CFNetwork in Safari in Apple Mac OS X before 10.5.3 automatically sends an SSL client certificate in response to a web server's certificate request, which allows remote web sites to obtain sensitive information (Subject data) from personally identifiable certificates, and use arbitrary certificates to track user activities across domains, a related issue to CVE-2007-4879.

Mozilla Firefox before Firefox 2.0.0.13, and SeaMonkey before 1.1.9, can automatically install TLS client certificates with minimal user interaction, and automatically sends these certificates when requested, which makes it easier for remote web sites to track user activities across domains by requesting the TLS client certificates from other domains.