fast-uri vulnerable to scheme injection via an unescaped first-segment colon in suffix references

Impact

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 serialization escapes that colon, but the suffix path does not.

For example, normalize('javascript:alert(1)', { reference: 'suffix' }) returns javascript:alert(1), which reparses as a live javascript: scheme; data: and vbscript: behave the same.

Applications that serialize or normalize a relative reference with reference: 'suffix' and then treat the result as a URI can turn inert input into a live javascript:, data:, or vbscript: URL (a cross-site scripting vector), and equal() can conflate a scheme-bearing reference with a relative one. Exposure is limited to applications that use the non-default reference: 'suffix' option.

Patches

Upgrade to fast-uri 2.4.8, 3.1.9, or 4.2.3.

Workarounds

Do not use the reference: 'suffix' option on untrusted input. If suffix output is required, percent-encode a colon in the first path segment of the result before treating it as a URI.