The problem came to light after a proof-of-concept attack tool called iDict was released on GitHub in early January.
Developed by a user who uses the online alias Pr0x13, the tool was described as "100% Working iCloud Apple ID Dictionary attack that bypasses Account Lockout restrictions and Secondary Authentication on any account."
It worked by trying out a large number of passwords for the targeted Apple IDs. By default the tool came with a file -- also called a dictionary -- containing 500 commonly used passwords, but the list could have easily been extended.
Online services like iCloud typically limit the number of failed log-in attempts per account in order to prevent brute-force attacks, but Pr0x13 allegedly found a way to bypass those protections.
Pr0x13 claims that he made iDict public in order to draw attention to the problem and force Apple to fix it. "This bug is painfully obvious and was only a matter of time before it was privately used for malicious or nefarious activities," he said in the tool's description.
Apple was relatively quick to act and started enforcing rate limiting for log-in attempts done with iDict a day after the tool became available.
"iDict is patched," Pr0x13 said on Twitter. "Discontinue its use if you don't want to lock your account."
This attack comes after the iCloud accounts of several celebrities including Jennifer Lawrence, Kate Upton and Kirsten Dunst were compromised and private photos were stolen from them in September.
The company said at the time that the compromises were the result of a targeted attack on user names, passwords and security questions. Following the incident Apple implemented two-factor authentication for iCloud accounts. Users are advised to turn on the feature.