We're pretty used to seeing security updates for Safari, Mail, QuickTime and the like, but Apple has released an update to Garage Band, it's music-making software, which includes a security fix. It turns out that - for some reason - Garage Band uses web cookies, and changes Safari's cookie security settings. As Apple says in its notes:
When GarageBand is opened, Safari's preferences are changed to always accept cookies. The default preference is to accept cookies only for the sites being visited. The altered setting may allow third parties and advertisers to track a user's web activity. This update addresses the issue by not changing the preference setting. Users who have run previous versions of GarageBand should confirm that their Safari preferences are set as desired.
Note that last sentence: you should check your Safari preferences, on the Security tab, and see what the current settings are for cookies.