A security vulnerability in Safari (iOS, OS X, and Windows) allows an outside party to access cookies belonging to other websites. This could be exploited to hijack logged-in sessions and view private data.
You can test if you are using a vulnerable browser below. The test will attempt to access your cookies belonging to "apple.com".
Click the "Start test". After a while you will see the test results in a frame below it (green text). If nothing appears, or you get a browser error, you are probably not vulnerable. You can see an example screenshot of a vulnerable Safari here.
Important: false negatives could result e.g. for the following reasons: