A little over a year ago we enabled Enhanced Tracking Protection (ETP) by default in Firefox. We did so because we recognize that tracking poses a threat to society, user safety, and the autonomy of individuals and we’re committed to protecting users against these threats by default. ETP was our first step in fulfilling that commitment, but the web provides many covert avenues trackers can use to continue their data collection.

Today’s Firefox release introduces the next step in providing a safer and more private experience for our users with Enhanced Tracking Protection 2.0, where we will block a new advanced tracking technique called redirect tracking, also known as bounce tracking. ETP 2.0 clears cookies and site data from tracking sites every 24 hours, except for those you regularly interact with. We’ll be rolling ETP 2.0 out to all Firefox users over the course of the next few weeks.

What is “redirect” tracking?

When we browse the web we constantly navigate between websites; we might search for “best running shoes” on a search engine, click a result to read reviews, and finally click a link to buy a pair of shoes from an online store. In the past, each of these websites could embed resources from the same tracker, and the tracker could use its cookies to link all of these page visits to the same person. To protect your privacy ETP 1.0 blocks trackers from using cookies when they are embedded in a third party context, but still allows them to use cookies as a first party because blocking first party cookies causes websites to break. Redirect tracking takes advantage of this to circumvent third-party cookie blocking.

Redirect trackers work by forcing you to make an imperceptible and momentary stopover to their website as part of that journey. So instead of navigating directly from the review website to the retailer, you end up navigating to the redirect tracker first rather than to the retailer. This means that the tracker is loaded as a first party and therefore is allowed to store cookies. The redirect tracker associates tracking data with the identifiers they have stored in their first-party cookies and then forwards you to the retailer.

A step-by-step explanation of redirect tracking: 

Let’s say you’re browsing a product review website and you click a link to purchase a pair of shoes from an online retailer. A few seconds later Firefox navigates to the retailer’s website and the product page loads. Nothing looks out of place to you, but behind the scenes you were tracked using redirect tracking. Here’s how it happened:

• Step 1: On the review website you click a link that appears to take you to the retail site. The URL that was visible when you hovered over the link belonged to the retail site.
• Step 2: A redirect tracker embedded in the review site intercepts your click and sends you to their website instead. The tracker also saves the intended destination—the retailer’s URL that you actually thought you were visiting when you clicked the link.
• Step 3: When the redirect tracker is loaded as a first party, the tracker will be able to access its cookies. It can associate information about which website you’re coming from (and where you’re headed) with identifiers stored in those cookies. If a lot of websites redirect through this tracker, the tracker can effectively track you across the web.
• Step 4: After it finishes saving its tracking data, it automatically redirects you to the original destination.

How does Firefox protect against redirect tracking?

Once every 24 hours ETP 2.0 will completely clear out any cookies and site data stored by known trackers. This prevents redirect trackers from being able to build a long-term profile of your activity.

When you first visit a redirect tracker it can store a unique identifier in its cookies. Any redirects to that tracker during the 24 hour window will be able to associate tracking data with that same identifying cookie. However, once ETP 2.0’s cookie clearing runs, the identifying cookies will be deleted from Firefox and you’ll look like a fresh user the next time you visit the tracker.

This only applies to known trackers; cookies from non-tracking sites are unaffected. Sometimes trackers do more than just track; trackers may also offer services you engage with, such as a search engine or social network. If Firefox cleared cookies for these services we’d end up logging you out of your email or social network every day. To prevent this, we provide a 45 day exception for any trackers that you’ve interacted with directly, so that you can continue to have a good experience on their websites. This means that the sites you visit and interact with regularly will continue to work as expected, while the invisible “redirect” trackers will have their storage regularly cleared. A detailed technical description of our protections is available on MDN.

ETP 2.0 is an upgrade to our suite of default-on tracking protections. Expect to see us continue to iterate on our protections to ensure you stay protected while using Firefox.