The finding was reported by Amazon employee Abby Fuller, who claimed that she gained access to another user's WhatsApp archive on her new phone.
"Yes it was a new device. It was not a second-hand SIM. Yes I'm sure they weren't my messages, or groups that I was added to," she said.
Fuller added that the messages were in plaintext, and were not restored by a backup.
Other users have reported similar incidents when setting up WhatsApp with a new phone number.
WhatsApp claims that a message history associated with a specific phone number is destroyed after the number is not used for 45 days.
However, Fuller said her number had been hers for over 45 days, meaning that the account should already have been cleared of all messages.
WhatsApp has not yet offered an official statement on the matter.
