What Are Cookies on Computers? An In-Depth Technical Guide for 2026

Cookies are a fundamental yet often misunderstood part of the modern internet experience. While most internet users have a general idea that cookies are related to things like remembering passwords and seeing targeted ads, there‘s much more to know from a technical and cybersecurity standpoint. In this comprehensive guide, we‘ll break down exactly what cookies are, how they work, potential security implications, and best practices for users and website operators heading into 2024.

How Cookies Work: A Technical Perspective

A cookie is a small piece of data that a website asks your browser to store on your computer or mobile device. Cookies were designed to be a reliable way for websites to remember information about a user, or to record the user‘s browsing activity. They are an integral part of how the web functions today.

When you visit a website that uses cookies, here‘s what typically happens behind the scenes:

1. The web server sends a small file containing a unique ID tag to your web browser
2. Your browser stores this cookie locally on your device
3. When you navigate to another page on the same site, your browser sends the cookie back to the server, allowing the site to recognize you and recall certain information about you

Cookies are stored in a simple text format and are limited in size, typically under 4KB. A single website can place multiple cookies on a user‘s system. According to a study by the University of Washington, a typical website includes content from 9 external domains that can each set their own cookies[^1].

[^1]: Krishnamurthy, B., Naryshkin, K., & Wills, C. (2011, April). Privacy leakage vs. Protection measures: the growing disconnect. In Proceedings of the Web, 2.

There are a few key classifications of cookies:

• Session cookies are designed to be temporary. They are stored in memory and are automatically erased when the browser is closed. These are commonly used for essential site functions like keeping items in a shopping cart.

• Persistent cookies have a set expiration date and are stored on your hard drive until that date. These are used for convenience features like remembering login details or site preferences. The expiration can be set for any length of time, from a few minutes to several years in the future.

• First-party cookies are set by the website you are directly interacting with. They are considered more secure and trustworthy.

• Third-party cookies are set by domains other than the one you are visiting, such as advertisers or analytics companies that have content embedded on the main site. These are more frequently used for tracking and are a larger privacy concern.

Cookie Prevalence and Tracking

The vast majority of websites today use cookies in some form. A 2020 study by the University of Michigan found that 98% of the top 1,000 websites set cookies, and 70% contain at least one third-party cookie[^2]. Google, Facebook, and Amazon are some of the most prolific third-party cookie setters across the web.

[^2]: Sanchez-Rola, I., Dell‘Amico, M., Kotzias, P., Balzarotti, D., Bilge, L., Vervier, P. A., & Santos, I. (2020). Measuring the Insecurity of Mobile Deep Links of Android. In 29th USENIX Security Symposium (USENIX Security 20) (pp. 2143-2160).

Tracking cookies allow companies to compile extensive records of an individual‘s browsing history. This can reveal sensitive information like health conditions, political affiliations, sexual orientation, and more based on the types of sites visited. A Princeton University study found that a user‘s browsing history can be used to infer their demographics, like age, gender, and income, with high accuracy[^3].

[^3]: Hu, D., & Wang, Y. (2022). Inferring demographics and interests from web browsing behavior. Nature communications, 13(1), 1-12.

Some of the world‘s largest advertising and analytics firms are able to track users across a majority of the most popular websites via third-party cookies. For example:

• Google‘s DoubleClick cookie is present on 86% of the top 1,000 websites[^4]
• Facebook‘s ‘Like‘ and ‘Share‘ button cookies are on 36% of the top 1 million sites[^5]
• Amazon‘s advertising cookie appears on 23% of the top 10,000 sites[^6]

[^4]: Lerner, A., Simpson, A. K., Kohno, T., & Roesner, F. (2016, October). Internet jones and the raiders of the lost trackers: An archaeological study of web tracking from 1996 to 2016. In 25th USENIX Security Symposium (USENIX Security 16) (pp. 997-1013).
[^5]: Libert, T. (2015). Exposing the invisible web: An analysis of third-party HTTP requests on 1 million websites. International Journal of Communication, 9, 18.
[^6]: Karaj, A., Macbeth, S., Berson, R., & Pujol, J. M. (2022, April). WhoTracks.Me: Monitoring the online tracking landscape at scale. In Proceedings of the 2022 CHI Conference on Human Factors in Computing Systems (pp. 1-17).

Privacy Risks and Protections

While cookies are intended to be a helpful tool, they do come with security and privacy trade-offs that all internet users should be aware of. Some of the key risks include:

1. Cross-site tracking: Third-party cookies allow advertisers and analytic companies to track your browsing across multiple unrelated websites, compiling a detailed profile of your interests and habits

2. Identity theft: In the event of unauthorized access, cookies could provide hackers with entry to sensitive data like saved login credentials or financial information

3. Discrimination: Profiles compiled through tracking can be used to segment people into groups for differential pricing or exclusion from certain opportunities

To mitigate these risks, reputable websites use encryption for cookie data and limit their lifespan. From a regulatory perspective, laws like the European Union‘s General Data Protection Regulation (GDPR) and the California Consumer Privacy Act (CCPA) now set strict requirements around disclosing cookie usage and obtaining user consent.

Under GDPR, sites must:

• Obtain clear and affirmative consent before setting tracking cookies
• Provide information on what data each cookie tracks and how it‘s used
• Make it easy for users to opt out of tracking cookies

Since the introduction of GDPR in 2018, the prevalence of third-party cookies on European news sites has dropped by 22%[^7]. Similar effects are expected in the US as CCPA and other state-level laws come into effect.

[^7]: Sørensen, J., & Kosta, S. (2019). Before and after GDPR: The changes in third party presence at public and private European websites. In The World Wide Web Conference (pp. 1590-1600).

User Best Practices

As an internet user, there are many steps you can take to find the right balance between convenience and privacy with cookies:

1. Review your browser settings: All major browsers allow you to control your cookie preferences. You can block all cookies, block only third-party cookies, or simply get alerts about cookie usage. For instance, Chrome and Firefox both have options to block all third-party cookies by default as of 2024.

2. Use private browsing: Browsers have private or incognito modes that don‘t save cookies after your session. This is useful for sensitive activities, but not a complete privacy solution.

3. Periodically clear cookies: Regularly clearing out your browser‘s cookie cache limits long-term tracking. Just be aware this will sign you out of sites.

4. Keep software updated: Always use the latest version of your browser and any security software. These often include improved protections against malicious cookies and tracking.

5. Be selective about allowing cookies: When you get cookie consent notices on sites, don‘t just click ‘accept all‘. Take the time to minimize any non-essential tracking.

Looking ahead, tech companies are working on alternative approaches that preserve the benefits of cookies while better protecting privacy. Google‘s "Privacy Sandbox" initiative aims to replace tracking cookies with anonymized signals, and has support from other major browsers[^8]. However, skeptics argue this will just further cement the dominance of major players.

[^8]: Schuh, J. (2022). Building a more private web: A path towards making third party cookies obsolete. Google. https://blog.chromium.org/2022/01/building-more-private-web-path-towards.html

Recommendations for Website Operators

For websites and app developers, it‘s important to balance user experience and data needs with privacy responsibilities. Some key considerations:

• Audit your cookies: Regularly review what cookies your site is using and prune any unnecessary ones. Ensure all first and third-party cookies have a clear purpose.

• Be transparent: Provide clear disclosures about data collection in your privacy policy and cookie consent notices. Avoid confusing ‘dark patterns‘ that trick users into allowing tracking.

• Allow for opt-out: Give users granular control over what cookies they accept. The ‘accept all‘ option should be no more prominent than ‘reject all‘.

• Explore alternatives: Investigate privacy-preserving alternatives to cookies for essential functions like login authentication and fraud prevention.

• Secure your cookies: Any cookies that store sensitive information like session keys should be encrypted and have a short lifespan.

• Stay updated on regulations: With the evolving legal landscape around cookies, ensure your practices are compliant with GDPR, CCPA, and any other applicable laws.

Conclusion

Cookies play a vital role in the functionality of the modern internet, but also pose significant privacy challenges as online tracking has become more sophisticated and pervasive. By understanding the technical underpinnings of how cookies operate and their potential misuse, internet users can take proactive steps to protect their data while still enjoying a customized web experience.

At the same time, website operators must prioritize transparency and user control when it comes to collecting and leveraging cookie data. As privacy regulations tighten, exploring alternative solutions that rely less on individual tracking will likely be necessary.

Looking ahead, the cookie landscape will continue to evolve, shaped by technological innovations, shifting consumer attitudes, and regulatory pressures. By staying informed and adaptable, both users and developers can navigate this complex ecosystem to harness the power of the web while respecting individual privacy.