In a newly released support document (via The Wall Street Journal), Apple has confirmed that it is aware of the "intermittent organized network attacks" on iCloud users, but says that its own servers have not been compromised.
Apple is deeply committed to protecting our customers' privacy and security. We're aware of intermittent organized network attacks using insecure certificates to obtain user information, and we take this very seriously. These attacks don't compromise iCloud servers, and they don't impact iCloud sign in on iOS devices or Macs running OS X Yosemite using the Safari browser.Apple's support document goes on to stress the importance of digital certificates, suggesting that users who see an invalid certificate warning in their browser while visiting iCloud.com should not proceed. The company also outlines how users can verify that their browser is connected to iCloud.com and not a third-party man-in-the-middle website.
Apple asks users to make sure that a green lock icon is visible in Safari and that the message "Safari is using an encrypted connection to www.icloud.com" is displayed when the lock icon is clicked. Apple also has verification instructions for both Chrome and Firefox.
Unfortunately, many of the victims falling prey to the fake iCloud sites are not using secure browsers that issue warnings