CS 3733 Operating Systems

Read Chapter 20 of SG

• Security Problems
  • Unauthorized reading of data (theft)
  • Unauthorized modification
  • Unauthorized destruction

• Authentication
  • Passwords are often easy to guess
  • If unencripted can be read by snoop programs
  • Crack programs can go through the complete password address space
  • Newer versions of Unix keep a shadow password table so that users can't directly see the password table.
  • Ways to improve security:
    • Force long passwords
    • Force frequent changes
    • System picks password
    • Use one-time passwords (credit-card calculators)

• Operating system threats
  • Trojan horse (e.g. fake login)
  • Trap door (cheat codes) could be hidden in the compiler.
  • Worms - standalone invader (discuss the famous Morris worm)
  • Viruses - code fragment embedded in a legitimate program. use safe computing practices

• Standard things to monitor for:
  • Short or easy passwords
  • Unauthorized set-uid programs
  • Unauthorized programs in system directories
  • Unexpected long-running processes
  • Improper directory protections
  • Improper protections on system files
  • Bad search paths
  • Invalid program checksums

• Security levels:
  • D -minimal MS-Dos and Windows 3.1
  • C
    • C1 - cooperating users access data at same level of security - UNIX
    • C2 - Individual access control (Secure Unix, well-configured NT)
  • B - C2 plus sensitivity label for each object
    • B1 - security label for each object in system
    • B2 - security label extended to each system resource
    • B3 - lists of users that are not given access

  • A -
    • A1 - functionally equivalent to B3 with formal specification and verification bult in.

• NT Security
  • User accounts
    • Identified by a unique security ID
    • At log in gets a security access token (SAT)
    • Every process that runs on behalf of user gets a copy of the SAT
    • A subject tracks and manages permissions for each program:
      • simple subject
      • Server subject

Skill: Appreciate basic issues in operating systems security.