Cookie stuffing, cookie dropping, forced clicks, or cookie sprinkling is a blackhat marketing method used by affiliates to record tracking cookies on the end user’s machine, without the prior consent of the user, and with the purpose of creating an impression that the customer/user referral is to be attributed to the blackhat affiliate’s marketing.
Looking into the problem of cookie stuffing, Ben Edelman starts from the essence of how the affiliate marketing mechanism works, and only after that, gets to the core of the issue. Edelman writes:
Affiliate tracking systems are intended to pay commissions to independent web sites (“affiliates”) when users click through these sites’ links to affiliate merchants. Merchants are not intended to pay commission when users merely visit affiliates’ sites. Instead, commission ordinarily only becomes payable in the event that a user 1) visits an affiliate’s site, 2) clicks through an affiliate link to a merchant, and 3) makes a purchase from that merchant.
However, some affiliates use “cookie-stuffing” methods to cause affiliate merchants’ tracking systems to conclude that a user has clicked through a tracking link (and to pay commissions accordingly) even if the user has not actually clicked through any such link. If the user subsequently makes a purchase from that merchant — immediately, or within the “return days” period specified by the merchant’s affiliate program — the affiliate then receives a commission on the user’s purchase. [source]
Essentially, what happens is an abuse of the system by taking advantage of (a) the technological imperfections, and (b) the “last cookie gets the credit” rule.
Edelman gets into greater details and examples in his above-quoted text, and if you want to get a full picture of how bad things can get, I highly encourage you to take a few minutes to read through his text, and watch the videos he is using to back up his observations.
Additionally, it is important for me to emphasize here that cookie stuffing affects both (i) the affiliates in the program that has the cookie stuffer on board, and (ii) the merchant’s overall online marketing. With other affiliates, the blackhat affiliate overwrites their tracking cookies that have already been recorded on the customer’s machines (though pop-ups, toolbars, etc), hijacking the commission that legitimately belongs to other affiliates. With merchants, on the other hand, the cookie stuffer cannibalizes the other marketing channels that the e-business is employing. Watch Haiko de Poel’s videos here, and pay a special attention to how both the affiliates, and the merchants (the SEO and PPC achievements of the latter) get hurt.
Conclusion: by keeping a cookie stuffer aboard an affiliate program, you let them (a) hurt the program’s healthy development, and (b) distort the metrics of your other online marketing campaigns (making you overspend too!). The only method that works here is spotting them, and booting them out of the program.