An analysis of automatic image filtering on WeChat Moments

We report results from a series of experiments that uncover mechanisms used to filter images on WeChat, the most popular social media platform in China. Our results inform strategies for evading image filtering on the application. By performing tests on a collection of politically sensitive images filtered by WeChat, we found that WeChat uses two different algorithms to filter, an Optical Character Recognition (OCR)-based algorithm that filters images containing sensitive text, and a visual-based algorithm that filters images that are visually similar to those on an image blacklist. The OCR-based algorithm has implementation similarities to many common OCR algorithms that allow us to create text images that evade filtering. We found that the visual-based algorithm does not use any machine learning approach that uses high level classification of an image to determine whether it is sensitive; however, we discovered multiple implementation details of the visual-based algorithm that inform the creation of images that are visually similar to those blacklisted but that evade filtering. This study is the first in-depth technical analysis of image filtering on WeChat, and we hope that our methods will serve as a road map for studying image censorship on other platforms.