Exploiting AWS S3 Bucket Misconfigurations

When companies migrate to the cloud, there is a dangerous assumption that because infrastructure is hosted by Amazon, Microsoft, or Google, it is automatically secure.

But the cloud operates on a Shared Responsibility Model. Amazon is responsible for the security of the cloud (the physical servers, the hypervisors). You are responsible for security in the cloud (your data, your passwords, your permissions).

The most devastating cloud breaches rarely involve sophisticated zero-day exploits. They usually involve a developer making a simple spelling mistake in an Access Control List (ACL), leaving massive databases completely open to the public.

What is an S3 Bucket?

In Amazon Web Services (AWS), an S3 (Simple Storage Service) bucket is essentially a massive, infinitely scalable folder in the sky. Companies use them to store website images, daily database backups, customer invoices, and internal source code.

Every bucket has a unique URL, usually formatted like this: https://[bucket-name].s3.amazonaws.com

By default, AWS makes all new S3 buckets strictly private. However, developers often need to make certain files (like website logos) public. The vulnerability occurs when they accidentally apply public permissions to the entire bucket rather than just the specific files.

The Flaw: The AnyAuthenticatedUser Trap

AWS permission policies use JSON. A common, fatal mistake happens when a developer tries to grant access to “all authenticated users” in their company. They select the built-in AWS group called AnyAuthenticatedUser.

The fatal misunderstanding: In AWS, AnyAuthenticatedUser does not mean “users who are logged into my company’s app.” It means “anyone on planet Earth who has a free, valid AWS account.”

If a bucket has this permission, a hacker can simply create a free AWS account, generate a set of access keys, and read the entire contents of the bucket.

The Attack Flow: Finding the Leaks

How do attackers find these invisible buckets?

Step 1: Reconnaissance (Bucket Hunting)

Attackers use automated tools to guess bucket names based on a company’s name. If a target is AcmeCorp, an attacker will use a tool like cloud_enum or lazys3 to rapidly check names like:

• acmecorp-backup
• acmecorp-dev
• acmecorp-staging
• acme-db-dumps

They can also use advanced Google Dorking: site:s3.amazonaws.com "acmecorp"

Step 2: The Check

Once a potential bucket URL is found, the attacker uses the official AWS Command Line Interface (CLI) to poke it. They attempt to list the contents (ls) without providing any password (--no-sign-request).

# Asking the bucket to list its files anonymously
aws s3 ls s3://acmecorp-backup --no-sign-request

If the bucket is misconfigured, the terminal will light up with a directory listing:

2026-03-10 14:00:00 4503221 production_db_backup.sql
2026-03-11 09:30:00 12055   employee_passwords.csv
2026-03-12 11:15:00 889422  customer_kyc_scans.zip

Step 3: Data Exfiltration (and worse)

The attacker simply uses the cp (copy) command to download the sensitive files to their local machine.

aws s3 cp s3://acmecorp-backup/employee_passwords.csv . --no-sign-request

The Escalation (Write Access): Sometimes, the misconfiguration is so bad that the bucket allows public Write access. If a company’s website serves its Javascript files from this bucket, an attacker can upload a malicious Javascript file, overwrite the legitimate one, and instantly infect every user visiting the company’s website (a Cloud-based supply chain attack).

Defense: Sealing the Bucket

Amazon has realized how common this is and introduced several heavy-handed defenses, but they must be enabled:

1. Block Public Access (BPA): AWS now offers a master switch at the account level called “Block Public Access.” If this is flipped on, AWS will actively override any individual bucket policy that tries to make data public.
2. Least Privilege Policies: Never use wildcards (*) in IAM policies or S3 Bucket Policies unless strictly necessary. Grant access to specific IAM roles, not to the public.
3. Cloud Security Posture Management (CSPM): Enterprise teams should use tools like AWS Macie or open-source tools like ScoutSuite to continuously scan their cloud environments and trigger alerts the second a bucket permission is changed to public.

Summary for the Exam

• S3 Bucket: AWS object storage, frequently misconfigured to allow public Read/Write access.
• Shared Responsibility Model: The cloud provider secures the hardware; the customer secures the data and permissions.
• Enumeration: Using brute-force tools (lazys3, cloud_enum) to guess bucket names.
• Impact: Massive data exposure, website defacement, or malware hosting if write access is allowed.