At TestingBot we make sure that all VMs running in our network are patched with the latest updates.
We make sure to update our Windows Virtual Machines as soon as possible after Patch Tuesday, when Microsoft releases new Windows updates.
This means our Windows VMs always have the latest security updates from Microsoft.
Flash player and Microsoft Silverlight are updated once a month, or sooner if there's a critical update.
We update our MAC VMs after Apple releases a new security update.
For our Linux VMs, we maintain a 2-week update schedule where we install the latest security updates.
This makes sure that all your tests run on up-to-date versions of the Windows/Mac/Linux operating systems.