fast-uri percent-decodes the scheme component with the legacy global unescape() before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but fast-uri does. For example, normalize('%6Aavascript:alert(1)') returns javascript:alert(1), resolve('https://a.example/', '%6Aavascript:alert(1)') returns javascript:alert(1), and parse('%6Aavascript:alert(1)').scheme returns …
fast-uri's serialize(), normalize(), and resolve(), when called with the non-default option reference: 'suffix' on a reference that has no authority, emit a relative reference whose first path segment contains a bare colon without escaping it. RFC 3986 section 3.3 requires the first segment of a path-noscheme reference not to contain a colon, precisely because a bare colon makes the string reparse as an absolute URI with a scheme. fast-uri's default …
fast-uri's normalize() and serialize() apply RFC 3986 dot-segment removal (remove_dot_segments) to the path of a relative-path reference. RFC 3986 section 5.2.4 defines that algorithm only for the merged target path during resolution, not for the reference itself, so applied to the reference it drops leading .. segments and can turn a document-relative reference into a root-relative one. For example, normalize('a/../b') returns /b and normalize('../x') returns x, so resolve(base, normalize(ref)) points …
fast-uri versions >= 2.4.2, <= 2.4.7, >= 3.1.3, <= 3.1.8, and >= 4.0.1, <= 4.2.2 can silently rewrite the host of an HTTP-family URL (http, https, ws, wss, or any scheme with domainHost: true). For those schemes canonicalizeHost() passes the parsed host to the WHATWG URL parser and adopts its .hostname without checking that the parser read the whole input as a host. RFC 3986 permits @ inside the host …
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= …
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 …
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) …
fast-uri serializes the port component of a URI without validating it. When recomposing the authority, fast-uri escapes the userinfo and host components but concatenates the port verbatim, so a port value that is not a sequence of digits can inject authority delimiters. For example, serializing a component whose port is @127.0.0.1:8124 produces http://trusted.example:@127.0.0.1:8124/app, demoting the intended host to userinfo and pointing the authority at an attacker-controlled host. Both fast-uri and …
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 …
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 …
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 …
fast-uri decodes percent-encoded characters in the scheme component with the legacy global unescape() and serializes the result back as raw characters, without re-escaping it or validating it as a scheme. A scheme that decodes to characters outside the RFC 3986 scheme grammar can therefore introduce structure the original input did not contain. For example, %2f%2fevil.example:/pwn parses with no authority (parse().host is undefined), but resolve() and normalize() return //evil.example:/pwn, which reparses …
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 …
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 …
fast-uri versions >= 2.3.1, <= 4.0.0 fail to canonicalize Unicode/IDN hostnames for HTTP-family URLs. The IDN conversion path calls URL.domainToASCII(…) on the global WHATWG URL constructor, where that helper does not exist. The resulting TypeError is silently routed into parsed.error, but parse(), normalize(), and equal() all return with the host left in its original Unicode form. For example, http://127。0。0。1/ is treated by fast-uri as host 127。0。0。1, while Node's WHATWG URL …
fast-uri v3.1.0 and earlier decodes percent-encoded path separators (%2F) and dot segments (%2E) before applying dot-segment removal in normalize() and equal(). This makes encoded path data behave like real / and .., so distinct URIs collapse onto the same normalized path. For example, http://example.com/public/%2e%2e/admin normalizes to http://example.com/admin, and equal() considers them the same URI. Applications that normalize or compare attacker-controlled URLs to enforce path-based policy can be bypassed. A path …
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 …