https://www.vice.com/en_us/article/v7ejmy/apple-scan-message-content-nudity

Apple plans to introduce a new feature that would scan messages sent to and by child users of iPhones to determine if the images contain nudity, the company announced on Thursday. The move is a major development in the ongoing debate around privacy and the inspection of communications.

Whereas previous features from other tech giants designed to detect child abuse images compare the cryptographic fingerprint of images to a list of known child abuse material, the new feature in the Messages app uses machine learning to determine whether a photo likely contains sexual material or not for all images. Some tech companies do aim to detect nudity in other forms of content. Facebook scans public posts for nudity, with varying degrees of success. Apple's planned feature concerns private messages sent between devices, and not publicly shared material.