what is acl in aws
ACL in AWS primarily refers to Access Control Lists , which are mechanisms to manage permissions for resources like S3 buckets/objects or VPC subnets. There are two main types: S3 ACLs for object-level access and Network ACLs (NACLs) for subnet traffic control.
S3 Bucket ACLs
These legacy ACLs define who can access S3 buckets or objects (e.g., READ,
WRITE permissions for AWS accounts or groups).
Each bucket/object has an attached ACL as a subresource; S3 checks it before
granting access.
Note : AWS recommends IAM policies over ACLs for finer control, as ACLs are being phased out for new buckets.
Permission| Action Examples| Use Case
---|---|---
READ| s3:GetObject| Public reads
WRITE| s3:PutObject| Uploads/modifies
READ_ACP| s3:GetBucketAcl| View ACL
FULL_CONTROL| All above| Complete access 7
Network ACLs (NACLs)
NACLs act as stateless firewalls at the subnet level in a VPC, filtering
inbound/outbound traffic by IP, port, protocol (ALLOW/DENY rules).
Rules process in numbered order (lowest first, e.g., rule 100 before 200); an
implicit DENY ends the list.
Unlike stateful Security Groups, NACLs require explicit rules for return traffic—great for broad subnet protection.
Example Inbound Rules (evaluated top-to-bottom):
| Rule # | Type | Protocol | Port | Source | Allow/Deny |
|---|---|---|---|---|---|
| 100 | HTTP | TCP | 80 | 0.0.0.0/0 | ALLOW |
| 200 | ALL | ALL | ALL | 0.0.0.0/0 | DENY |
| * | ALL | ALL | ALL | 0.0.0.0/0 | DENY |
Key Differences: ACLs vs. Security Groups
Feature| S3 ACLs| NACLs| Security Groups
---|---|---|---
Scope| Buckets/objects| Subnets (VPC)| Instances/ENIs
Stateful?| N/A| No| Yes
Granularity| Account/group perms| IP/port rules| Instance-level
Default| Private| Allow all| Allow all 39
Best Practices & Trends (2026)
- Prioritize IAM/Bucket Policies over S3 ACLs for security (ACLs disabled by default on new buckets since 2023).
- For NACLs, use low rule numbers for DENYs first; associate with multiple subnets but monitor ephemeral ports (1024-65535).
- Trending : Forums note rising NACL use in hybrid VPCs amid 2025 AWS VPC updates—pair with WAF for app-layer defense. No major 2026 changes reported.
TL;DR : ACLs grant/deny access—S3 for storage perms, NACLs for network firewalls. Use modern IAM where possible.
Information gathered from public forums or data available on the internet and portrayed here.