Virtualization technology is progressively becoming the authority on which platform security is built and clouds are secured. Hyper-V, Microsoft's virtualization stack, is the backbone to Azure and held to a high security standard. Microsoft offers a bug bounty program with rewards up to $250,000 USD for vulnerabilities in Hyper-V. The hypervisor provides a calling mechanism for guests referred to as hypercalls. Not only could hypercalls offer an avenue for VM escapes, but with the introduction of virtualization-based security (VBS) hypercalls may be abused to bypass Virtual Secure Mode (VSM). In this presentation, we'll discuss our research into developing Hyperseed, our format-aware hypercall fuzzer. We'll dive into the hypercall interface detailing the classes of hypercalls Hyper-V supports, the design of hyperseed, and culminate with details on vulnerabilities we found in hypercall handlers.