WebAssembly is a growing technology to build cross-platform applications. We aim to understand the security issues that developers encounter when adopting WebAssembly. We mined WebAssembly questions on Stack Overflow and identified 359 security-related posts. We classified these posts into 8 themes, reflecting developer intentions, and 19 topics, representing developer issues in this domain. We found that the most prevalent themes are related to bug fix support, requests for how to implement particular features, clarification questions, and setup or configuration issues. We noted that the topmost issues attribute to authentication in Blazor WebAssembly. We discuss six of them and provide our suggestions to clear these issues in practice.