At the same time as Apple released iOS 9, its next big update for its mobile operating system, the company has also put out an update for iTunes. Version 12.3 of the software adds support for both iOS 9 and Mac OSX El Capitan, fixes a selection of small bugs, and — most importantly — enables two-factor authentication. The 233 MB update, the first for major update iTunes since late June, is now available to download.
In order to take advantage of the extra security measure, you'll need to enable two-factor authentication on your Apple ID. Select "Manage your Apple ID" on the relevant page, choose to turn two-factor on, and then wait for the three-day security period to be up. Once activated, you'll need a second code, generated by a trusted device such as your phone, to log into your iCloud, iTunes, and other Apple accounts.
Apple has been steadily adding support for two-factor authentication to its software and services, updating iMessage and FaceTime with the enhanced verification in February, after introducing it to iCloud websites and apps in July last year. Apple originally added two-factor verification to Apple IDs in 2013, but as shown by the "Celebgate" leaks last year, in which nude photos were reportedly taken from iCloud accounts, not all iPhone, iPad, and Mac users had enabled the security feature. Reminder — if you haven't yet added two-factor authentication to your accounts, it's a very good idea to do so now.