An access control list (ACL) is a set of rules associated with a system resource that specifies which users, devices, processes, or other entities may access it and what they are allowed to do. In short, it connects an entity, a resource, and permitted access rights.
What is an access control list?
NIST defines an ACL as a mechanism that controls access to a system resource by listing the entities permitted to access it and the access modes granted to each. An entity might be a person, a group, a machine, or a process; a resource might be a file, application, service, or network interface. The allowed access modes depend on the system—for example, a resource may distinguish reading from changing it.
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Think of an ACL as a permissions list attached to something being protected: it records who or what can use that resource and which rights apply. The term is used across different technologies, so an ACL does not have one universal format or set of rules. NIST’s glossary definition of access control list provides the general meaning.
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When a subject—such as a user or process—tries to use a resource, the system evaluates the relevant access rules to decide whether the requested operation is allowed. Access control is the broader decision to permit or deny a subject access to system objects; an ACL is one mechanism for expressing permissions that inform such decisions. NIST’s access control definition describes that broader decision.
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For example, a file ACL might associate a user or group with rights to read or modify that file. A network ACL can instead contain rules about traffic, such as which addresses or protocols are permitted. Both are called ACLs because they express access rules, but their entries and behavior differ.
What does an ACL entry contain?
At a conceptual level, an ACL entry ties an entity to one or more rights on a particular resource. The details vary by implementation, but the key questions are:
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- Resource: What object or traffic is protected?
- Entity or match: Which user, group, process, device, address, or other attribute does the rule apply to?
- Right or action: What access is granted or denied?
These questions are useful for understanding an ACL, but they do not imply that every ACL uses the same fields, syntax, or evaluation logic.
How do network ACLs differ from file or operating-system ACLs?
The resource and the rule format change with the implementation. In a filesystem or operating-system context, rules commonly concern entities and their rights to an object. In Cisco’s documented NCS 1010 networking example, an ACL is a sequential list of permit and deny statements applied to packet addresses and upper-layer protocols. Cisco also states that the list must be referenced by a command that accepts it before it affects traffic. That behavior describes this network context, not every ACL in every system. See Cisco’s NCS 1010 ACL documentation.
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| Aspect | File or operating-system ACL | Cisco NCS 1010 network ACL example |
|---|---|---|
| Protected resource | A file or other system object | Network traffic |
| Rule target | An identity, such as a user, group, machine, or process | Packet addresses and upper-layer protocols |
| Rule form | Permissions associated with an object; exact format depends on the implementation | A sequential list of permit and deny statements |
| How rules take effect | Depends on the operating system or product | The ACL must be referenced by a command that accepts it |
| Conflict and order behavior | Not uniform across implementations, according to NIST SP 800-12 | Sequential evaluation is described for this Cisco example |
Do ACLs always apply rules in the same order?
No. Rule ordering, conflict resolution, and default behavior depend on the implementation. NIST’s SP 800-12 handbook cautions that implementations do not share uniform rules for resolving apparently conflicting ACL entries. Cisco’s sequential permit/deny description is specific to its documented network ACL context; it should not be assumed to describe file permissions or another platform. For a real system, consult its own documentation before relying on a particular rule order or default.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are ACLs good at—and what can be harder?
ACLs make it natural to ask who can access a particular resource and what rights that access includes. The resource-centered design can make a different audit question more difficult: identifying every privilege one user holds across many separate objects. NIST SP 800-12 discusses this distinction and the variation among implementations. Because that handbook is legacy general guidance, use current product documentation for platform-specific administration.
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