Most of the source code for Chrome is open-source under the Chromium project, but some features are only available on official Chrome builds. One exclusive feature is Chrome Sync, which synchronizes your bookmarks, history, tabs, and other data across all your devices. Third-party Chromium browsers were never supposed to have access to Chrome Sync, and now Google is cutting access.
"During a recent audit," Google said in a blog post, "we discovered that some third-party Chromium based browsers were able to integrate Google features, such as Chrome sync and Click to Call, that are only intended for Google’s use. This meant that a small fraction of users could sign into their Google Account and store their personal Chrome sync data, such as bookmarks, not just with Google Chrome, but also with some third-party Chromium based browsers."
Google said it would cut third-party browsers' access to Chrome Sync starting on March 15th. No user data will be lost, since data is stored in the cloud, but data will no longer synchronize with non-Google browsers. Google didn't state which browsers were discovered using Chrome Sync — most major Chromium-based browsers either have their own sync engines (Opera, Brave, etc.) or don't offer cloud sync at all.
Google cut off third-party browsers on Android from using Chrome Sync in 2017, after a change in Chrome's code blocked non-Google browsers from generating the required login tokens. Google said at the time that it never officially supported Chrome Sync for third-party browsers.
- Chromium Blog