First-Party vs Third-Party Cookies

Understanding the distinction between first-party and third-party cookies is crucial for privacy, security, and marketing strategies.

Definitions

A cookie set by the same domain that the user is currently visiting.

Example:

Characteristics:

A cookie set by a different domain than the one the user is currently visiting.

Example:

Characteristics:

How to Identify

Common Scenarios

Scenario 1: E-commerce Site with Analytics

Setup:

Cookies:

Result:

Sequence Diagram:

sequenceDiagram participant User participant Shop as Shop Site
(shop.example.com) participant Analytics as Analytics
(analytics.google.com) participant Browser User->>Shop: Visit shop.example.com Shop->>Browser: HTML with analytics script tag Browser->>Browser: Load page Shop->>Browser: Set-Cookie: cart=item1,item2
Domain=shop.example.com
SameSite=Lax Browser->>Browser: Store first-party cookie Browser->>Analytics: GET /analytics.js
Cookie: (analytics cookies if any) Analytics->>Browser: Set-Cookie: ga_id=abc123
Domain=analytics.google.com
SameSite=None
Secure Browser->>Browser: Store third-party cookie Note over Browser: First-party cookie: shop.example.com
Third-party cookie: analytics.google.com

Scenario 2: Embedded Social Media Widget

Setup:

Cookies:

Result:

Sequence Diagram:

sequenceDiagram participant User participant Blog as Blog Site
(blog.example.com) participant Facebook as Facebook
(facebook.com) participant Browser User->>Blog: Visit blog.example.com Blog->>Browser: HTML with Facebook widget iframe Blog->>Browser: Set-Cookie: theme=dark
Domain=blog.example.com
SameSite=Lax Browser->>Browser: Store first-party cookie Browser->>Facebook: GET /widget (iframe)
Cookie: (facebook cookies if SameSite allows) Facebook->>Browser: Set-Cookie: fb_id=xyz789
Domain=facebook.com
SameSite=None
Secure Browser->>Browser: Store third-party cookie Note over Browser: First-party: blog.example.com
Third-party: facebook.com
(may be blocked by browser)

Scenario 3: Pixel Tracking

Setup:

Cookies:

Result:

Sequence Diagram:

sequenceDiagram participant User participant Newsite as Newsite
(newsite.com) participant Tracker as Tracker
(tracker.adnetwork.com) participant Browser User->>Newsite: Visit newsite.com/page Newsite->>Browser: HTML with tracking pixel Newsite->>Browser: Set-Cookie: session=def456
Domain=newsite.com
SameSite=Lax Browser->>Browser: Store first-party cookie Browser->>Tracker: GET /pixel?id=123
Cookie: (tracker cookies if any) Note over Browser,Tracker: Third-party request
(different domain) Tracker->>Browser: Set-Cookie: tracker_id=789abc
Domain=tracker.adnetwork.com
SameSite=None
Secure Browser->>Browser: Store third-party cookie
(may be blocked by browser) Tracker->>Browser: HTTP 200 OK
1x1 GIF

Browser Restrictions

Modern Browser Behavior

Third-Party Cookie Blocking:

First-Party Cookie Treatment:

SameSite Attribute Impact

SameSite=Strict

SameSite=Lax

SameSite=None

Marketing and Advertising Implications

Why Third-Party Cookies Matter

Advertising Use Cases:

  1. Cross-Site Tracking: Follow users across different websites
  2. Retargeting: Show ads based on previous site visits
  3. Attribution: Track which ads led to conversions
  4. Audience Building: Create user profiles across sites

Current Challenges:

Alternatives to Third-Party Cookies

  1. First-Party Data: Collect data directly on your site
  2. Server-Side Tracking: Track via server logs
  3. Federated Identity: Login-based tracking (e.g., Google, Facebook)
  4. Contextual Advertising: Target based on page content, not user history
  5. Privacy Sandbox: Browser APIs for privacy-preserving advertising

Testing in the Playground

Test First-Party Cookies

  1. Visit https://site-a.cookie-playground.pun7o.click
  2. Set cookie: Domain=site-a.cookie-playground.pun7o.click or leave empty
  3. Cookie is first-party for this domain
  4. Accessible by JavaScript on this domain

Test Third-Party Cookies

  1. Visit https://site-a.cookie-playground.pun7o.click
  2. Load pixel from different domain: <img src="https://cookie-playground.pun7o.click/pixel">
  3. If cookie-playground.pun7o.click sets a cookie, it's third-party for site-a.cookie-playground.pun7o.click
  4. Browser may block it depending on SameSite settings

Cross-Subdomain Testing

Scenario: Are cookies between site-a.cookie-playground.pun7o.click and cookie-playground.pun7o.click first or third-party?

Answer: It depends on browser interpretation:

Privacy Considerations

User Privacy

First-Party Cookies:

Third-Party Cookies:

GDPR/CCPA Compliance

Requirements:

Best Practices:

Practical Guidelines

When to Use First-Party Cookies

✅ Good Use Cases:

When Third-Party Cookies Are Used

⚠️ Common Use Cases (becoming less viable):

Note: With browser restrictions, these use cases are declining.

Next Steps