Language: Python
Label: vulnerable
CWE: CWE-444 \u2014 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
The product acts as an intermediary HTTP agent
         (such as a proxy or firewall) in the data flow between two
         entities such as a client and server, but it does not
         interpret malformed HTTP requests or responses in ways that
         are consistent with how the messages will be processed by
         those entities that are at the ultimate destination.

Code:
    def _real_extract(self, url):
        qs = parse_qs(url)
        src = urllib.parse.unquote(traverse_obj(qs, ('url', 0)) or '')
        if src and YoutubeTabIE.suitable(src):
            return self.url_result(src, YoutubeTabIE)
        return self.url_result(smuggle_url(
            urllib.parse.unquote(traverse_obj(qs, ('src', 0), ('url', 0))),
            {'http_headers': {'Referer': url}}))