CVE 2010-2264
The Cascading Style Sheets (CSS) implementation in WebKit in Apple Safari before 5.0 on Mac OS X 10.5 through 10.6 and Windows, and before 4.1 on Mac OS X 10.4, does not properly handle the :visited pseudo-class, which allows remote attackers to obtain sensitive information about visited web pages via a crafted HTML document.
See the 
CVE page on Mitre.org
for more details.
