Techniques are described for implementing a cloud computer system to provide access to a custom execution environment for execution of custom executable instructions. Users may be able to configure one or more different types of template execution environments, e.g., a virtual machine environment, each of which can be used to establish a type of custom execution environment. Users may configure the template execution environment with regard to settings, states, resources, permissions, or other criterion related to an execution environment. Upon request, a custom execution environment (e.g., a child execution environment) may be established for running one instance of a custom executable instruction. The custom execution environment may be based on the template execution environment. The custom execution environment may provide a secure, isolated environment for execution of a custom executable instruction. Access permissions for the custom execution environment may be configurable depending on a type of execution environment desired.