  HTTP client hints are a set of standardized HTTP request headers designed to
modernize and potentially replace the traditional user agent string. While the
user agent string exposes a wide range of information about the client's
browser and device, client hints provide a controlled and structured approach
for clients to selectively disclose their capabilities and preferences to
servers. Essentially, client hints aim at more effective and privacy-friendly
disclosure of browser or client properties than the user agent string.
  We present a first long-term study of the use of HTTP client hints in the
wild. We found that despite being implemented in almost all web browsers,
server-side usage of client hints remains generally low. However, in the
context of third-party websites, which are often linked to trackers, the
adoption rate is significantly higher. This is concerning because client hints
allow the retrieval of more data from the client than the user agent string
provides, and there are currently no mechanisms for users to detect or control
this potential data leakage. Our work provides valuable insights for web users,
browser vendors, and researchers by exposing potential privacy violations via
client hints and providing help in developing remediation strategies as well as
further research.
