Page 59 - DCAP516_COMPUTER_SECURITY

Unit 5: Access Control Mechanism

3. Directory: no access, read (read/execute files in directory), list, add, add and read, change Notes
(create, add, read, execute, write files; delete subdirectories), full control, special access

Abbreviated ACL + Full ACL

Many systems augment abbreviation of ACL with full blown ACL. In this scheme the
abbreviations e used as default permissions control. And the explicit overrides the default as
desired.
Example (IBM AIX)

Example (IBM-INX):
1. Base permissions are abbreviated ACL, extended permissions are ACLs with user group
2. ACL entries can add rights, but on deny, access is denied.

5.5 Creation and Maintenance of Access Control Lists

Specific implementations of ACLs differ in details. Some of the issues are:
1. Which subjects can modify an object’s ACL?
2. If there is a privileged user such as root on a UNIX system or administrator on a WINDOWS
system, do the ACLs apply to that user?
3. Does the ACL support groups or wildcards, i.e. can users be grouped into sets based on a
system notion of “group”?

4. How are contradictory access control permissions handled? For example, if one entry
grants read privileges only and another grant write privileges only, which right does the
subject have over the object?

5. If a default setting is allowed, do the ACL permissions modify it, or is the default used
only when the subject is not explicitly mentioned in the ACL?
These issues are critical to the correct implementation and use of ACLs on a system.

Modify

When an ACL is created, rights are instantiated. Chief among these rights is the one called own.
Possessors of the own right can modify its ACL. Creating an object also creates its ACL, with
some initial value, possibly empty but more usually the creator is initially given all rights,
including own over the new object. By convention the subject with own rights is allowed to
modify the ACL. However, some systems allow anyone with access to manipulate the rights.

Privileged User

Many systems have users with extra privileges. The two best know are the root super-user on
UNIX systems and the administrator user on Windows systems. Typically, ACLs or their degenerate
forms are applied in a limited fashion to such users.

Example: Solaris UNIX systems use both the abbreviations of ACLs standard to UNIX
systems and a full-blown ACL. The abbreviations of ACLs are ignored when root is the subject.
However, the full ACLs apply even to root.

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