Computer security techniques are described. One example provides a security module. The security module executes on a computing system and determines whether to allow a user or a program (e.g., native executable, script, etc.) associated with the user to access a resource, such as by reading, writing, or executing a file. This decision is based at least in part on whether an access control list that is associated with the resource specifies that a source (e.g., IP address, hardware address) that is associated with the user is allowed to access the resource. This decision can also or instead be based on whether the computing system is executing in maintenance mode, such as in single-user diagnostic mode.