It seems like only last week that Apple launched a new version of Safari, however that hasn't stopped the laptop, software, phone and iPod maker releasing what it claims is a must download update for the browser.
The 40.2MB (strangely fitting 4.0.2 number sequence) promises to update a potential cross-site scripting attack which according to Apple means:
"An issue in WebKit's handling of the parent and top objects may result in a cross-site scripting attack when visiting a maliciously crafted website. This update addresses the issue through improved handling of parent and top objects".
The issues affect all operating systems Safari is available for rather than just Windows users.
You will have to perform a restart once the update is complete.