Tom Landesman, a security researcher at Cloudmark, said it appears someone got a hold of many iCloud accounts and pushed out spam via iMessage peddling links to discount sale websites for several designer brands such as Oakley, Ray-Ban, and Michael Kors.

Nearly two-thirds of the messages had links pointing to fake Oakley sites. These sites could be linked back to China-based domains.

An iOS app, iMessage can be used by any Apple device, including MacBooks and iMacs, to send messages for free.

Cloudmark found that this iMessage spam accounted for 28 percent of all mobile messages reported in July and was the second most prolific form of mobile spam that month.

Since the messages had a "Read Receipt", the spammers could make out a massive list of valid phone numbers for iPhones. Landesman said: "This list would be a hot sell on the underground market for other spammers (or malicious attackers) looking to explicitly target the iOS platform with 100 percent accuracy,"

Users can disable read receipts in iMessage by going into Settings on their iPhones or iPads. The slider to turn off "Send Read Receipts" is located under the "Messages" option.