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Security Design

Updated Jan 30, 2024 ·

Overview​

Security design involves integrating protection measures within a system. Whether added during initial design or later, effective strategies require thoughtful planning to safeguard against threats.

Bolt-on Security​

Bolt-on security involves adding protective measures to a system after it has been designed. While it can be a quick fix for existing systems, it is usually more costly and less effective than integrating security from the beginning.

  • Often used for legacy systems where redesign is not feasible
  • Can result in higher costs and complexities
  • May lead to vulnerabilities if not implemented carefully

Subject-Object Model​

The subject-object model is used in access control to define who is making an access request and what resource is being requested. This framework helps clarify and enforce access policies.

  • Subject: requesting the access
  • Object: resource being requested

Example:

  • A user requests access to a file
  • A process requests access to a memory location

It's useful to describe access requests in these terms to make the requests explicitly clear.

Failure Modes​

Failure modes define how systems behave during failures.

  • Fail-open

    • Allows all traffic through if a failure occurs
    • Does not inspect or filter traffic
    • Prevents network service disruptions

  • Fail-closed

    • Blocks all traffic during a failure
    • Maintains network security
    • May cause temporary service disruption

  • Fail Reset

    • Automatically restarts when a failure is detected
    • Restore operations without verifying secure access
    • Helps recover quickly but may expose some risks

  • Fail-secure

    • Defaults to the highest security level during faults
    • Ensures predictable and secure system behavior
    • Prevents unauthorized access in error conditions

  • Fail safe

    • Mechanism fails to a state that ensures personnel or equipment safety
    • Example would be a normally locked door in a data center, which would fail to an unlocked state if a fire alarm is triggered.

  • Fail open

    • Mechanism would fail in the event of a fault
    • Moves to a state that protects people or equipment
    • Prioritizes safety over normal operation
    • Example: A data center door unlocks during a fire alarm

Example: Firewall

  • Fail-open can allow unfiltered traffic to reach the network
  • Fail-closed ensures security is maintained during downtime
  • Decision depends on the priority of security versus availability

Example: IDS

  • Failure costs might not justify a fail-secure mode
  • Continuation of monitoring is important
  • Fail-open can ensure monitoring without disrupting operations

Isolation and Segmentation​

Isolation and segmentation divide systems to limit risks and contain threats, enhancing overall security by controlling access.

  • Network Segmentation

    • Divides a network into smaller, isolated segments
    • Limits exposure to threats
    • Restricts access to critical systems
    • Uses firewalls and access controls

  • Process Isolation

    • Keeps processes independent
    • Prevents interference between processes
    • Increases system stability

  • Memory Segmentation

    • Divides memory into separate sections
    • Protects against unauthorized access
    • Helps prevent buffer overflow attacks

  • Virtual Machine Isolation

    • Separates VMs from one another
    • Ensures one VM’s compromise doesn't affect others
    • Provides a secure environment for applications