GPUs have become an integral part of modern systems, but their implications for system security are not yet clear. This paper demonstrates both that discrete GPUs cannot be used as secure co-processors and that GPUs provide a stealthy platform for malware. First, we examine a recent proposal to use discrete GPUs as secure co-processors and show that the security guarantees of the proposed system do not hold on the GPUs we investigate. Second, we demonstrate that (under certain circumstances) it is possible to bypass IOMMU protections and create stealthy, long-lived GPU-based malware. We demonstrate a novel attack that compromises the in-kernel GPU driver and one that compromises GPU microcode to gain full access to CPU physical memory. In general, we find that the highly sophisticated, but poorly documented GPU hardware architecture, hidden behind obscure close-source device drivers and vendor-specific APIs, not only make GPUs a poor choice for applications requiring strong security, but also make GPUs into a security threat.