Patching is a critical security service that keeps computer systems up to date and defends against security threats. Existing patching systems all require running systems. With the increasing adoption of virtualization and cloud computing services, there is a growing number of dormant virtual machine (VM) images. Such VM images cannot benefit from existing patching systems, and thus are often left vulnerable to emerging security threats. It is possible to bring VM images online, apply patches, and capture the VMs back to dormant images. However, such approaches suffer from unpredictability, performance challenges, and high operational costs, particularly in large-scale compute clouds where there could be thousands of dormant VM images. This paper presents a novel tool named N