Cookie stuffing or cookie dropping is a Black hat SEO technique used to
generate fraudulent affiliate sales. It involves placing an affiliate
tracking cookie on a website visitor's computer without their knowledge,
which will then generate revenue for the person doing the cookie
stuffing. Income is generated when the affected user visits the target
affiliate site and either creates an account or makes a purchase,
depending on the terms of the affiliate agreement. This not only
generates fraudulent affiliate sales, but also has the potential to
overwrite legitimate affiliates' cookies, essentially stealing their
legitimately earned commissions.

Operators of websites that allow user-generated content, such as forums
that allow users to post, should be aware of this technique in order to
protect their visitors from this attack. Cookie stuffing can be
accomplished with as little as including an image in a forum post.