Privacy on the web | MDN

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People use websites for several important tasks such as banking, shopping, entertainment, and paying their taxes. In doing so, they are required to share personal information with those sites. Users place a certain level of trust in the sites they share their data with. If that information fell into the wrong hands, it could be used to exploit users, for example by profiling them, targeting them with unwanted ads, or even stealing their identity or money.
Modern browsers already have a wealth of features to protect users' privacy on the web, but that's not enough. To create a trustworthy and privacy-respecting experience, developers need to educate their site users in good practices (and enforce them). Developers should also create sites that collect as little data from users as possible, use the data responsibly, and transport and store it securely.
In this article, we:
Before we look at the various privacy and security features available to use on the web, let's define some important terms.
It is hard to talk about privacy without also talking about security — they are closely related, and you can't really create privacy-respecting websites without good security. Therefore, we shall define both.
Privacy refers to the act of giving users the right to control how their data is collected, stored, and used, and not using it irresponsibly. For example, you should clearly communicate to your users what data you are collecting, who it will be shared with, and how it will be used. Users must be given a chance to consent to your terms of data usage, have access to all of their data that you are storing, and delete it if they no longer wish you to have it. You must also comply with your own terms: nothing erodes user trust like having their data used and shared in ways they never consented to. And this isn't just ethically wrong; it could be against the law. Many parts of the world now have legislation that protects consumer privacy rights (for example the EU's GDPR).
Security is the act of keeping private data and systems protected against unauthorized access. This includes both company (internal) data, and user and partner (external) data. It is no use having a robust privacy policy that makes your users trust you if your security is weak and malicious parties can steal their data anyway.
Personal information is any information that describes a user. Examples include:
Private information is any information that users do not want shared publicly and must be kept private (i.e., information that is accessible only by a certain group of authorized users). Some private data is private by law (for example medical data), and some is private more by personal preference.
Following on from the above section, personally identifiable information (PII) is information that can be used, in whole or in part, to track down and/or identify a specific person. For example, if a site leaks a list of users' names and zip codes online, a bad actor could almost certainly use this information to find their full addresses. Even if a full-scale leak does not happen, it is still possible to identify users through less obvious means, such as the browsers they are using, the devices they are using, specific fonts they have installed, and so on.
Tracking refers to the process of recording a user's activity across many different websites. This can be done in various ways, for example:
Tracking data can be used to build a profile of a user and their interests and preferences, which is usually bad and can be annoying to various degrees. For example:
A process very closely related to tracking is fingerprinting: this specifically refers to identifying users by building up a store of data points about them that differentiate them from other users. This could be anything from cookie contents to what browser they are using and what fonts they have installed locally.
Modern browsers take steps to help prevent fingerprinting-based attacks by either not allowing information to be accessed or, where the information must be made available, by introducing variations or "noise" that prevent it from being used for identification purposes.
For example, if a website queries a user's browser for the elapsed time, a comparison of that time to the time reported by the server might be useful as a factor in fingerprinting. Because of this, browsers typically introduce a small amount of variability to timers to make them less useful for identifying the user's system.
Note: See Fingerprinting on web.dev for additional useful information.
Browser vendors are aware of the need to protect user privacy and the negative effects of tracking, fingerprinting, etc., on user experience. To this end, they have implemented various features that enhance privacy protection and/or mitigate threats. In this section, we look at different categories of privacy protection that browsers apply automatically.
Transport Layer Security (TLS) provides security and privacy by encrypting data during transport over the network and is the technology behind the HTTPS protocol. TLS is good for privacy because it stops third parties from being able to intercept transmitted data and use it maliciously, for example for tracking.
All browsers are moving towards requiring HTTPS by default; this is practically the case already because you can't do much on the web without this protocol.
Related topics are as follows:
An open standard for monitoring and auditing certificates, creating a database of public logs that can be used to help identify incorrect or malicious certificates.
HSTS is used by servers to let them protect themselves from protocol downgrade and cookie hijack attacks by letting sites tell clients that they can only use HTTPS to communicate with the server.
While HTTP/2 technically does not have to use encryption, most br