Abstract : This work aims at proving that security and flexibility can coexist in an operating system kernel. We show that both security and flexibility can be guaranteed by separating the management of the security policy from the tools used to enforce it. Our work is composed of two parts. In the first part, we propose elementary protection tools which protect the system against selected types of attacks while remaining completely independent from the security policy. In the second part, we describe the security software framework we have implemented, including the component in charge of managing security policies which we call the security manager. Our results are validated on examples showing the flexibility of protection in the system.