Bounce tracking mitigations

Bounce tracking defined

Bounce tracking (sometimes called redirect tracking) is an abuse of cross-site navigation in which a tracker redirects a user to their website for the purpose of setting a first-party cookie to track that user across other sites. The redirection can happen so quickly that a user may not even notice it.

In effect, bounce tracking enables tracking cookies to be set even if the browser has third-party cookies disabled.

Bounce tracking can be done in a couple of different ways:

1. As a "bounce back". In this case, the user starts on a website (site1.example), is navigated to a tracker site (tracker.example) where the tracking cookie is set, and then redirected back to site1.example.

   

2. As a "bounce through". In this case, the user starts on a website (site1.example), is navigated to a tracker site (tracker.example) where the tracking cookie is set, and then redirected to another site (site2.example).

   

In both instances, users may be unaware they have visited tracker.example. They may believe they have only visited site1.example or tried to navigate to site2.example.

How Bounce tracking mitigations work

Bounce tracking mitigations work by identifying tracker sites via a heuristic and periodically deleting cookies and other state data associated with them (other examples include localStorage, IndexedDB, Cache API, and network state data). The feature avoids using block or allow lists to decide which websites are affected.

It is critical that, while defending against bounce tracking, the browser doesn't interfere with legitimate, non-tracking redirection flow uses. For example, single sign-on (SSO), identity federation, and payment services generally involve redirecting the user to another site where they perform an action, updating state information as a result, then redirecting the user back to the original site.

The process works as follows:

1. The browser monitors navigations and flags sites that are part of a "bounce", meaning sites through which a navigation was redirected. This includes both server-initiated redirections and client-side redirections where JavaScript programmatically triggers a navigation.
2. The browser periodically examines its list of flagged sites and checks to see if the user has actively used the site by interacting with it within the last 45 days. Example interactions include clicking a button, entering data into a form, and scrolling the site. The interaction can occur before, during, or after the bounce was detected.
3. If the site does not have any user interaction and third-party cookies are blocked, then its state will be deleted.

The heuristic operates on sites defined by eTLD+1. As a result, both foo.example.site and bar.example.site are treated as example.site.

Specifications

Browser support

• Chromium's implementation of bounce tracking mitigations was shipped in version 116.
• Firefox also supports it.
• WebKit first shipped bounce tracking protection in ITP 2.0.