Third-Party Cookie
About 4 min read
Last updated: 2026-08-15
What Are Third-Party Cookies
A third-party cookie is a cookie set by a domain different from the site (first party) the user is visiting.
For example, while browsing news site A, an ad banner from ad network B embedded on the page sets a cookie under ad network B's domain. This is a third-party cookie. When the user visits another site C that also displays ads from ad network B, the earlier cookie is read, recording the browsing history that "the user visited both site A and site C."
Through this mechanism, ad networks gain detailed insight into users' cross-site browsing behavior, enabling them to deliver targeted advertising based on interests.
Major Browser Responses
Advertising Technologies Beyond Third-Party Cookies
Driven by Safari's default blocking, Firefox's cookie isolation, and tightening regulation, the advertising industry is exploring targeting methods that do not depend on third-party cookies.
- Topics API (Privacy Sandbox): Developed by Google as a cookie replacement, but after Chrome decided to keep third-party cookies, Google announced the retirement of the main Privacy Sandbox APIs in October 2025.
- First-party data: Leverages user data collected directly on the company's own site (membership information, purchase history, etc.). Not affected by cookie regulations since it does not rely on third parties.
- Contextual advertising: Delivers ads based on the content of the page being viewed rather than user behavior history. Has the least privacy impact.
- Server-side tracking: Processes tracking pixel requests on the server side, bypassing browser cookie restrictions. Privacy concerns remain.
Countermeasures for Users
The following measures are effective for preventing tracking via third-party cookies:
- Block in browser settings: In Chrome, go to "Settings" → "Privacy and Security" → "Third-party cookies" to enable blocking. Safari blocks them by default, and Firefox's default Total Cookie Protection prevents cross-site tracking.
- Install an ad blocker: Extensions like uBlock Origin block requests to ad network domains entirely, preventing tracking methods beyond just cookies.
- Regular cookie deletion: Periodically delete cookies in your browser settings, or set up automatic deletion using extensions like Cookie AutoDelete.
However, even blocking third-party cookies does not prevent tracking methods that do not rely on cookies, such as browser fingerprinting and tracking pixels. Combining multiple countermeasures is important.
To learn more about this topic, see Cookies and Tracking: How Your Online Behavior Is Being Monitored.
Common Misconceptions
- Once third-party cookies are deprecated, online tracking will disappear
- Third-party cookies are just one tracking method. Cookie-independent tracking technologies such as browser fingerprinting, tracking pixels, and server-side tracking will continue to be used.
- Blocking third-party cookies will break websites
- Most third-party cookies are used for advertising and tracking purposes. Blocking them will not affect the majority of websites. In rare cases where login integration or embedded content is affected, individual exception settings can be configured.
Difference Between First-Party and Third-Party Cookies
First-Party Cookie
Set by the site you are visiting. Used for basic functions like login persistence and settings storage. Cannot be used for cross-site tracking. Less likely to be blocked by browsers.
Third-Party Cookie
Set by a domain different from the site you are visiting. Used for ad targeting and cross-site tracking. Blocked by default in Safari, isolated per site in Firefox; Chrome confirmed in 2025 it would keep them.