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HomeCybersecurity 101
What is an ACL?

What's an Access Control List (ACL)?

Written by: Brenda Buckman

Published: May 22, 2025

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Access control list defined

An Access Control List is essentially a set of rules that define who or what can access specific resources, such as files, directories, or networks. Originally, ACLs were introduced as a foundational security mechanism, providing a straightforward way to restrict unauthorized access. While newer technologies like RBAC and zero-trust frameworks have emerged, ACLs maintain their relevance due to their simplicity, flexibility, and effectiveness in both small-scale and enterprise-wide applications.

Think ACLs are outdated? Think again.

The continued use of ACLs speaks volumes to their ability to protect while also meeting these needs:

  • Granular control: ACLs allow administrators to specify access permissions at a detailed level.

  • Versatility: They operate across various platforms, including file systems, routers, and operating systems.

  • Cost-effective security: ACLs don’t require additional licenses or complex configurations, making them a budget-friendly tool for small businesses and large enterprises alike.

Two main types of ACLs

Though the term “ACL” applies to multiple scenarios, the two primary types are Filesystem ACLs and Networking ACLs.

Filesystem ACLs

These control access to files and directories within an operating system. Permissions for different users or processes are defined, determining actions like reading, writing, or executing files. For instance:

  • Linux ACLs: Allow administrators to add more granular permissions beyond basic categories (user, group, others).

  • Windows ACLs: Present a user-friendly interface to define file permissions, though they offer less complexity compared to their Linux counterparts.

Networking ACLs

Networking ACLs filter traffic at the router or switch level based on predefined rules. They can determine which data packets are allowed to travel through or be blocked. Two common types include:

  • Standard ACLs: Focus only on the source IP addresses. These are simpler but limited in scope.

  • Extended ACLs: Offer greater flexibility by filtering based on both source and destination IP addresses, protocols, and port numbers.

    • Numbering Ranges for Networks:

      • Standard ACLs: 1–99, 1300–1999

      • Extended ACLs: 100–199, 2000–2699

4 reasons why ACLs are used

ACLs provide several key benefits that help organizations of all sizes strengthen their cyber defenses and enhance performance. These include:

1. Traffic control for networks

ACLs filter unnecessary or harmful traffic at the router level, improving network efficiency and minimizing vulnerabilities. For example, blocking malicious IP addresses reduces the risk of DDoS (Distributed Denial of Service) attacks.

2. Protecting sensitive data

With file system ACLs, organizations can enforce restrictions on which employees have access to confidential files. Only authorized users can view, edit, or delete critical data.

3. Enhancing visibility

By implementing ACLs, organizations gain insight into who's accessing systems or data, and how. This helps with compliance and auditing processes, ensuring accountability.

4. Supporting encryption

When paired with technologies like Virtual Private Networks (VPNs), ACLs specify what types of encrypted traffic are allowed on the network, bolstering secure communication.

Behind the scenes — how ACLs work

At their core, ACLs work by applying rules to determine permissions for users, systems, and traffic. Here’s how that process typically unfolds:

  1. Permission definition: ACL rules specify who or what (user, IP address, system) is permitted to interact with a resource.

  2. Rule matching: Requests for access or data packets are evaluated against the ACL rules.

  3. Allow or deny: Based on the criteria, access is either granted or denied.

For example, a network ACL might have an entry stating, “Allow all traffic from 192.168.1.0/24 to port 80,” ensuring web traffic is accessible to the internal network but restricting other protocols.

ACL best practices

Proper implementation of ACLs is essential to ensure they function effectively without negatively impacting performance. Here are some best practices:

  • Apply ACLs thoughtfully: Use them sparingly on critical resources and interfaces to prevent unintended disruptions.

  • Order rules strategically: Place the most frequently triggered rules at the top of the list to speed up processing.

  • Regular audits: Document and review ACLs periodically to ensure continued relevance and security effectiveness.

  • Group logically: Use logical groupings of similar rules to make ACL management simpler and more efficient.

ACL vs. RBAC

Role-Based Access Control (RBAC) is a security model used to manage user permissions based on their role within an organization, rather than assigning permissions to each individual user.

Key components of RBAC:

  1. Roles: A collection of permissions tied to a job function.

  2. Users: The people or identities assigned to one or more roles.

  3. Permissions: Access rights to specific systems or data.

  4. Sessions: A mapping between a user and an activated subset of roles.

Benefits of RBAC:

  • Scalability: Easily manage access for large numbers of users.

  • Security: Ensures only users with appropriate roles can access sensitive data.

  • Efficiency: Simplifies permission updates when roles change.

  • Compliance: Helps meet regulatory standards like HIPAA, GDPR, or SOX by enforcing least privilege.

RBAC vs ACL:

  • RBAC: Access is granted based on the user’s role. Better for organization-wide permissions.

  • ACL (Access Control List): Access is granted on a per-user or per-object basis. Better for fine-grained control.

While ACLs shine in controlling specific user access, RBAC is ideal for larger organizations that need to manage access at scale. The choice often depends on the extent and complexity of your access requirements.

ACLs will continue to have a role in security

Despite the rise of new access control methods, ACLs aren't disappearing anytime soon. They remain a valuable tool in the IT security toolbox for cases requiring simplicity, precision, and cost-effectiveness. Whether it’s integrating ACLs with VPNs for secure traffic or enforcing file permissions to protect sensitive data, they offer reliable security enhancements.

Still, no organization can solely rely on ACLs. Pairing them with broader identity management systems like RBAC or zero-trust frameworks can deliver a more comprehensive defense.

Security isn’t static, and neither should your policies be. A proactive approach leads to a more secure environment, giving you the competitive advantage you need.

Access Control Lists (ACLs) might sound old school in the era of advanced firewalls and sophisticated Role-Based Access Control (RBAC) mechanisms. However, their ability to provide granular control over access to systems and data has cemented their importance in IT infrastructure. From securing networks to protecting sensitive files, ACLs remain a powerful tool and should not be slept on. 

In this guide, you should expect to learn what an ACL is, the different types, why many businesses still rely on them, and how they stack up against other access management tools. 

Whether you're an IT professional, cybersecurity analyst, or business leader, understanding ACLs is a crucial step in enhancing your organization's security posture.

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