Inspired by a conversation with Instacart's @nickelser on HackerOne, I've optimised and published Sandcastle – a Python script for AWS S3 bucket enumeration, formerly known as bucketCrawler.
The script takes a target's name as the stem argument (e.g. shopify) and iterates through a file of bucket name permutations, such as the ones below:

-training
-bucket
-dev
-attachments
-photos
-elasticsearch
[...]

Getting started
Here's how to get started:

1. Clone this repo (PyPi distribution temporarily disabled).
2. Run sandcastle.py with a target name and input file (grab an example from this repo)
3. Matching bucket permutations will be identified, and read permissions tested.

usage: sandcastle.py [-h] -t targetStem [-f inputFile]

arguments:
  -h, --help            show this help message and exit
  -t targetStem, --target targetStem
                        Select a target stem name (e.g. 'shopify')
  -f inputFile, --file inputFile
                        Select a bucket permutation file (default: bucket-
                        names.txt)

   ____             __             __  __
  / __/__ ____  ___/ /______ ____ / /_/ /__
 _\ \/ _ `/ _ \/ _  / __/ _ `(_-</ __/ / -_)
/___/\_,_/_//_/\_,_/\__/\_,_/___/\__/_/\__/

S3 bucket enumeration // release v1.2.4 // ysx


[*] Commencing enumeration of 'shopify', reading 138 lines from 'bucket-names.txt'.

[+] Checking potential match: shopify-content --> 403

An error occurred (AccessDenied) when calling the ListObjects operation: Access Denied

Status codes and testing

Status code	Definition	Notes
404	Bucket Not Found	Not a target for analysis (hidden by default)
403	Access Denied	Potential target for analysis via the CLI
200	Publicly Accessible	Potential target for analysis via the CLI

AWS CLI commands
Here's a quick reference of some useful AWS CLI commands:

• List Files: aws s3 ls s3://bucket-name
• Download Files: aws s3 cp s3://bucket-name/<file> <destination>
• Upload Files: aws s3 cp/mv test-file.txt s3://bucket-name
• Remove Files: aws s3 rm s3://bucket-name/test-file.txt

What is S3?
From the Amazon documentation, Working with Amazon S3 Buckets:

Amazon S3 [Simple Storage Service] is cloud storage for the Internet. To upload your data (photos, videos, documents etc.), you first create a bucket in one of the AWS Regions. You can then upload any number of objects to the bucket.

In terms of implementation, buckets and objects are resources, and Amazon S3 provides APIs for you to manage them.

Closing remarks

• This is my first public security project. Sandcastle is published under the MIT License.
• Usage acknowlegements:
  • Castle (icon) by Andrew Doane from the Noun Project
  • Nixie One (logo typeface) free by Jovanny Lemonad