navigator.buildID now returns a fixed timestamp

Published: | Categories: DOM, Privacy & Security


The non-standard navigator.buildID property, that returns the 14-digit build identifier of Firefox, is now fixed at "20181001000000". The change has been made in an effort to better protect user privacy.

Since Firefox 16, the minor version number has been excluded from the browser’s user agent (UA) string that can be obtained with the navigator.userAgent property or the User-Agent HTTP header. Also, since Firefox 25, the build ID in the UA string has been frozen at 20100101. However, the navigator.buildID property had kept exposing the actual build ID that varied by the minor release of Firefox and platform, that could be a fingerprinting factor.

Given that the property may be used by some websites for browser detection, it has been fixed instead of being removed, though exposing a non-standard property to web content is not ideal.

Update: The earlier version of this article said the property was removed, but it has actually been kept to avoid potential compatibility issues. We have corrected the title and description.