This issue was a nice catch, discovered by Aaron Sigel who has a detailed explanation, video demo and proof-of-concept on his blog. It probably goes without saying, but Safari users should run Software Update as soon as possible.
Thanks to Safari’s nifty AutoFill feature, it has long been susceptible to an information disclosure vulnerability which could allow an malicious web page to extract various details stored in your personal vCard in Address Book.
The issue exists due to the way that Safari tries (by default) to auto-populate some of your details, including name, address, telephone number, etc, when you fill out forms. This can only happen if you have ‘AutoFill web forms’ enabled in Safari’s preferences, as shown in the screenshot below:
Uncheck these boxes to prevent this attack… but note that you’ll have to type your own info in afterwards! It’s not a high-risk vulnerability, but if you’re concerned about your privacy whilst browsing and in general, do what I do and don’t actually set an empty card as your personal card in Address Book. You can do this by creating a new card (enter some dummy info if you want), selecting it, and then choosing “Make this my card” from the Card menu.
Apple’s been notified of the issue, however as this is a ‘feature’ and not a bug, it’ll be interesting to see whether they’ll actually choose to do anything about it.