"Safari ... is vulnerable to an attack that allows a malicious web site to read files on a user's hard drive without user intervention," Mastenbrook wrote on his blog.
"This can be used to gain access to sensitive information stored on the user's computer, such as emails, passwords, or cookies that could be used to gain access to the user's accounts on some web sites."
While he understandably does not go into detail he says that people using OS X 10.5 are vulnerable no matter what browser they are using and PC users are at risk if using the Safari browser.
He has published a suggested workaround for Apple users but advises PC users of Safari to change browser for the moment.
Mastenbrook is seen as a reliable source who has in the past found other flaws with Apple’s software.