INVITATION TO CYBERSECURITY 206 control over the file, and that Alice and the Administrators group also have been assigned certain permissions for the file. Figure 8.7 The access control list for a Python script in a Linux OS. 8.2.2 Multi-level Security Most OSs use DAC, but for some highly sensitive information environments, mandatory access control (MAC) is preferred. In a MAC system, permissions are managed exclusively by administrators. This makes it much easier to monitor who has access to what resources because users are not able to assign permissions to other users. The United States Department of Defense (DOD) uses MAC to protect national security information. They control access using a multi-level security (MLS) scheme that assigns permissions based on information sensitivity levels. The levels they use are Unclassified (U), Confidential (C), Secret (S), and Top Secret (TS). The levels are called clearances for subjects and classifications for objects. Data is classified based on the damage it could “reasonably be expected to cause” if it were disclosed: U poses no risk, C could cause “identifiable damage,” S could cause “serious damage,” and TS could cause “exceptionally grave damage” to national security.1 Examples of TS information might be the names and locations of spies abroad and the details of a military or intelligence operation. As we saw in Chapter 3, cyber operations are intelligence and military operations that take place in and through cyberspace. Information surrounding cyber operations are highly likely to be classified TS. People are provided clearances based on what they need to know in order to do their jobs. Since it is a MAC system, all authorizations are approved and managed by administrators. The higher the clearance, the more vetting necessary for the subject. TS security clearances require an extensive background investigation that can take months to complete. The subject must disclose the places he has lived, the foreign countries he has visited, and the jobs he has held. Investigators interview people that know the subject to gather additional information and to verify facts. Investigators look for ties to foreign countries, any illegal activities the subject may have engaged in, and anything in his background that could make him susceptible to blackmail, such as significant financial debt, a history of 1 From the United States Code of Federal Regulations Title 41 105-62.101
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