Cookie stuffing

On the World Wide Web, cookie stuffing (also cookie dropping) is an affiliate marketing technique in which, as a result of visiting a website, a user receives a third-party cookie from a website unrelated to that visited by the user, usually without the user being aware of it. If the user later visits the target website and completes a qualifying transaction (such as making a purchase), the cookie stuffer is paid a commission by the target. Because the stuffer has not actually encouraged the user to visit the target, this technique is considered illegitimate by many affiliate schemes.

Websites that run an affiliate program pay a commission to affiliates for introducing visitors who then complete one or more qualifying transactions. Other website owners often join affiliate programs to earn the commission, usually by simply sending visitors to the site running the affiliate program via a special link or advertisement. When the user clicks this special link, a single cookie is usually placed on a user's computer; this is not cookie stuffing. This is considered normal practice and is how affiliate marketers generate genuine income. By definition, cookies can only be considered to be stuffed when one or more is placed on a user's computer purely as a result of viewing a page, or more than one is added at a time as a result of a single click. Taken to the extreme dozens of cookies can be stuffed in a scattergun approach in the hope that the user will visit one of the several target affiliate sites and complete a qualifying transaction.

Cookie stuffing is often referred to as a blackhat online marketing technique. This not only has the potential to generate fraudulent affiliate income for the cookie stuffer, but may also overwrite legitimate affiliate cookies, essentially stealing the commission from another affiliate. It is perfectly normal for a user to visit a website, click on a link and be directed to a target affiliate site but not complete a qualifying transaction at that time. That user may revisit the target affiliate website at some later time and complete a qualifying transaction. The original referring affiliate would be credited with the transaction and make a commission. However, many affiliate programs award the commission to the most recent referring affiliate, not the original referring affiliate.

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Wikipedia