What Are Filter Lists, and Why Are They at Risk?

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Almost all threats that our devices face come via the internet so security solutions to malicious websites have never been more important. Other than antivirus software and browser-based scanning like Microsoft SmartScreen and Google Safe Browsing, home users have few realistic options to block these malicious connections.


DNS and content blocking browser extensions offer amazing additional protection and have one thing in common—filter lists. Filter lists are crucial, but they face an uncertain future. So what are they? What might the future hold in store?


What Are Filter Lists?

Filter lists are text files of rules that tell solutions what to block and how they should block it. They are absolutely essential to content blockers and DNS services with any kind of blocking capability as they wouldn’t work without them. The easiest way to think of the Domain Name System is the concept of an internet phone book. You visit a domain, like “makeuseof.com”, and your device asks the DNS for its IP address, so you can connect.

Users get to choose from thousands of filter lists to block security threats, trackers, pornography, gambling, dating, and other inappropriate content to make the internet safer for themselves, children on the network, or businesses.

Filter lists and the hosts file on your operating system have very similar blocking capabilities but with different formats and intended uses. The hosts file tells your operating system what a domain leads to, i.e. an IP address. To block a domain with your hosts file, you can use the correct formatting to tell your operating system to lead the domain to a fake IP address like 0.0.0.0 instead of the real one.

Your computer will visit 0.0.0.0 instead of the harmful IP address and your browser will take you to an error page as it doesn’t exist, and so keeps you safe. DNS blocking does the same thing, by not giving your computer the IP address then you can’t connect to the dangerous website.

Your hosts file and DNS services essentially do the same thing, except hosts files find themselves unused in most systems. In the 2000s, it was common for professional users to download community-made hosts files from the internet to block malware, ads, and other unwanted content before alternative DNS services and content blockers became readily available.

While you still can use hosts file lists, they aren’t updated as much, aren’t as easy to use compared to DNS services, and sometimes cause slowdowns. A DNS can block millions of domains and make no performance difference as the blocks are on the DNS server; your hosts file blocking thousands may cause a noticeable performance decrease. Many old hosts file lists have been converted into filter lists, to be used with content blockers and DNS solutions, giving you more choice and flexibility.

Why Is the Future of Filter Lists at Risk?

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Most public filter lists are volunteer projects, with some exceptions like ones backed by larger companies.

The largest and most used filter lists are the volunteer projects, and they usually have communities of contributors. These communities make sure the filter lists are always updated, and that any false positives are removed. This is good for users, as there isn’t one company behind most of these lists which decides what is and isn’t blocked. This is especially important in filter lists for ads and trackers. With most public filter lists, anybody can contribute to, add, or suggest the removal of individual filters.

However, this does come with drawbacks. Volunteer filter lists rely on donations and motivation from individual contributors to continue being effective at blocking and staying updated. For the most part, there isn’t a concern about a lack of contributors, or even a lack of donations. However, there are concerns about the money these projects do have not being sustainable in the long term as issues that arise may be costly to mitigate.

Spikes in traffic from region-specific browsers and a general increase in users have put strain on these projects with limited budgets. Even by blocking some of this traffic, hundreds of terabytes can be used on bandwidth to deliver block pages. Furthermore, Content Delivery Networks (CDNs) like Cloudflare don’t want to proxy content such as text files; they want to proxy normal websites. Without CDNs, the hosting costs for filter lists would increase dramatically. This was a problem that a popular filter list, EasyList, had, as explained by AdGuard.

Cloudflare also has its own free DNS service which solves numerous DNS privacy risks. It is one of the fastest services around, competing with (and sometimes beating) Google depending on your location.

An illustration of a lens scanning digital devices

Future incidents like these could cause hosting costs to increase, resulting in massive slowdowns so downloading and updating filter lists is much slower. It could even cause some projects to go into a dormant state if the hosting costs get too high.

Thankfully, the incident involving EasyList was resolved by Cloudflare, but it showed just how quickly issues like this can occur. There shouldn’t be anything for you to worry about as the contributors volunteering their time to make these filter lists amazing will continue, but it is a situation worth monitoring if you use these filter lists through DNS or content blockers.

Anybody can make a filter list, and there are thousands to choose from, but only some have large enough communities and contributors to have hundreds of thousands of rules to be as effective as they are. Filter lists in general aren’t going anywhere, especially those backed by companies like AdGuard, but we do need to appreciate community projects out there leading the way.

You will always have antivirus software and browser scanning to keep you safe. Browser extensions made by antivirus companies will likely be using their own lists, or check websites you visit against the antivirus cloud directly. Content blockers from larger companies likely combine their own lists with public ones too.

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