A Practical UNIX Capability System Adam Langley <
[email protected]> 22nd June 2005 ii Abstract This report seeks to document the development of a capability security system based on a Linux kernel and to follow through the implications of such a system. After defining terms, several other capability systems are discussed and found to be excellent, but to have too high a barrier to entry. This motivates the development of the above system. The capability system decomposes traditionally monolithic applications into a number of communicating actors, each of which is a separate process. Actors may only communicate using the capabilities given to them and so the impact of a vulnerability in a given actor can be reasoned about. This design pattern is demonstrated to be advantageous in terms of security, comprehensibility and mod- ularity and with an acceptable performance penality. From this, following through a few of the further avenues which present themselves is the two hours traffic of our stage. Acknowledgments I would like to thank my supervisor, Dr Kelly, for all the time he has put into cajoling and persuading me that the rest of the world might have a trick or two worth learning. Also, I’d like to thank Bryce Wilcox-O’Hearn for introducing me to capabilities many years ago. Contents 1 Introduction 1 2 Terms 3 2.1 POSIX ‘Capabilities’ . 3 2.2 Password Capabilities . 4 3 Motivations 7 3.1 Ambient Authority . 7 3.2 Confused Deputy . 8 3.3 Pervasive Testing . 8 3.4 Clear Auditing of Vulnerabilities . 9 3.5 Easy Configurability .