An S3 bucket policy is a resource-based policy: instead of attaching permissions to a user or role, it lives on the bucket itself and names who — the Principal — is allowed to do what. That inversion trips people up constantly, especially the difference between a bucket-level ARN (arn:aws:s3:::my-bucket, needed for actions like listing) and an object-level ARN (arn:aws:s3:::my-bucket/*, needed for reading or writing individual objects), and the fact that a principal of "*" with no restricting condition means the bucket, or the objects in it, are reachable by literally anyone on the internet. S3 Bucket Policy Generator is a focused form for building exactly this shape of policy correctly.
You pick a principal type — Everyone (the public wildcard), a specific AWS account, or a specific IAM role or user ARN — enter the bucket name, choose which common S3 actions to allow from checkboxes (s3:GetObject, s3:PutObject, s3:ListBucket), and decide whether the statement should apply to the bucket itself, to the objects inside it, or to both — which controls whether the generated Resource field is the bare bucket ARN, the bucket ARN with a trailing /*, or an array containing both. An optional condition lets you restrict access further, most commonly by source IP address, without leaving the specialized form.
The one piece of real logic beyond simple field assembly is the public-access check. After building the statement, the tool inspects the resolved Principal and Condition: if the principal resolves to the AWS-wide wildcard * and there is no condition block restricting it in any way, a prominent red warning banner appears above the JSON output, explaining plainly that this statement grants access to anyone on the internet with no restriction. This is not a static disclaimer printed regardless of input — it is computed from the actual statement you built, so scoping the principal to a specific account or role, or adding an IP restriction, makes the warning disappear because the underlying condition genuinely no longer holds. That mirrors, on a much smaller scale, the kind of check AWS's own S3 Block Public Access and Access Analyzer features perform, though this tool only inspects the single statement you are currently building — it does not evaluate a bucket's full existing policy, its Block Public Access settings, ACLs, or any cross-account access that other statements or IAM policies elsewhere might grant.
Once built, the same structural validator used across these tools runs against the assembled document — confirming Version is present, the statement has a recognized Effect, an Action, and either a Resource or the Principal that resource-based policies use in its place. That check, plus the public-access warning, is the full scope of what gets verified: nothing here calls the AWS API, checks whether the bucket actually exists, or accounts for other policies, ACLs, or account-level Block Public Access settings that would also affect real-world access.
The result is a ready-to-attach bucket policy JSON document, with stat cards summarizing the statement and a clear warning when the combination you built amounts to public access — built and checked entirely in your browser, with the bucket name and any account IDs you enter never leaving the page.