HTTP Cookies: Basics for QA Specialists

What are cookies and how are they transmitted in the HTTP protocol?

Cookies are small files that websites send to users' devices through the HTTP(S) protocol. Their main purpose is to store data between interactions (sessions) between the client and the server, making them essential for personalization and session state management (e.g., maintaining user login status).

When a browser sends a request to a server, for example, to load a webpage, the server can respond with a Set-Cookie header. This header contains the cookie information:

Set-Cookie: session_id=abc123; HttpOnly; Secure; SameSite=Lax; Max-Age=3600

This means the server is asking the browser to store a cookie with the name session_id, the value abc123, and a set of attributes that control how this cookie behaves. The next time the browser sends a request to the same server, it will include the cookie in the Cookie header:

Cookie: session_id=abc123

In this way, cookies help the server "remember" the user and maintain context, such as the login state, user preferences, and other settings.

What kind of data is stored in cookies?

Cookies can store a variety of data depending on what the server wants to "remember" for the user:

• Session identifiers (e.g., session_id) to keep users logged in.
• User preferences, such as language settings or theme choices.
• Analytics data, like unique IDs for tracking user activity.

Typically, the data stored in cookies is in the form of key-value pairs, where the key is the cookie name, and the value is the associated data. This data can also be encrypted to protect user privacy.

Structure of cookie data

A cookie consists of the following components:

1. Name: The unique identifier for the cookie, e.g., session_id.
2. Value: The data associated with the name, e.g., abc123.
3. Domain and Path: Specifies which domains and paths the cookie applies to.
4. Expiration (Expires/Max-Age): Determines how long the cookie will be stored in the browser.
5. Security Attributes:

How do cookies work in browsers?

Browsers store cookies on the client side and use them to pass information between requests. The browser's main role is to save cookies and send them back to the server when needed.

1. Storage: Browsers store cookies in special file structures. Depending on the browser, this might be an SQLite database (e.g., Chrome), a file (e.g., Firefox), or another storage mechanism.
2. Access: Cookies can be accessed via JavaScript using the document.cookie property unless they are marked with the HttpOnly attribute.
3. Transmission to the server: The browser automatically sends all relevant cookies (those that match the domain and path) with each request to the server.
4. Control and deletion: Users can manage cookies through browser settings, delete them manually, or block new cookies.

Why do browsers need cookies?

For browsers, cookies serve several purposes:

• Session management: Users can stay logged in across sessions.
• Personalization: User settings (e.g., language) are saved between visits.
• Analytics: Services can track how users interact with the website through unique IDs in cookies.

Where are cookies stored in the browser?

• Temporary cookies: Stored in memory and deleted when the browser is closed (session cookies).
• Persistent cookies: Stored on the hard drive and may persist for a long time (until expiration or user deletion).
• Storage APIs: In addition to cookies, browsers also use other technologies to store data, such as LocalStorage or IndexedDB.

Cookies play an important role in web technologies, and it's crucial for QA specialists to understand how they work to test their security and proper usage.