What is an Access Control List (ACL)? Basics &
Best Practices

Access control lists (ACLs) support enterprise security by helping organizations manage who can access a given resource and what privileges they have if granted access. 

ACLs might apply to a broad system or network, or even just one file folder. Plus, they may be used alongside other security mechanisms to safeguard sensitive resources and efficiently filter network traffic.  

In this guide, we’ll explore what access control lists are, how they work, and best practices for their implementation and use for optimal security results. We’ll even discuss some of the more advanced access control systems that are better suited to modern organizations.

An access control list (ACL), also referred to as an access list, is a set of rules that specify which users can access which resources. Traditionally, an ACL dictates how a given system should filter traffic on a specific network. ACLs enhance digital security, working to protect sensitive company resources like documents, data, networks, and more by limiting their access to authorized users only.

The basic idea is that an ACL will evaluate incoming and outgoing data packets and compare their attributes against pre-set rules to determine whether to grant or deny access. At one point, ACLs were the primary mechanism for firewall protection.

Though ACLs are now an outdated model of access control, they were once implemented by enterprises as a straightforward way to manage access to resources and filter network traffic. ACLs are favored for offering benefits like:

• Enhanced Security: Prevent intrusions or unwanted traffic to protected resources from unauthorized users. Only users with the proper permissions are granted access. Others are denied.
• Course-Grained Control: Administrators get some level of control over who has access to specific resources, and which actions they’re permitted to do (e.g. view, modify, execute).
• Scalability: New lists can be configured as needed to accommodate an expanding network, supporting security for new applications, documents, files, etc. As new employees are hired or as tenured workers change roles, ACLs can be updated to reflect the new permissions.

In older, more basic systems, administrators could set up a static list of permissions for each document or resource, dictating which users or groups were authorized to access it.

For example, maybe a financial services institution allows all employees in the “teller” role to access basic customer account and transaction information. But, only those in the “lender” position are able to view a customer’s credit history and comprehensive financial profile. Thus, the ACL would need to be configured properly to reflect these permissions.

ACLs were able to enhance security for simpler networks and systems. However, as security threats have evolved and enterprise systems have grown more complex, more sophisticated access control models like role-based access control (RBAC) and policy-based access control (PBAC) are better suited to provide enterprise-level security.

An access control list typically consists of a few key elements, including:

1. Sequence Number: A series of numbers used to identify an ACL entry.
2. Access Control List Name: A unique identifier given to a specific ACL to distinguish it from others. Can be used in place of a sequence number, though you may be able to create a name that uses a combination of letters and numbers.
3. Remark Statement: Additional information or a description that you may add to ACL entries. This may explain the purpose or rationale behind a given rule within the list.
4. Network Protocol: How the ACL allows or denies access to network protocols like IP, TCP, UDP, etc., based on access policies or other criteria.
5. Source and Destination Address: A list of permitted IP addresses as a source or destination to filter out unwanted traffic or unauthorized users and apply the appropriate permissions.
6. Wildcard Masks: Used to identify which bits in an IP address should be matched to the ACL.
7. Action (Permit or Deny): Access to a certain resource will be permitted or denied based on the IP address and rules of the ACL. Certain systems may allow you to set custom statements that appear to users when an action is determined.

There are multiple types of access control lists, and they are each better suited to certain scopes and use cases. These include:

Standard Access Control Lists

A standard access control list is the most common variation. It’s applicable in basic systems and networks, filtering just the source address of a data packet. Given the simplistic nature of a standard ACL, they require less processing power. However, their scope is limited.

Extended Access Control Lists

Extended ACLs are also commonly used. However, they can evaluate both the source and destination IP addresses and port numbers to filter traffic. Thus, they are more resource-intensive, though they do offer additional layers of security than a standard access control list.

Named Access Control Lists

As the term might suggest, a named access control list allows administrators to assign a name to specify a list. Named lists can either be standard or extended ACLs. They make it easier for organizations to manage access for large groups, as they can be applied to multiple interfaces or devices without the need to recreate the entire list each time.

Dynamic Access Control Lists

Also referred to as lock-and-key lists, a dynamic access control list can filter traffic from a specified source or destination with user authentication. They are particularly useful in retail and financial services industries that handle large volumes of sensitive information.

Dynamic ACLs provide an additional layer of security to protect data from unauthorized access while still ensuring the appropriate employees can access the resources they need to fulfill their duties.

Reflexive Access Control Lists

Reflexive access control lists are a version of extended ACLs that offer IP session-based control. The purpose of these lists is to allow IP traffic from internal networks, and deny traffic that originates from external or unknown IP addresses.

These are particularly useful for combatting spoofing attacks that are initiated by external users, but meant to deceive by appearing as an in-network user.

An access control list notifies a network’s operating system of the specific permissions that each user has for a given resource or object, filtering traffic accordingly. Every object within a system, be it a file, document, or application, has a security framework connecting it to its ACL, which details the specific rights that each user possesses.

Typically, the main privileges users can have include the ability to read or view a file, write a file, or execute a file. To get a better idea of how an ACL operates and grants these privileges, here’s an overview of how it works to process traffic within network devices.

1. The user makes an access attempt (they want to edit a file, use an application, etc.).
2. Devices like a firewall or router inspect the incoming data packet, noting details like the IP address or port number.
3. The system references the ACL for the applicable entry based on the packet’s attributes.
4. The data packet is compared against the list of rules in the ACL to look for any matching criteria.
5. The system will look for the first rule that matches and enforce the appropriate action. If there is no match, the access request will generally be denied. Otherwise, the packet can continue on to the desired object.

As we’ve mentioned throughout, there are more sophisticated mechanisms available to enterprises for managing user access. However, those who still use access control lists can adhere to the following tips and best practices for effective implementation and maintenance.

Regular Review and Updates

Administrators should ensure access control lists are up-to-date with current security needs. This can help curb issues of permissions creep or excessive privileges, where users have more permissions than are necessary to complete their duties.

Thus, teams should regularly review ACL rules and adjust them as needed to reflect updated company policies or changes to organizational structure.

Minimizing Complexity

Rules in an ACL should be clear and specific to help avoid security vulnerabilities. However, they also shouldn’t be overly complex or restrictive, which can lead to friction in the user experience and be harder to manage or update. Rules that are too complex may even cause a higher risk of error, as it may be more difficult to identify potential gaps in security.

Documentation

Those in charge of maintaining access control lists should thoroughly document all rules within an ACL, providing insights into the purpose and reasoning behind them. It can also be beneficial to keep audit logs of all access attempts, which can help organizations investigate security incidents or threats more thoroughly.

Testing Access Control Lists

Implementing ACLs shouldn’t be a “set it and forget” security approach. Administrators should regularly test the list to ensure that it’s producing the proper results, like denying or permitting the appropriate actions. Otherwise, the ACL may require a closer look.

Especially after making an update to any rules within the ACL, running a test first can help ensure that it’s providing the desired protection. This can also ensure your ACLs are ordered correctly, making sure no policies are ignored or unenforced.

Organizations can configure ACLs using Linux or Windows. Each platform offers unique value and benefits that might be better for certain use cases, such as:

• Linux Access Control Lists: Linux is known for being the more flexible environment for building ACLs. However, the tradeoff of greater customizability and granularity comes with higher complexity. Thus, these systems can be more complex to maintain than Windows.
• Windows Access Control Lists: Windows is an easier-to-manage platform, though it doesn’t offer the same flexibility and versatility as Linux. For instance, the ability to make kernel modifications is only supported in Linux.

As we alluded to earlier, role-based access control, or RBAC for short, is often a better model for modern enterprises to manage user access than ACLs alone.

The key difference between the two is that with RBAC, users are granted specific permissions based on their role in the company rather than on an individual level, like with an ACL. Thus, RBAC can be more scalable, as it’s not as tedious for companies to manage. Administrators can apply broad rules to all employees in the same position, keeping them from managing individual access rights for each user.

In general, RBAC is more useful in situations like:

• Organizations that have standardized job functions, meaning roles can be created around specific duties and applied to each user in a given position.
• Large enterprises with hundreds or thousands of employees, in which case it wouldn’t be practical to apply individual access permissions to each user.
• Industries with strict auditing requirements, making it easier to track permissions and demonstrate compliance with industry standards.

Still, an ACL could be the better choice in scenarios such as:

• Environments where administrators need granular control over individuals’ access to highly specialized resources.
• Cases when users only need access to a given resource temporarily, like for an ad hoc project.
• Smaller organizations that only have a few employees or groups, meaning it’s more feasible to control access on an individual basis.

Fortunately, organizations don’t need to choose between one or the other. In fact, they can enjoy more comprehensive access control by combining both RBAC and ACLs into one security framework.

ACLs are a critical foundation for modern enterprise security. They help prevent unauthorized access to sensitive company resources and give organizations granular control over network traffic.

However, in today’s dynamic environment, mechanisms like RBAC or PBAC are much better equipped to mitigate growing security threats and scale with operations — all while delivering a seamless user experience.

Thus, while ACLs can certainly lay the groundwork for a solid fraud prevention strategy, they should be just one aspect of an organization’s broader security infrastructure. A modern approach to cybersecurity moves away from a “trust but verify” perimeter-centric approach to a “never trust, always verify” model to keep bad actors out and prevent cyber attacks.

With an identity access management (IAM) solution like Ping Identity, you can seamlessly integrate this Zero Trust model throughout the entire security stack, ensuring only the right people have access to the right things in the proper context.