Access Control

This service protects against the unauthorized use of LAN resources, and can be provided by the use of access control mechanisms and privilege mechanisms. Most file servers and multi-user workstations provide this service to some extent. However, PCs which mount drives from the file servers usually do not.

Users must recognize that files used locally from a mounted drive are under the access control of the PC. For this reason it may be important to incorporate access control, confidentiality and integrity services on PCs to whatever extent possible.

According to [NCSC87], access control can be achieved by using discretionary access control or mandatory access control. Discretionary access control is the most common type of access control used by LANs.

The basis of this kind of security is that an individual user, or program operating on the user’s behalf is allowed to specify explicitly the types of access other users (or programs executing on their behalf) may have to information under the user’s control.

Discretionary security differs from mandatory security in that it implements the access control decisions of the user. Mandatory controls are driven by the results of a comparison between the user’s trust level or clearance and the sensitivity designation of the information.

Access control mechanisms exist that support access granularity for acknowledging an owner, a specified group of users, and the world (all other authorized users). This allows the owner of the file (or directory) to have different access rights than all other users, and allows the owner to specify different access rights for a specified group of people, and also for the world.

Generally access rights allow read access, write access, and execute access. Some LAN operating systems provide additional access rights that allow updates, append only, etc. A LAN operating system may implement user profiles, capability lists or access control lists to specify access rights for many individual users and many different groups.

Using these mechanisms allows more flexibility in granting different access rights to different users, which may provide more stringent access control for the file (or directory). (These more flexible mechanisms prevent having to give a user more access than necessary, a common problem with the three level approach.)

Access control lists assign the access rights of named users and named groups to a file or directory. Capability lists and user profiles assign the files and directories that can be accessed by a named user. User access may exist at the directory level, or the file level.

Access control at the directory level places the same access rights on all the files in the directory. For example, a user that has read access to the directory can read (and perhaps copy) any file in that directory. Directory access rights may also provide an explicit negative access that prevents the user from any access to the files in the directory.

Some LAN implementations control how a file can be accessed. (This is in addition to controlling who can access the file.) Implementations may provide a parameter that allows an owner to mark a file sharable, or locked. Sharable files accept multiple accesses to the file at the same time.

A locked file will permit only one user to access it. If a file is a read only file, making it sharable allows many users to read it at the same time. These access controls can also be used to restrict usage between servers on the LAN. Many LAN operating systems can restrict the type of traffic sent between servers.

There may be no restrictions, which implies that all users may be able to access resources on all servers (depending on the users access rights on a particular server). Some restrictions may be in place that allow only certain types of traffic, for example only electronic mail messages, and further restrictions may allow no exchange of traffic from server to server.

The LAN policy should determine what types of information need to be exchanged between servers. Information that is not necessary to be shared between servers should then be restricted. Privilege mechanisms enable authorized users to override the access permissions, or in some manner legally bypass controls to perform a function, access a file, etc.

A privilege mechanism should incorporate the concept of least privilege. [ROBA91] defines least privilege as "a principle where each subject in a system be granted the most restrictive set or privileges needed for the performance of an authorized task."

For example, the principle of least privilege should be implemented to perform the backup function. A user who is authorized to perform the backup function needs to have read access to all files in order to copy them to the backup media. (However the user should not be given read access to all files through the access control mechanism.)

The user is granted a ’privilege’ to override the read restrictions (enforced by the access control mechanism) on all files in order to perform the backup function. The more granular the privileges that can be granted, the more control there is not having to grant excessive privilege to perform an authorized function.

For example, the user who has to perform the backup function does not need to have a write override privilege, but for privilege mechanisms that are less granular, this may occur. The types of security mechanisms that could be implemented to provide the access control service are listed below.

  • Access control mechanism using access rights (defining owner, group, world permissions)
  • Access control mechanism using access control lists, user profiles, capability lists
  • Access control using mandatory access control mechanisms (labels)
  • Granular privilege mechanism