Beyond Cryptography
A notable non-cryptographic man-in-the-middle attack was perpetrated by one version of a Belkin wireless network router in 2003. Periodically, it would take over an HTTP connection being routed through it: this would fail to pass the traffic on to destination, but instead itself respond as the intended server. The reply it sent, in place of the web page the user had requested, was an advertisement for another Belkin product. After an outcry from technically literate users, this 'feature' was removed from later versions of the router's firmware.
Another example of a non-cryptographic man-in-the-middle attack is the "Turing porn farm." Brian Warner says this is a "conceivable attack" that spammers could use to defeat CAPTCHAs. The spammer sets up a pornographic web site where access requires that the user solves the CAPTCHAs in question. However, Jeff Atwood points out that this attack is merely theoretical — there was no evidence by 2006 that any spammer had ever built such a system. However, it was reported in October 2007 that spammers had built a Windows game in which users are asked to interpret CAPTCHAs acquired from the Yahoo webmail service, and are rewarded with pornographic pictures. This allows the spammers to create temporary free email accounts with which to send out spam.
Read more about this topic: Man-in-the-middle Attack