Thank you for posting this. The manuscript cited by the Forbes article is at:
https://www.usenix.org/system/files/sec22summer_jain.pdf. To quote the conclusion of the manuscript:
'In this paper, we introduced the first framework to evaluate the robustness of perceptual hashing-based client-side scanning against adversarial attacks. We proposed a general black-box attack and showed that >99.9% of images can be successfully modified while preserving the image content. We also show our attack to generate diverse perturbations preventing straightforward mitigation strategies such as expanding the database with modified images. We finally propose two white-box attacks, providing a theoretical basis for attacks.
Taken together, our results shed strong doubt on the robustness to adversarial black-box attacks of perceptual hashing-based client-side scanning as currently proposed. The detection thresholds necessary to make the attack harder are likely to be very large, probably requiring more than one billion images to be wrongly flagged daily, raising strong privacy concerns.'
In short, Apple's CSAM could trigger a computational arms race between those trying to detect illegal images and those wanting to hide them. Because Apple's system is based fundamentally on similarity between scanned images and known illegal images, paedophiles are likely to modify images. If they do so according to the algorithms described in the manuscript, Apple would have to make thresholds more lax to detect these modified images (which look to the human eye like the original BTW) resulting in many more false positives. In short you don't get a computational something for nothing. If you want to detect images modified to avoid detection, your thresholds have to be more lenient and this entails more false positives.
For those of you who stated that people who posted with concerns about Apple's CSAM system simply did not know what they were talking about, we await your apologies after you read the document above, so that you know what you're talking about.