Cookies: Web Tracking & Session Storage That Powers the Internet

Cookies are small client-side data files stored in the browser to maintain state, identify users, and preserve interaction history across web sessions—solving the stateless nature of HTTP and enabling persistent login, personalization, analytics, and advertising workflows.

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Core Characteristics

Cookies enable websites to store key-value data associated with user behavior, device profile, authentication state, and preference settings.
They allow applications to track returning visitors, maintain shopping carts, remember account sessions, or deliver targeted content.

Cookies contain structured metadata such as:

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Modern implementations require strict rule enforcement to protect against session hijacking, CSRF, replay attacks, and unauthorized tracking.

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Essential Cookie Categories

• Session Cookies — Temporary, deleted when browser closes; used for short-lived state.
  
  
• Persistent Cookies — Stored long-term with expiration; support returning user personalization.
  
  
• Authentication Cookies — Maintain login status and verify identity across pages or tabs.
  
  
• Preference Cookies — Save UI settings like language, theme, and currency.
  
  
• Analytics Cookies — Track user interactions for UX optimization and performance insights.
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• Tracking / Third-Party Cookies — Used for cross-domain user profiling in marketing ecosystems.

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Implementation and Behavior

Client-Side Example

document.cookie = "user=alex; max-age=86400; secure; samesite=Strict";

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Server-Side Example (Express.js)

res.cookie("session", token, {
    secure: true,
    httpOnly: true,
    sameSite: "Strict",
    maxAge: 3600000
});

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Browsers enforce storage limits (typically 4 KB per cookie, 50–180 cookies per domain) and isolate access between domains to prevent unauthorized leakage.

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Security Considerations

To mitigate abuse, cookies rely on policy enforcement:

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Cookies remain a common entry point for credential theft if not configured properly.

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Privacy Compliance and Regulations

Regulatory frameworks define limitations for tracking and require user consent:
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• GDPR (EU)
  
  
• CCPA (California)
  
  
• ePrivacy Directive ("Cookie Law")
  
  

Websites now deploy consent banners, opt-in models, storage anonymization, and expiration governance.
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Third-party tracking cookies face increasing deprecation due to browser restrictions and privacy-first initiatives.

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Usage Scenarios

• Authentication session persistence
  
  
• E-commerce cart state retention
  
  
• Cross-page personalization
  
  
• Conversion tracking and attribution modelling
  
  
• AB-testing and analytics frameworks
  
  

Cookies remain foundational to digital personalization and measurement despite growing privacy constraints and emerging alternatives like local storage, session storage, and token-based authentication.

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Related Terms

• Session Storage
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• Token Authentication