Unseen but Deadly: How Forgotten S3 Buckets Become Breach Gateways

Introduction

Amazon Simple Storage Service (S3) is a foundational cloud storage solution used by organizations worldwide. Its scalability, reliability, and ease of use make it the backbone of cloud storage architectures. However, these same strengths can turn into security threats when misconfigured.

Over the years, numerous high-profile breaches have been traced back to unsecured S3 buckets. Organizations have accidentally exposed sensitive corporate data, personally identifiable information (PII), and even government records, making them prime targets for cybercriminals. Attackers exploit misconfigurations to exfiltrate data, inject malicious payloads, or hijack cloud infrastructure.

One of the most dangerous threats is Bucket Takeover, where attackers claim the names of deleted S3 buckets and intercept sensitive data, leading to serious breaches. This attack is especially insidious because it leverages the automation scripts and configurations that organizations forget to update after bucket deletion. Attackers can register a deleted bucket name, steal incoming data, and inject malicious payloads or hijack brand domains.

In this article, we break down the risks of misconfigured S3 buckets, explore real-world attack techniques, and outline best practices to secure AWS cloud storage.

Understanding the Risks of Amazon S3's Global Namespace

Unlike other AWS services that operate within specific regions, S3 uses a global namespace for bucket names. This design choice simplifies access but introduces security risks.

Key Risks of S3’s Global Namespace:

• • Bucket Takeover: When an organization deletes an S3 bucket, an attacker can claim the same bucket name. If automated scripts continue uploading data to this bucket, sensitive files may end up in an attacker-controlled environment.
  • Data Leakage: Publicly exposed S3 buckets can allow unauthorized users to access and download confidential data.
  • Subdomain Hijacking: Organizations often map S3 buckets to subdomains (uploads.company.com). If the bucket is deleted and an attacker claims the name, they can serve malicious content under the company’s domain.
  • Unintended Public Access: Misconfigured permissions (such as Authenticated Users access) can expose sensitive data to all AWS users.

How Attackers Exploit Misconfigured S3 Buckets

While AWS provides robust security controls, customers are responsible for enforcing them. Below are common misconfigurations and how attackers exploit them:

Common S3 Misconfigurations:

• Publicly Accessible Buckets: Buckets accidentally left open to the public expose critical files to the internet.
• Overly Permissive IAM Policies: Weak access controls grant unauthorized users excessive privileges.
• Lack of Encryption: Unencrypted data in S3 can be easily accessed if an attacker gains access to the bucket.
• Disabled Logging: Without AWS CloudTrail or S3 server access logging, organizations lack visibility into unauthorized access attempts.
• Misconfigured Cross-Account Access: Excessive permissions granted to external AWS accounts increase security risks.

Attack Scenario: S3 Bucket Takeover

One of the most dangerous attack vectors against S3 is bucket takeover, where attackers claim the names of deleted buckets and intercept sensitive data.

Step 1: Identifying a Deleted Bucket

Attackers use OSINT (Open-Source Intelligence), brute-force techniques, and automated tools to find previously used bucket names.

Techniques Used by Attackers:

• Scanning Public Code Repositories

1. 1. • Developers often hardcode bucket names in repositories like GitHub.
      • Attackers use tools like GitRob or TruffleHog to search for exposed S3 references.
      • Example command: git clone https[:]//github.com/target-org/repo.git && grep -Ri "s3://"

• Brute-Forcing Bucket Names

• • • Attackers try common naming patterns (e.g., company-logs, prod-backups).

1. 1. • Bucket Finder automates this process.
      • Example command: python bucket_finder.py -w bucket-wordlist.txt -t 10

• Querying AWS Directly

1. 1. • Attackers attempt to list S3 buckets using the AWS CLI.
      • Example command: aws s3 ls s3://company-backups
      • If AWS returns NoSuchBucket, it indicates the bucket is unregistered and potentially available for takeover.

Step 2: Registering the Bucket

Once attackers confirm a bucket is available, they register it under their AWS account.

Why This Attack Works:

• Automation Scripts Still Reference Old Buckets: Organizations often fail to update scripts after bucket deletion.
• CloudFront & APIs Still Point to the Bucket: If a company mapped CloudFront or APIs to an S3 bucket, they unknowingly interact with the attacker’s new bucket.

Example command to register a stolen bucket: aws s3 mb s3://company-backups --region us-east-1

Step 3: Data Interception & Exploitation

Once attackers gain control of the bucket, they can:

• Steal Incoming Data: Sensitive logs, backups, and documents are automatically uploaded by unsuspecting systems.
• Inject Malicious Payloads: If applications pull assets from an S3 bucket, attackers can inject malware into the stored files.
• Hijack Brand Domains: If the bucket is linked to a corporate subdomain, attackers can host phishing pages under a trusted domain.

Mitigating S3 Bucket Takeover Attacks

Organizations must take proactive measures to prevent bucket takeover risks:

• Monitor & Remove References to Deleted Buckets

1. • Update automation scripts after deleting an S3 bucket.
   • Scan CI/CD pipelines and codebases for hardcoded bucket names.
   • Use AWS CloudTrail & Config Rules to track bucket creation/deletion.

• Enable S3 Bucket Ownership Controls

• • Enforce bucket ownership settings to prevent unauthorized account claims.

• Restrict Access with IAM Policies

1. • Apply the principle of least privilege to IAM roles.

Cyngular Security's CIRA Platform

To further secure your cloud environment, consider integrating Cyngular Security's CIRA platform. It enhances your security posture by providing advanced investigation and response capabilities, enabling your team to address threats swiftly and effectively. By adopting Cyngular Security's CIRA, you empower your organization with proactive and automated security measures that protect your cloud assets.

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