host-policy check can be routed to a local or private address such as loopback (::1), unique-local, or link-local. Because parse().error is unset for these inputs, checking it does not protect the consumer.

Patches

Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. Malformed IPv6 literals are now rejected with a host error instead of being normalized to a valid address.

Workarounds

Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.

Impact

fast-uri decodes a hostname's percent escapes twice in a single normalize() or resolve() call: once during parsing and again during authority recomposition. A nested percent-encoded host therefore survives the first decode and is turned into a live destination by the second, so normalize('http://%256c%256f%2563%2561%256c%2568%256f%2573%2574/') returns http://localhost/. Applications that normalize or resolve an untrusted URI before an SSRF check, redirect validation, or host allowlist can be steered to a different destination, including internal addresses such as loopback or a cloud metadata endpoint, than the encoded input appeared to contain. This is an incomplete-fix variant of CVE-2026-6322, whose encoded-authority-delimiter fix introduced the second decode.

Patches

Fixed in fast-uri 2.4.5, 3.1.6, and 4.1.3.

Workarounds

Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().

Impact

fast-uri canonicalizes a host to its ASCII form only when the input carries an explicit scheme. When resolve() resolves a scheme-relative reference (//host/) against a scheme-bearing base, it still emits the host verbatim even though the effective scheme is known, so re-parsing the resolved URI yields a different host than the one resolve() returned. An application that resolves an untrusted reference with fast-uri and then checks or routes on the resulting host can make a policy decision on one host and reach another. This is an incomplete-fix variant of CVE-2026-13676, whose IDN canonicalization was applied only to the scheme-bearing form.

Patches

Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. resolve() now canonicalizes the host once the effective scheme is known, and fails closed if a raw non-ASCII host cannot be converted.

Workarounds

Resolve scheme-relative references against a base that carries a scheme before performing any host-policy or origin check.

Impact

fast-uri v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) normalizes \ to / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input string.

For example, http://evil.com\@allowed.com is treated by fast-uri as host allowed.com with userinfo evil.com\, while Node's WHATWG URL parser and fetch() see host evil.com with path /@allowed.com.

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, including cloud metadata endpoints, loopback, or internal hosts.

Patches

Upgrade to fast-uri v4.1.1, v3.1.4, or v2.4.3.

Workarounds

None. Upgrade to the patched version.

Impact

fast-uri v3.1.1 and earlier decodes percent-encoded authority delimiters (%40 as @, %3A as :) inside the host component and serializes them back as raw characters. This changes the URI structure, turning a hostname into userinfo plus a different host.

For example, http://trusted.com%40evil.com/ normalizes to http://trusted.com@evil.com/, which reparses as host evil.com with userinfo trusted.com.

Applications that normalize untrusted URLs before host allowlist checks, redirect validation, or outbound request routing can be steered to a different authority than the original URL appeared to contain.

Patches

Upgrade to fast-uri >= 3.1.2.

Workarounds

None. Upgrade to the patched version.