Access Control

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Who and what has access to which resource has to be controled on every IT System.


Controlled are issues like

  • access to files
  • access to memory
  • execution of programs
  • sharing data with other principals

Access is controlled at different levels:

  • application
  • middleware
  • operating system
  • hardware

The complexity of administering Access Control is growing complexity.

Hardware Protection

Protection Problem: preventing one process from interfering with another

Confinement Problem: preventing programs communicating outwards through other than authorized channels (e.g. memory overwriting)

Intel 80x86 (Pentium) Processors

8088/8086: any running program controlled the whole machine
80286: protected segment addressing and rings, operating systems could run proper
80386: built-in virtual memory and large memory segments, treated as a 32-bit flat-address machine


  • process in ring 0 (kernel) manages privilege level of other processes
  • ring 1, 2 usually system processes (e.g. win32 subsys, virtual DOS)
  • ring 3 user programs
  • gates between rings for executing code at an other level

Other Procssors

Acorn Risc Machine (ARM)

  • most commonly licensed to third-party vendors of embedded systems
  • 32-bit processor
  • separate banks of registers for user and system processes
  • hardware protection can be customized

Security Processors

  • hardware security support for cryptography and access control
  • authorized state
  • password covered memory access


Operating Systems

  • access control for files and processes, ring management, IO-management, memory, processors s.o. as deep as the hardware permits it
  • matrix is often used to manage this
  • not usable for large organizations, because the administration becomes to difficult for humans

Do it by groups and/or roles !

Groups and Roles

  • every user fits into one or some categories
  • rights have to be defined for these categories
  • user gets role and fits in group

What is the difference ?

There‘s no final definition.

Access Control Lists

  • just one column of the access control matrix stored for every resource
  • not very performant
  • difficult to administrate


  • simple list: owner, group, world for user – not for programs
  • indirect method for programs: SUID and SGID
  • or by user dummy

Win NT

  • more attributes (take ownership, change permission, delete)
  • arrangement in domains with trust between them


  • just one row of the access control matrix stored for every resource
  • some experimental implementatios in the 70s
  • today a comeback in the form of public key certificates

Win 2k/XP

  • used combined with ACL
  • group policies
  • active directory


  • control access at the right level
  • file access ↔ database file
  • different systems, different access controls
Sandboxing access to a restricted environment
Proof-Carrying-Code tests the behaviour of a program
Object Request Brokers controlling calls for several objects/resources


  • problem if any level doesn’t controll access
  • "Every system has at least one bug – Windows much more."
  • "The most serious bug is sitting in front of the monitor."
  • smashing the stack
  • bypassing denied permissions
  • trojans
  • structural defects in operating systems (Windows user has to be admin for installation)


Decisions were made one time, consequences work eternally, but environment changes very rapidly.

Sometimes, also a developer uses the easier way to reach a goal.

Sometimes it‘s just user friendly.