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attack-surface.md

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Attack Surface Analysis for thephpleague/flysystem

Attack Surface: 1. Path Traversal Vulnerability via Local Adapter and API

  • Description: Improper handling of user-controlled input in file paths, when used with Flysystem's Local adapter and API functions, allows attackers to access files and directories outside the intended scope. Flysystem, by design, operates on paths provided to it. If these paths are not properly validated by the application, path traversal vulnerabilities can arise when using the Local adapter.
  • Flysystem Contribution: Flysystem's Local adapter directly interacts with the filesystem based on paths provided through its API. If the application using Flysystem passes unsanitized user input as part of these paths to functions like read(), write(), delete(), etc., Flysystem will operate on the attacker-controlled path, enabling traversal.
  • Example: An application uses user-provided filenames in URLs to serve downloads using Flysystem's Local adapter. The code directly uses $filesystem->read($_GET['filename']) without validation. An attacker can craft a URL with filename=../../../../etc/passwd to attempt to read the system's password file via Flysystem.
  • Impact: Reading sensitive files, potentially arbitrary file write/overwrite leading to code execution or system compromise.
  • Risk Severity: Critical to High
  • Mitigation Strategies:
    • Strict Input Validation and Sanitization: Thoroughly validate and sanitize all user-provided input used to construct file paths before passing them to Flysystem API functions. Use whitelisting of allowed characters and patterns.
    • Path Normalization: Normalize paths using built-in functions to remove path traversal sequences (like ..) before using them with Flysystem.
    • Restrict Application Access: Implement application-level access control to limit which files and directories users should be able to access, regardless of path manipulation attempts.

Attack Surface: 2. Exposed Cloud Provider Credentials in Application Configuration

  • Description: Cloud provider credentials (API keys, service account keys) required for Flysystem's cloud adapters are insecurely managed within the application's configuration or deployment. This allows attackers who gain access to the application's configuration to retrieve these credentials and compromise the associated cloud storage and potentially other cloud resources. Flysystem relies on these credentials to function with cloud storage.
  • Flysystem Contribution: Flysystem's cloud adapters (e.g., for AWS S3, Google Cloud Storage, Azure Blob Storage) require cloud provider credentials to be configured. If the application using Flysystem stores these credentials insecurely (e.g., hardcoded, in easily accessible config files), it directly contributes to the attack surface by making credential theft possible, which then compromises the storage Flysystem manages.
  • Example: AWS access keys are hardcoded directly into a PHP configuration file that is deployed with the application. An attacker gains access to the application's codebase (e.g., via a separate vulnerability) and retrieves the AWS keys from the configuration file, allowing them to access the S3 bucket used by Flysystem.
  • Impact: Full compromise of cloud storage managed by Flysystem, potential access to other cloud resources, data breach, resource hijacking, financial damage.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Utilize Environment Variables or Secure Secrets Management: Store cloud provider credentials as environment variables or use dedicated secure secrets management solutions (like HashiCorp Vault, cloud provider secret managers).
    • Avoid Hardcoding Credentials in Configuration Files: Never hardcode credentials directly in application code or configuration files that are deployed or version controlled.
    • Principle of Least Privilege for Credentials: Grant the application's service account or IAM role only the minimum necessary permissions required to interact with the cloud storage via Flysystem.

Attack Surface: 3. Arbitrary File Upload leading to Remote Code Execution or XSS

  • Description: Applications using Flysystem for file uploads, without proper validation and security measures, can allow attackers to upload malicious files. These files, if processed or served incorrectly by the application or web server, can lead to remote code execution or cross-site scripting (XSS) vulnerabilities. Flysystem is the mechanism used to store these uploaded files.
  • Flysystem Contribution: Flysystem provides the functionality to write uploaded files to storage using methods like writeStream() and write(). If the application using Flysystem does not implement sufficient validation before using these methods, it becomes vulnerable to accepting and storing malicious files. Flysystem itself doesn't inherently prevent malicious uploads; it's the application's responsibility to validate before using Flysystem to store the files.
  • Example: An application allows users to upload avatar images using Flysystem. It lacks proper file type validation and stores uploaded files directly in a web-accessible directory. An attacker uploads a PHP script disguised as an image (malicious.php.jpg). If the web server executes PHP files in the upload directory, accessing malicious.php.jpg in the browser will execute the attacker's PHP code on the server.
  • Impact: Remote code execution, website defacement, malware distribution, cross-site scripting (XSS), denial of service.
  • Risk Severity: Critical to High
  • Mitigation Strategies:
    • Robust File Type Validation: Implement strong file type validation based on file extensions, MIME types, and ideally, file content (magic numbers). Use whitelisting of allowed file types.
    • Input Sanitization for Filenames: Sanitize uploaded filenames to prevent injection of malicious characters or path manipulation.
    • Secure Upload Directory Configuration: Store uploaded files outside of the web server's document root if possible. If not, configure the web server to prevent script execution within the upload directory (e.g., using .htaccess or web server configuration directives).
    • Content Security Policy (CSP): Implement CSP headers to help mitigate the impact of potential XSS vulnerabilities arising from uploaded content.
    • Regular Security Scanning: Implement regular security scanning for uploaded files, including malware and vulnerability scans.