4. SecuritySecurity is a very critical issue in many systems, especially distributed systems. This is because the system is distributed over a network across many devices and therefore there may be multiple ways for outsiders who do not have permission to access the system. A security breach is considered whenever an unsafe state is reached within the system. An insecure state is one in which data can be illegally intercepted, disrupted, modified, or fabricated. Since a distributed system is composed of various smaller systems with the ability to manage each system separately, it becomes very important that all parts of the distributed system are managed uniformly and comprehensively to prevent security breaches. Furthermore, in case a security breach is made on a subsystem, it becomes very likely that the attacker can access other parts of the system. In this case, the attacker may also be able to make it appear that other parts of the system were accessed legally [1, 6, 7]. Access control, bracketing capabilities, and the formal security model are methods to ensure a safe and secure distributed system environment. 4.1 Access Control Access control is described as “the process of regulating the type of access (e.g. read access, write access, access) that an entity has to system resources” [7]. Access Control Lists can then prevent and allow parts of systems to perform certain actions and access specific files and data. Access Control Lists are used to store information about the privileges that they specify whether an entity has the right to access, write or execute certain sections of a system [8].A comm...... middle of paper ......a , 2008.[6] Benson. A Formal Security Model in Centralized, Parallel, and Distributed Systems. University of California , Irvine, California. 2001.[8] Access control lists. Microsoft Development Network. Internet: http://msdn.microsoft.com/en-us/library/aa374872(VS.85).aspx. [Oct. 12, 2011].[9] How permissions work. MicrosoftTechNet. Internet: http://technet.microsoft.com/en-us/library/cc783530(WS.10).aspx. [Oct. 12, 2011].[10] Clarskon, Michael. Access control. Cornell University. Internet: http://www.cs.cornell.edu/courses/cs513/2007fa/NL.accessControl.html. [Oct. 12, 2011].[11] Evered, Marco. Bracket functionality for distributed system security. University of New England, Armidale, Australia. 2002.