Should We Chat, Too? Security Analysis of WeChat's MMTLS Encryption Protocol
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Presented at Black Hat Asia 2025 by
WeChat, with over 1.2 billion monthly active users, stands as the most popular messaging and social media platform in China and third globally. Instead of TLS, WeChat mainly uses a proprietary network encryption protocol called "MMTLS". We performed the first public analysis of the security and privacy properties of MMTLS and found it to be a modified version of TLS 1.3, with many of the modifications that WeChat developers made to the cryptography introducing weaknesses.We also discovered a second layer of encryption inside MMTLS which we refer to as "Business-layer encryption". We analyzed the security and privacy of Business-layer encryption and found serious issues including metadata leak, forgeable integrity check signatures, potential AES-CBC padding oracle and Key, IV re-use in block cipher mode. These issues are not directly exploitable thanks to the protection of outer MMTLS encryption.Finally, we hypothesize that WeChat's double-layer encryption is a technical debt, and discuss the wider trend of Chinese apps rolling their own crypto.