Short definition of Access Control List
An Access Control List (ACL) is a rule set that decides who (users, groups, devices) can do what (read, write, execute, forward, deny) to a specific resource (file, folder, database table, API, subnet, port).
What does an Access Control List (ACL) do?
An ACL defines and enforces permissions. It tells systems who can access a resource and what they are allowed to do (e.g., view, edit, delete, forward traffic). Without ACLs, sensitive data or systems could be open to anyone.
What is the access control model ACL?
The ACL model is a list-based access model. Each resource (a file, folder, device, or subnet) has its own permission list, specifying what actions each user or group can take.
Example: “Finance group = Read/Write, HR group = Read only, Guests = Deny.”
This differs from RBAC (Role-Based Access Control) or ABAC (Attribute-Based Access Control), which apply rules more broadly. ACLs are more resource-specific and granular.
What are the 4 types of ACLs?
There are two common ways to categorize ACLs, depending on the context:
1. By Resource/Scope (general IT/security view)
When discussing ACLs broadly across files, networks, cloud, and apps, the “types” usually mean where ACLs are applied:
File System ACLs (Windows NTFS, Linux POSIX)
Network ACLs (router/firewall/cloud subnet rules)
Application/API ACLs (software/services)
Cloud/Object ACLs (AWS S3, Azure Blob, GCP Buckets)
This is the cross-domain view, mostly talked about and applied by enterprise IT consultants, because it ties ACLs to business services and compliance.
2. By ACL Functionality (network/security engineering view)
In network engineering (Cisco, firewall, router, AWS VPC, etc.), the “four types of ACLs” are:
Standard ACLs → filter only by source IP.
Extended ACLs → filter by source + destination IP, protocol, port numbers.
Dynamic ACLs → require user authentication before access is granted.
Reflexive ACLs → session-aware; allow only return traffic from established sessions.
This is the network-specific view, very accurate when discussing firewalls, routers, and packet filtering.
Both are correct: the first view is broader (useful for IT leaders), while the second is more technical (used by network/security engineers).
How to set up access control lists?
Setting up an ACL involves four steps:
Identify the resource you want to protect (file, folder, subnet, or app).
Define principals (users, groups, IP ranges, or devices).
Assign permissions (allow/deny + actions like read/write/execute/forward).
Test & monitor to confirm rules behave as expected and don’t accidentally block valid access.
Best practice: Start with “deny all by default”, then add explicit allow rules.
What Is a Network Access Control List (ACL)?
A network ACL filters packet flows at IP/port level. They are stateless (especially in cloud contexts like AWS), meaning inbound and outbound rules must be defined separately.
Example: Allow HTTPS (443) from branch office, deny everything else.
Network ACLs are widely used for segmentation, compliance, and baseline traffic filtering.
How ACLs Work
Rule-Based Permissions: Ordered lists of allow/deny entries.
Resource-Specific Application: Rules apply to the resource they’re attached to.
Filtering & Enforcement: Requests are matched top-down; first hit applies, otherwise a default deny closes the gap.
Key Benefits of Access Control List (ACL)
Least-Privilege Security: Tight, resource-level control reduces blast radius.
Compliance Alignment: Map-able to SOC 2, HIPAA, GDPR access-control requirements.
Forensics & Auditability: Clear, reviewable rules; log -able decisions.
Segmentation: Network ACLs help isolate environments (prod vs. dev, PCI zones).
Operational Safety: Prevents accidental changes or exfiltration by limiting write/export paths.
Common Pitfalls of Access Control Lists (and how to avoid them)
Over-permissive “allow any” rules → Start with deny-by-default, permit known flows only.
Shadowed rules (never hit due to order) → Use tooling to detect and prune.
Stale principals (ex-employees, retired service accounts) → Automate off-boarding hooks from HR/IdP.
Config drift across sites/clouds → Source-control ACLs; standardize via templates and CI checks.
Relying on ACLs alone → Pair with MFA, RBAC/ABAC, and continuous monitoring (SIEM).
ACLs vs. Similar Controls (quick guide)
Firewall policies: broader security feature set; often stateful. ACLs are the building blocks.
Security Groups (cloud): stateful instance-level rules; NACLs are stateless subnet rules.
RBAC/ABAC/IAM: scalable identity-centric authorization; use ACLs for resource-level nuance.
Zero Trust/Micro-segmentation: strategy; ACLs enforce its “verify explicitly, least privilege” principle.
When to Apply Access Control Lists (ACLs)
M&A/user migrations, cloud re-segmentation, or audit findings.
You suspect excessive access but lack visibility/tooling.
You need cross-platform consistency (Windows, Linux, network, cloud).
Business Value of Access Control List
Access Control Lists (ACLs) are foundational to Zero Trust security. Mid-market companies often lack visibility into “who has access to what.” A structured ACL strategy reduces risk exposure and audit pain.
