Does 'Client Hints' Leave Fingerprints?

A new proposed web standard, Client Hints, is a new standard for HTTP communication. It has separate aspects that are dealt with in different Internet standards bodies (i.e. the IETF and W3C respectively.)

It first became a topic of discussion in 2015, with the aim of creating images that would adapt in size based on the user's device width. The idea was to always show images in an appropriate resolution and size.

The concept had browsers and servers sharing information like screen width, viewport width, and device pixel ratio (DPR). If there was a way to share this at the HTTP header level, a server would only have to share the right size of image, which would lead to minimal delay and content negotiation.

Client Hints proposes that servers send an HTTP header to a browser, in advance of the actual web page.

Also, website owners can also tell browsers to share Client Hints with all the the third-party domains used by their website. This could mean that third-party services would obtain in-depth details about another site's visitors.

But Brave has noticed that this may happen and argues against it in their blog post.

They think the parameters that the mechanism would be sending allow for "fingerprinting" of the browser being served, especially if things like user-agent details are moved into the mechanism. Brave says that, "Prior research and organizations have documented that the values transmitted through Client-Hints can be used to identify and track Web users with shockingly high accuracy. For example, Eckersley (2010) found that screen resolution is highly identifying, and Laperdrix et al (2016) found that device color depth (along with screen resolution) is also highly identifying." Also, additional parties in the web like "TLS-terminators" (i.e. servers between the client and the website) would be able to track users in a simple manner. TLS-terminating parties like CDNs and proxies would have new passive and consistent access to identifying information about the browsers.

Brave is not entirely negative on some of the privacy aspects of the proposed standard.

They say that, "moving the User-Agent HTTP header to a client hint is one such example where, if coupled with removing the navigator.userAgent property, Client-Hints could be useful for improving privacy online. (Removing User-Agent would require effort sustained over a long term, among all browsers and maintained websites and services.) At the moment though, most of the suggested values shared in Client-Hints are privacy harming, and so we are negative on the proposal in general."

But when Brave sees what is being proposed, they "do not think the potential performance improvements in the proposal are worth the risk to Web privacy."

Yet they say that they applaud and appreciate that the Client Hints authors are working towards an important goal.

But if you already use anti-fingerprinting extensions or browser settings which stop intrusive JavaScript tracking scripts, as is currently done, Client Hints gives an alternate way for websites to track users.

Currently, Chromium browsers are the only ones that support this mechanism (Mozilla has also spoken against it), but that may change when Edge starts to use the Chromium framework.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.