What are Access Control Lists (ACLs) | Definition and Meaning

Access Control Lists, often abbreviated as ACLs, are crucial components in the realm of computer security and network management. They serve as a fundamental mechanism for implementing security policies by defining who can access specific resources within a system or network.

What is an Access Control List (ACL)?

An Access Control List (ACL) is a data structure that specifies permissions on an object or a resource. ACLs are widely utilized in various operating systems, network devices, and software applications to ensure that only authorized users or processes can access certain resources.

ACL Structure and Functionality

At its core, an ACL is a list of entries that stipulate which users or user groups have what kind of access rights to a specific resource. Each entry typically contains the following elements:

• Subject: This can be a user, group, or process that requests access.
• Action: The type of permission granted, such as read, write, execute, or delete.
• Resource: The specific object or resource to which access is being controlled.

ACLs operate by evaluating access requests against its defined rules, allowing or denying access based on the matching entries.

Types of Access Control Lists

Access Control Lists can be categorized in various ways, including:

1. Discretionary Access Control Lists (DACLs): These allow users to manage access rights to their own resources.
2. Mandatory Access Control Lists (MACLs): This type enforces stricter access controls laid down by a central authority, limiting user discretion.
3. File System ACLs: These are used to control access to files and directories at a system level, often seen in operating systems like Windows and Linux.

Examples of Access Control Lists

In practice, ACLs can take various forms. Here are a few common examples:

• Windows ACLs: These allow users to set permissions on files and folders, controlling access for different users or groups.
• Network Device ACLs: Common in routers and switches, these manage traffic flow based on varied conditions, such as IP addresses and protocols.

Example entry in a file system ACL:

User: Alice 
Permission: Read, Write

Access Control Lists vs. Firewalls

While both ACLs and firewalls contribute to network security, they serve different purposes. ACLs typically manage permissions at the data access level, controlling who can view or modify files and resources. In contrast, firewalls protect networks by monitoring and controlling incoming and outgoing network traffic based on predetermined security rules.

Access Control Lists can be used within firewalls to define more granular rules, allowing for a layered security approach. For instance, a firewall might utilize ACLs to permit or deny traffic based on IP addresses.

Access Control List Configuration

Configuring ACLs involves defining rules that lay down which user or processes have access to specific resources. This can include setting default ACL settings, removing ACL permissions, or adjusting configurations in systems like Windows or Linux.

# Example of setting ACL on a Linux file
setfacl -m u:alice:rw file.txt

Restoring Access Control List Permissions

Access rights can sometimes be misconfigured or altered due to system migrations, failures, or deliberate changes. It’s essential to maintain realistic and effective ACL permissions to ensure security.

Tools and commands are available across platforms for restoring default ACLs, such as icacls in Windows or getfacl/setfacl in Linux. These commands allow administrators to reset permissions or adjust them to adhere to an organization’s security policies.

Known Vulnerabilities in ACLs

While Access Control Lists are fundamental to network security, they are not without their vulnerabilities. Several well-known brands have reported vulnerabilities related to ACL misconfigurations or flaws, which could allow unauthorized access or control over network resources. Below are notable examples:

1. CVE-2024-20263: A vulnerability in the access control list (ACL) management within Cisco Business 250 and 350 series switches could allow an unauthenticated remote attacker to bypass protection offered by a configured ACL. This vulnerability arises due to incorrect processing of ACLs when the primary or backup switches undergo a full stack reload or power cycle.

2. CVE-2024-20291: This Cisco vulnerability affects the ACL programming for port channel subinterfaces of Nexus 3000 and 9000 series switches. It allows an attacker to send traffic that should be blocked through the affected device, potentially granting access to network resources that should be protected.

3. CVE-2023-20190: In Cisco IOS XR software, a vulnerability in the classic ACL compression feature could allow an unauthenticated remote attacker to bypass the protections offered by a configured ACL. This is due to incorrect encoding in the compression module that affects destination address ranges.

4. CVE-2023-30245: This vulnerability in Cisco’s Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software could allow attackers to bypass configured ACL rules through a logic error in the per-user-override feature.

5. CVE-2023-28357: In Rocket.Chat, the ACL checks in the slash command /mute occur after checking whether a user is a member of a channel, enabling unauthorized users to enumerate channel membership.

Understanding these vulnerabilities is crucial for system administrators, as it highlights the importance of regular updates and proper configuration of ACLs to mitigate risks and strengthen security practices.

Access Control Lists (ACLs) represent a pivotal mechanism in safeguarding resources across software, operating systems, and networked environments. By providing a manageable way to define and enforce access policies, ACLs help to ensure that sensitive information is only available to authorized users. Understanding how ACLs work, their types, and their relation to other security measures such as firewalls is essential for anyone involved in IT security or network management. Proper ACL configuration and management can significantly enhance the security posture of an organization, protecting it against unauthorized access and data breaches. However, awareness of known vulnerabilities is equally vital, underscoring the need for vigilance and proactive security measures.