HTTP Referrer spoofing with malformed URLs — Mozilla

Security researcher Gregory Fleischer demonstrated a problem with the HTTP Referer: (sic) header sent with requests to URLs containing Basic Authentication credentials with empty usernames. In these cases a number of leading characters, based on the length of the password in the URL, are removed from the referrer hostname. Fleischer pointed out that websites which only check the Referer: header to protect against Cross-Site Request Forgery (CSRF) could be attacked using this flaw. This concept was based on and expanded from a post to the sla.ckers.org forum by security researcher RSnake.