Privacy glossary

Filter list

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What is a filter list?

A filter list is a list of criteria that ad blockers and tracker blockers use to determine which content to block. This can include ads, trackers, pop-ups, cookie consent notifications, or other website annoyances.

How do filter lists work?

Webpages are written in HTML, a system of notation that specifies what a page should look like and how it should work. Some parts of the HTML tell the browser to load content—such as images, videos, and ads—from other URLs. Filter lists identify content to block based on the URL it’s loaded from, or characteristics of the HTML, or both.

For example, a website like “somenewssite.com” may try to load ads from “ads.adnetwork.net.” If you have an ad blocker in your browser, it might use a filter list that lists ads.adnetwork.net. In this case, if you went to somenewssite.com in your browser, the filter list rule would prevent the ads from loading, while the rest of the page loaded as normal.

In addition to looking at content URLs, filter lists can include rules that look for clues in the HTML that indicate something is an ad. For example, some parts of the HTML can have labels that tell the browser how to lay out that part of the page. A filter list might include a rule that says “don’t show parts of the page whose label includes the characters ‘-ad-’.”

Who makes filter lists?

A mostly-volunteer community makes and maintains filter lists. These volunteer maintainers don’t build blocker software themselves, but instead publish their lists for blocker users and developers to use. Browser and extension developers may sometimes contribute their own changes to popular community lists.

Some browsers—such as Brave—supplement third-party lists with their own lists, while some only use third-party lists, and some don’t have built-in blocking at all (instead allowing users to install extensions).

What goes into making and maintaining a filter list?

It takes a lot of work to keep filter lists up to date. There are an enormous number of ways for ads and trackers to show up, and they’re constantly changing. Much of the work is manual: list maintainers monitor what their browsers are doing as they browse the Web, and investigate any traffic that looks like it may be an ad or tracker. They also collect reports from the open-source community.

Maintaining a filter list is a balancing act: It should block undesired content without interfering with anything else. If a list is overly aggressive, it may cause websites to not work properly. Some blockers, like Brave’s built-in Shields, offer users a choice of how aggressively to block.

Can I use a custom filter list?

Browsers with blocking functionality, and blocker extensions, usually allow you to customize the filter lists they use. You can add your own entries, or use alternative lists that others have created. (Brave, for example, allows custom filter list subscriptions.)

Why would I use a custom filter list?

There are several reasons to use a custom filter list. One common reason is language: Filter list maintainers concentrate on the part of the Web that is in the primary language of the list’s users. If you often browse websites in a language other than English, you may find that a browser or extension’s default list lets through some ads in that language. In this case you might want to use a language-specific list.

Custom filter lists can also be set to more aggressively block specific things (such as adult content) or to block other kinds of annoyances that aren’t ads or trackers (like cookie-consent popups).

What to consider when choosing a blocker or filter list

Some list maintainers work with ad networks (the tech companies that serve ads) to deliberately let some ads through, sometimes in exchange for payment. They may have standards for which ads are allowed—for example, being visually unobtrusive—but the fact remains that advertisers sometimes influence these lists.

Blocker extensions, like any other extensions, come with privacy and performance risks of their own. They are developed by third-party developers who may or may not be trustworthy. They may collect data about your browsing activity. They may slow down your browser, and drain your battery life. In other words, they can cause some of the same problems that ads do.

Some browsers try to mitigate these risks by limiting what they allow extensions to do. However, that creates a different problem: With limited capabilities, extensions might not be able to do comprehensive, fully effective ad and tracker blocking. There’s a fundamental tradeoff between safe extensions and powerful extensions.

Blockers built directly into the browser, like Brave Shields, don’t have to make this tradeoff: They can do powerful, flexible blocking without hurting privacy or performance.

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