A cookie is a small note a website leaves in your browser so it can recognise you next time. Affiliate programs use cookies to remember which affiliate sent a visitor, so a purchase made a few days later is still credited, as long as the note has not expired or been deleted.
An HTTP cookie is a small piece of data a server asks the browser to store and send back with later requests to the same domain, as specified in RFC 6265. A first-party cookie belongs to the site the user is visiting; a third-party cookie is set by another domain loaded on that page, such as an ad server or a network’s tracking pixel. Cookies can be set by the server in a response header or by JavaScript running in the page, and each carries an expiry date.
Affiliate tracking was built on cookies. When the visitor clicks a tracking link, the network or the advertiser stores a cookie recording the affiliate; if the visitor buys within the cookie duration, the affiliate is credited. Program terms state that window, which ranges from a single day to several months and matters most for products people think about before buying. Pixel-based conversion tracking reads the same cookie on the thank-you page.
Browsers have steadily narrowed what cookies can do. Safari’s Intelligent Tracking Prevention, introduced in 2017, blocks third-party cookies and caps cookies created by JavaScript at seven days, and at 24 hours in some link-decoration cases. Firefox turned on Total Cookie Protection for all users in 2022, isolating cookies per site. Chrome, after years of plans to remove third-party cookies, said in 2025 that it would keep them without a new choice prompt, and later began retiring most Privacy Sandbox APIs. In the EU, non-essential cookies also need the user’s consent.
That is why server-to-server postbacks keyed on a click ID became the primary method in performance marketing: they do not depend on the browser keeping anything. Cookies still matter for content affiliates working with retail programs, for long purchase cycles and for advertisers whose platforms only support pixel tracking, so a program’s cookie duration and tracking method remain part of judging whether an offer is worth promoting.
An offer receives 1,000 clicks and 40 buyers, spread across browsers in proportion to the clicks. The advertiser can credit sales either through a third-party cookie set by the network’s pixel domain, or through a click ID stored server-side on its own domain and returned by postback.
| Browser share | Buyers | Third-party cookie | First-party click ID + postback |
|---|---|---|---|
| Chrome, 60% | 24 | 24 credited | 24 credited |
| Safari, 30% | 12 | 0 (blocked) | 12 credited |
| Firefox, 10% | 4 | 0 (blocked) | 4 credited |
| Total | 40 | 24 | 40 |
On identical traffic the cookie-only setup credits 60% of the real sales, and the gap grows with the share of iPhone users. Before scaling, ask the network how the advertiser tracks conversions. Shares and counts are illustrative.
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A small file stored in the visitor’s browser after a click on an affiliate link. It records which affiliate referred them, so purchases made within the cookie duration are credited.
The period after the click during which a purchase still counts for the affiliate. Each program sets it, from one day to several months.
No. Google dropped that plan in 2025 and keeps third-party cookies on by default, while Safari and Firefox continue to restrict them.
Less so. Postbacks match conversions by click ID on the server. Cookies remain useful for long purchase cycles and for advertisers that only support pixels.
References are listed as plain text on purpose; look them up by title and publisher. Updated: 2026-10-09.
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