This invention relates to a method and apparatus for updating software. In particular this invention relates to a method, system and computer program for updating an operating system in a hypervisor comprising: determining a new version of a component of the operating system; installing the new component version;measuring an identifying characteristic of the component and making it available to an attestation system; notifying the attestation system that a component has been updated to a new version whereby, when the attestation system finds that the identifying characteristic of the new component does not match a pre-stored attestation value it is aware that a legitimate mis-match could have occurred. The installing of the new version of the component comprises: identifying an updater associated with new version of the component; measuring an identifying characteristic of the identified updater; loading and installing the new version of the component; and making both the identifying measurement of the updater and the new version of the component available to the attestation system.