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Privacy on the web


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:

Define privacy and important related terms.

Examine browser features that automatically protect user privacy.

Look at what developers can do to create privacy-respecting web content that minimizes the risk of users' personal information/data being obtained unexpectedly by third parties.



In this article

Defining privacy terms and concepts

Privacy features provided by browsers

Privacy considerations for client-side developers

Collect data ethically

Cut down on tracking

Carefully manage third-party resources

Protect user data

See also



Defining privacy terms and concepts

Before we look at the various privacy and security features available to use on the web, let's define some important terms.



Privacy and its relationship with security

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 and private information

Personal information is any information that describes a user. Examples include:

Postal address, email address, phone number, or other contact information

Passport number, bank account, credit card, social security number, or other official identifiers

Physical attributes such as height, gender expression, weight, hair color, or age

Health information such as medical history, allergies, or ongoing conditions

Usernames, when they can be linked to an individual

Hobbies, interests, or other personal preferences

Biometric data such as fingerprints or facial recognition data

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.



Personally identifiable information

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

Tracking refers to the process of recording a user's activity across many different websites. This can be done in various ways, for example:

Looking at multiple third-party cookies set across different sites where third-party content is embedded to find out various information points about the user.

Looking at the Referer header to see where a user has navigated from.

Including parameters on the URLs of inbound links (for example in embedded ads linking to product pages, or marketing emails) that can reveal to the linked site where the link originated from, what marketing campaign it is part of, the email address or other identifier of the user that clicked on it, etc. This process is referred to as link decorating, and results in link URLs that look like this: https://example.com/article/?id=62yhgt1a&campaign=902.

Redirect tracking, which involves trackers momentarily (and imperceptibly) redirecting a user to their website to use first-party storage to track that user across websites. This allows trackers to get around third-party cookies being blocked. For example, if you have read a product review and want to click through to buy it, you might unwittingly navigate to the redirect tracker first, then to the retailer. This means the tracker is loaded as a first party, and can associate tracking data with the identifiers they have stored in their first-party cookies before forwarding you to the retailer.

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:

Targeted ads: Everyone has had the unnerving experience of researching some items to buy on one device and then suddenly being bombarded by adverts for the same products on all their other devices.

Selling or sharing data: Some third parties have been known to compile tracking data and then sell it to/share it with others to use for various purposes, like targeted ads. This is obviously highly unethical and may also be illegal, depending on where in the world it happens.

Prejudice via data: In the worst cases, sharing data could result in the user being unfairly disadvantaged. For example, imagine an insurance company finding out data points about a potential customer that they didn't consent to share, and using them as a justification for increasing insurance premiums.



Fingerprinting

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.




Privacy features provided by browsers

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.



HTTPS by default

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:
Certificate Transparency
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.
HTTP Strict Transport Security (HSTS)
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.
HTTP/2
While HTTP/2 technically does not have to use encryption, most browser developers support it only when used with HTTPS; so in that regard, it can be thought of as a feature to enhance security/privacy.



Opt-in for "powerful features"

So-called "powerful" web API features that provide access to potentially sensitive data and operations are available only in secure contexts, which basically means HTTPS-only. Not only that, but these web features are gated behind a system of user permissions. Users have to explicitly opt in to features like allowing notifications, accessing geolocation data, making the browser go into fullscreen mode, accessing media streams from webcams, using web payments, etc.



Anti-tracking technology

Browsers have implemented several anti-tracking features that automatically enhance their users' privacy protection. Many of these block or limit the ability of third-party sites embedded in <iframe>s to access cookies set on the top-level domain, run tracking scripts, etc.

The Set-Cookie header SameSite attribute's default value has been updated to Lax, to provide better protection against tracking and CSRF attacks. See Controlling third-party cookies with SameSite for more information.

Browsers have all started to block third-party cookies by default. See How do browsers handle third-party cookies? for more details.

Browsers are implementing technologies to allow third-party cookies only in certain circumstances that do not damage privacy, or to implement common use cases that currently require third-party cookies in alternative ways. See Transitioning from third-party cookies.

Several browsers strip out known tracking parameters from URLs — this includes Firefox, Safari, and Brave. Browser extensions also help to do this, for example ClearURLs.

Browsers have implemented redirect tracking protection.



Privacy considerations for client-side developers

There are several actions web developers can and should take to improve privacy for their users. The below sections discuss the most important ones. Some of the categories are not purely technical tasks as such and will involve collaboration with other team members.



Collect data ethically

Companies collect lots of different data from their users for a variety of different reasons:

Usernames, passwords, emails, etc. for authentication purposes.

Emails, postal addresses, and phone numbers for communication.

Age, gender, geographical location, favorite pastimes, and a host of other PII for anything from site personalization to customer satisfaction surveys.

Browsing habits on their sites and other sites, to measure page and feature success metrics.

And so much more.

When collecting data from your customers, you have an opportunity to behave with integrity, show them that you are trustworthy, and build a great relationship with them, in turn, improving your brand and your chance of success.

The ethics of data collection can be broken down into three simple principles:

Don't collect more data than you need

Communicate clearly how you are going to use the data you collect

Delete the data once you have finished with it


Note: The tips provided below make for a better, more privacy-aware user experience, but many of them are required by law to comply with regulations, for example the GDPR in the EU. You should make sure to find out what regulations apply to you in your locale, and what you need to do to comply with them.




Don't collect more data than you need

It is tempting to ask for a lot of data from your users because you think it might be useful in the future. However, every bit of extra data you collect adds risk to your users' privacy and increases the chance that they will abandon the step they are performing (whether it is filling out a survey or signing up for a service).

It is good to anonymize data. You should also consider whether you can get what you need by making your data request less granular. As an example, instead of asking a user their favorite products, you could ask them to select between more general categories.

The best way to protect user privacy though, is to minimize the data you collect. Referring to the previous example, you could infer the same data by looking at user purchase history. As another example, users appreciate being able to buy products anonymously. You shouldn't force them to sign up for an account; if it's not necessary for the service to operate, it should be their choice.



Communicate clearly how you are going to use the data you collect

Once you have decided what data you are going to collect, you should publish a privacy policy on your site that clearly states:

Data that you collect
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