S3's file storage system employs a flat namespace architecture, where all objects are stored as key-value pairs without hierarchical folders. While most interfaces simulate folder hierarchies using / as a path delimiter, this abstraction can introduce security risks when designing access policies for list operations.

List Operation Behavior in Flat Namespaces

In traditional folder hierarchies, listing a directory reveals child files and subdirectories. S3's flat structure requires a different interpretation:

• Files begin with a prefix and do not contain a /
• Directories begin with a prefix and end with a /

This mapping ensures expected behavior when using prefix-based listing (e.g., s3:prefix conditions in IAM policies).

Critical Security Vulnerability in Policy Design

When implementing secure home directory storage in S3 (where user home paths follow home/${username}), the following policy is initially used:

{
  "Effect": "Allow",
  "Action": "s3:ListBucket",
  "Resource": "arn:aws:s3:::bucket-name",
  "Condition": {
    "StringLike": {
      "s3:prefix": ["", "home/", "home/${aws:username}/*"]
    }
  }
}

This policy permits listing contents under a user's home directory. However, it fails to address two critical scenarios:

1. Missing trailing /: Users may omit the trailing slash (e.g., home/${username}), enabling recursive listing of other users' directories
2. Substring conflicts: If usernames are non-unique (e.g., user and user_with_user_prefix), listing one user's directory may inadvertently expose another's contents

Vulnerability Example

When user lists their home directory without a trailing /, the S3 list operation may unexpectedly reveal: home/user_with_user_prefix/... This occurs because S3's flat namespace treats user_with_user_prefix as a valid prefix starting with the substring user.

Mitigation Strategy

To prevent cross-user leakage, the policy must explicitly support both prefix patterns:

{
  "Effect": "Allow",
  "Action": "s3:ListBucket",
  "Resource": "arn:aws:s3:::bucket-name",
  "Condition": {
    "StringLike": {
      "s3:prefix": ["home/${aws:username}", "home/${aws:username}/*"]
    }
  }
}

This solution requires string sanitization of usernames to avoid substring conflicts.

Key Takeaways

1. Never rush security implementations – Prefix-based listing policies require thorough validation
2. Prioritize flat namespace awareness over hierarchical abstractions when designing S3 access policies
3. Sanitize user identifiers to prevent substring-based access leakage (e.g., ensure usernames do not share prefixes)
4. Validate edge cases through concrete examples before deploying policies

This approach aligns with AWS best practices while addressing the unique challenges of S3's flat file model.

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