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Access Control List

TL;DR: An Access Control List (ACL) is a security mechanism that defines rules for controlling access to resources by specifying which users or systems can perform specific actions.

What is an Access Control List (ACL)?

An Access Control List (ACL) is a set of rules used to manage permissions for users, groups, or systems attempting to access a resource, such as files, networks, or applications. ACLs are commonly used in operating systems, network security, and various controlled environments to enforce security policies and restrict unauthorized access.

For example, in a network firewall, an ACL can determine which IP addresses are allowed or denied access to a server. In controlled entry systems, an ACL dictates who can enter specific areas based on predefined permissions.

How Does an ACL Work?

An ACL works by evaluating incoming requests against predefined rules. These rules define:

  1. Subjects: The users, groups, or systems that request access.
  2. Objects: The resources being accessed (e.g., files, networks, secured areas).
  3. Actions: The permitted or denied operations (e.g., read, write, enter, execute).
  4. Rules Order: ACLs are often evaluated sequentially, meaning the first matching rule determines access.

For example, in an entry control system:

  • Allow access to employees with valid keycards between 8 AM - 6 PM.
  • Deny access to non-registered visitors.

Types of ACLs

  1. File System ACLs – Used in operating systems (e.g., Windows NTFS, Linux ext4) to manage file and folder permissions.
  2. Network ACLs – Used in routers, firewalls, and cloud security groups to filter traffic based on IP addresses, ports, and protocols.
  3. Entry Control ACLs – Used in controlled environments to restrict access to buildings or specific areas based on credentials.
  4. Database ACLs – Restrict database access based on user roles and privileges.
  5. Application ACLs – Define access rules within software applications, such as role-based permissions in enterprise applications.

How to Implement an ACL

  1. Identify Resources – Determine which resources require access control (e.g., files, networks, restricted areas).
  2. Define Rules – Set up rules specifying who can access what, and under what conditions.
  3. Apply ACLs – Configure ACLs in firewalls, operating systems, or controlled access systems.
  4. Monitor and Audit – Regularly review and update ACLs to ensure security compliance and efficiency.

For example, in a keycard system, access control lists can be programmed to:

  • Grant employees access to office areas during working hours.
  • Allow cleaning staff access to designated areas during specified time slots.
  • Restrict unauthorized personnel from entry at all times.

Conclusion

Access Control Lists (ACLs) are essential for managing security by controlling access to resources. Whether applied in file systems, networks, or controlled environments, ACLs help enforce security policies, protect sensitive data, and mitigate unauthorized access risks. Regular audits and updates ensure ACLs remain effective against evolving threats.