Provided are methods, apparatus and computer programs for identifying vulnerabilities to viruses of hacking. Hash values are computed and stored for resources stored on systems within a network. If a first resource or a collection of resources (such as files comprising an operating system, Web Browser or mail server) is associated with a vulnerability, hash values for the first resource or collection of resources are compared with the stored hash values to identify systems which have the vulnerability. Messages may be sent to the people responsible for the vulnerable systems, or the vulnerability may be removed by automatic downloading of patches or service packs.