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# 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
- BTM Spec: https://privacycg.github.io/nav-tracking-mitigations/#bounce-tracking-mitigations
- BTM Explainer: https://github.com/privacycg/nav-tracking-mitigations/blob/main/bounce-tracking-explainer.md
- Chrome Status Listings:
- Original BTM Feature: https://chromestatus.com/feature/5705149616488448
- Stateless Bounce Extension: https://chromestatus.com/feature/6299570819301376
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