Petmail (http://petmail.lothar.com) is a proposed anti-spam email system. In the description the author hypothesizes the following man-in-the-middle attack against CAPTCHAs from free email account vendors. First, the spam sender creates a site that will attract visitor; the author suggests a site with pornographic photos. Second, the spammer requires people to solve CAPTCHA in order to enter the site and see the photos. At the moment a user requests access, the spam originator automatically generates a request to create a new email account (Hotmail, for example). Hotmail presents a CAPTCHA, which the spammer then presents to the pornography requester. When the requester enters the solution, the spammer forwards that solution back to Hotmail. If the solution succeeds, the spammer has a new account and allows the user to see the photos; if the solution fails, the spammer presents a new CAPTCHA challenge to the user. In this way, the attacker in the middle splices together two interactions by inserting a small amount of the account creation thread into the middle of the photo access thread. The user is unaware of the interaction in the middle.
Charles, P. P., Shari L. P., Jonathan M. (2015). Security in Computing: Fifth Edition (pp. 240). Upper Saddle River, NJ: Prentice Hall.