The criminals who posted online nude celebrity photos, including those of Oscar-winner Jennifer Lawrence, stole some of the images in a "very targeted attack" on user credentials for Apple iCloud.
Apple said in a statement Tuesday that the massive privacy breach discovered over the Labor Day weekend did not stem from a compromise of any of the systems used for the cloud storage service.
"After more than 40 hours of investigation, we have discovered that certain celebrity accounts were compromised by a very targeted attack on user names, passwords and security questions, a practice that has become all too common on the Internet," the company said. "None of the cases we have investigated has resulted from any breach in any of Apple's systems including iCloud or Find my iPhone."
Several media reported Sept. 1 that the hackers might have exploited a flaw in Apple's Find My iPhone service that allowed an attacker to try an unlimited number of passwords until the right one was found. Apple has since fixed the vulnerability that enabled the so-called brute-force attack.
Other celebrities whose privacy was violated included Kate Upton, Kelly Brook, Kim Kardashian, Kirsten Dunst, Mary Elisabeth Winstead, Ariana Grande and Victoria Justice. The breach is under investigation by the FBI.
The photos were posted on the image bulletin board 4chan, which is used to post and discuss a wide variety of pictures from Japanese anime and animals to pornography and gore. The hackers asked for payment in bitcoin to view the photos.
Some of the nude pictures were found to be fake. Grande said on Twitter that the photos of her were "completely fake," while Justice tweeted "so called nudes of me are FAKE people. Let me nip this in the bud right now pun intended."
As to how the credentials were stolen, experts say it could have been through an email phishing attack that tricked the celebrities into inputting their usernames and passwords on a fake login page.
If the attackers knew the victims' email addresses, then the criminals could have used Apple's "I forgot my password" link, assuming the celebrities did not have two-factor authentication set up for iCloud, Trend Micro said in a blog post.
Without the added layer of protection, the attackers would have received a "security question" instead and the answer may have been available through a Web search.
Another possibility is the celebrities reused their credentials on other sites that were less secure than iCloud and more easily compromised.
As to how sensitive data got on iCloud to begin with is understandable, because Apple's iPhone saves photos to the service by default. Rivals do the same. Google's Android saves photos on Google+ and Microsoft's Windows Phone to OneDrive.
People who delete photos on their phones often do not realize that they remain in the cloud until deleted from the service separately, experts say.
"With today's devices being very keen to push data to their own respective cloud services, people should be careful that sensitive media isn't automatically uploaded to the web, or other paired devices," Chris Boyd, malware intelligence analyst at Malwarebytes, said. "People should also investigate the deletion procedures for online storage."
To bolster security in general when cloud services are involved, experts recommend using a unique password for every site, choosing security questions only known to the user and taking advantage of two-factor authentication whenever it is offered.
Also, because no cloud service is 100 percent safe, experts recommend that people avoid storing potentially embarrassing photos.
"You could also argue that smartphones, which are continually connected to the Internet, are not the best place for nude pictures," Boyd said.
For businesses, the latest privacy breach is a reminder that organizations need to determine whether any executives have photos or anything else online that could be potentially embarrassing or taint the organization's brand, Rick Holland, analyst for Forrester Research said.
Companies should also educate executives on the risk associated with online services. In addition, executive protection services are available to help determine the safety of online activities.
"I think organizations are largely blind to the enterprise risks associated with consumer cloud technology," Holland said. "Security groups need to inventory the technologies used by their high-value targets, so they can understand the risk picture better."