Bounce Tracking Mitigations (BTM)
This directory contains the code for Chromium's Bounce Tracking Mitigation (BTM) feature. BTM aims to mitigate the privacy impact of “bounce tracking,” a technique used to track users across websites without relying on third-party cookies.
What is bounce tracking?
Bounce tracking involves redirecting users through a tracker website, often without their knowledge or interaction. This allows the tracker to set or access first-party cookies, effectively circumventing third-party cookie restrictions and user privacy preferences.
How does BTM work?
BTM detects potential bounce tracking by analyzing website behavior, including:
- Short dwell times on a website before redirecting.
- Programmatic redirects (as opposed to user-initiated ones).
- Writing to storage (cookies, etc.) before redirecting.
If BTM determines that a website is likely involved in bounce tracking and there‘s no indication of legitimate user interaction with the site, it automatically deletes the site’s storage (eTLD+1) after a brief grace period.
Goals of BTM
- Reduce cross-site tracking: Limit the ability of bounce trackers to identify and track users across different contexts.
- Protect user privacy: Prevent bounce tracking from circumventing third-party cookie restrictions.
- Maintain compatibility: Avoid disrupting legitimate use cases like federated logins and payment flows that rely on redirects.
- Adaptability: Mitigate tracking by short-lived domains that may evade traditional blocklist-based approaches.
Non-Goals
- Replacing third-party cookie blocking: BTM is primarily designed for environments where third-party cookies are already restricted.
- Mitigating tracking by sites with significant first-party activity: BTM focuses on incidental parties (sites without meaningful user interaction) and may not be effective against sites with substantial first-party engagement.
Further Reading