WeChat's OCR filter performs blob merging to reconstruct characters from disconnected components. Filling character strokes with tiled letter patterns evaded OCR filtering in 100% of tests (vs. 92% for tiled square patterns), because tiled letters distract the blob-merging stage into finding the letter tiles rather than the composed characters.

WeChat's OCR-based image filter converts images to grayscale using the luminosity formula (0.299r + 0.587g + 0.114b) before text recognition. All 150 test images with colored text on a luminosity-matched gray background evaded OCR filtering, while average and lightness formulas failed to evade filtering for most colors.

§3.1.1 detection

WeChat normalizes uploaded images by their shortest dimension before blacklist comparison. Adding blank space equal to 50–200% of the longest dimension caused 4/5 wide images and 3/5 tall images to evade filtering; adding space along the shortest dimension never evaded filtering, consistent with a shortest-dimension resize hypothesis.

§3.2.4 detection

WeChat's visual-based image filter compares uploads against a specific blacklist using a perceptual similarity metric rather than ML classification. Semantic-preserving transformations — mirroring, cropping, adding whitespace — evaded all 15 tested blacklisted images, and images filtered visually were typically removed within 10 seconds, faster than OCR-filtered images (5–30 seconds).

§3.2.2 detection

WeChat's visual filter compares images as a whole rather than via a sliding window. Adding 3 or more duplicate copies of a blacklisted image to its canvas caused 8/10 images to evade filtering with no evasion attributable to compression artifacts (Figure A.13), whereas blank canvas extensions evaded mostly only through compression artifacts.

§3.2.5 detection

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Simple character-level perturbations (English) and homophone substitutions (Chinese), combined with LLM instruction-following prompts directing the model to use word substitutions in its output, successfully bypassed all input and output filters for all 41 input-blocked and 197 output-blocked queries across five major Chinese LLM services (Baidu-Chat, DeepSeek, Doubao, Kimi, Qwen). Every input-blocked query contained at least one keyword combination that alone triggered the filter, confirming keyword-matching rather than semantic classification.

2026-ablove-characterizing §VII-D keyword-filtering

Cross-national experiments conducted from Singapore, South Korea, and Taiwan during February 19–24, 2025 found no variance in blocking implementations, event syntax, or server infrastructure across all five Chinese LLM services. Input blocking was enacted identically in all three international locations, and services connected to the exact same server IP addresses globally — Kimi and Baidu-Chat connected to identical IPs and DeepSeek to the same two addresses across all tested locations.

2026-ablove-characterizing §VIII-A keyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi

DeepSeek, Kimi, and Doubao all transmit analytics logs to the same Autonomous System (AS24429, Zhejiang Taobao Network Co., Ltd., a ByteDance/Volcengine subsidiary), with one monitoring endpoint IP directly overlapping between DeepSeek and Doubao. Additionally, all four non-Qwen services maintain connections with servers physically located in China throughout the chat session, transmitting user IDs, session IDs, viewport data, language preferences, and in Baidu-Chat's case, the full query text via URL-encoded CAPTCHA requests.

2026-ablove-characterizing §VI-D keyword-filtering

All five Chinese LLM services transmit partial or complete responses to the client machine even when output blocking is triggered, representing a major information leak. For DeepSeek and Qwen, truncated blocked responses are on average close in token length to full successful responses. For Baidu-Chat, the complete response is transmitted to the client but only partially rendered in the browser UI, with only a word or two visible on screen.

2026-ablove-characterizing §VI-C keyword-filtering