differer finds how URLs are parsed by different languages in order to help bug hunters break filters.
Differer project aims to help to find differences between several languages and libraries URL parsers. Not all of them behave in the same way and it might lead to unexpected vulnerabilities.
URLs format is defined in RFC 3986, however there are small differences between languages, libraries and how they deal with incorrect URLs. Some of them report an error to the caller, other raise exceptions and other go with the best-effort approach and try to fix them for you. It is exactly there where unexpected security issues might arise.