Ethereum smart contract enables developers to enforce access control policies of critical functions using built-in signature verification interfaces, i.e., ecrecover. However, due to the lack of best practices for these interfaces, improper verifications commonly exist in deployed smart contracts, leaving potential unauthorized access and financial losses. Even worse, the attack surface is ignored by both developers and existing smart contract security analyzers. In this paper, we take a close look at signature-related vulnerabilities and de-mystify them with clear classification and characterization. We present Siguard, the first automatic tool to detect these vulnerabilities in real-world smart contracts. Specifically, \tool explores signature-related paths in the smart contract and extracts data dependencies based on symbolic execution and taint analysis. Then, it conducts vulnerability detection based on a systematic search for violations of standard patterns, including EIP-712 and EIP-2621. The preliminary evaluation validated the efficacy of Siguard by reporting previously unknown vulnerabilities in deployed smart contracts on Ethereum. A video of Siguard is available at https://youtu.be/xXAEhqXWOu0.