Matter is emerging as a unifying standard in the IoT industry, designed to enhance interoperability among diverse smart home products, ensuring secure and seamless integration. As major IoT vendors increasingly adopt Matter in consumer IoT products, we conduct a systematic study to investigate security risks in the Matter discovery process. By analyzing Matter controllers in the wild, we uncover a novel design flaw in the Matter discovery and connection process, termed MDB. This flaw represents exploitable vulnerabilities in the design space, significantly undermining the control and monitoring capabilities of Matter-enabled devices for IoT users. To facilitate the detection of MDB, we developed an automatic tool, MDB-Checker. Using our tool, we conducted proof-of-concept attacks on 16 real Matter controllers from 13 leading vendors, confirming that MDB is both practical and widespread. We responsibly disclosed the security risks of MDB to CSA and affected vendors, with acknowledgments received from CSA, Tuya, among others. Additionally, we proposed a fix for the Matter standard and developed a mitigation tool to address these security issues.