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Attack Surface Analysis for blueimp/jquery-file-upload

Attack Surface: 1. Malicious File Execution (Code Injection)

  • Description: Attackers upload files containing executable code (e.g., PHP, ASP, JSP, Python scripts, or even HTML with embedded JavaScript) that the server then executes or the client renders.
  • How jquery-file-upload Contributes: The library is the direct mechanism for file uploads, providing the entry point for this attack. While it offers client-side file type validation (easily bypassed) and suggests server-side validation, the core vulnerability lies in how the uploaded file is handled after jquery-file-upload delivers it.
  • Example: An attacker uploads a file named malicious.php.jpg (double extension) or malicious.php (claiming it's an image via Content-Type). If the server doesn't properly validate the file type and executes it, the attacker's code runs. jquery-file-upload facilitated the upload.
  • Impact: Complete server compromise, data theft, website defacement, malware distribution.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strong Server-Side Validation: Never trust the filename or Content-Type header. Use a whitelist of allowed extensions (e.g., .jpg, .png, .gif). Inspect the file contents using a library like libmagic to determine the true file type.
    • Secure File Storage: Store uploaded files outside the web root, if possible. If within the web root, configure the server to not execute scripts in the upload directory.
    • Unique Filenames: Generate unique filenames on the server (e.g., using UUIDs) to prevent attackers from overwriting existing files or predicting filenames.
    • Correct Content-Type: Serve uploaded files with the correct Content-Type (e.g., application/octet-stream for unknown types) and the X-Content-Type-Options: nosniff header.

Attack Surface: 2. Cross-Site Scripting (XSS) via Uploaded Files

  • Description: Attackers upload HTML files (or files that can be interpreted as HTML) containing malicious JavaScript. If the application serves these files without proper sanitization or a strong Content-Security-Policy, the script executes in the context of the victim's browser.
  • How jquery-file-upload Contributes: The library directly facilitates the upload of these potentially malicious HTML files. It's the mechanism by which the attacker gets the malicious file onto the server.
  • Example: An attacker uploads an HTML file containing <script>alert('XSS');</script>. If a user views this file directly through the web application (because the server didn't prevent it from being served as HTML), the script executes. jquery-file-upload was the upload tool.
  • Impact: Session hijacking, cookie theft, website defacement, phishing attacks, redirection to malicious sites.
  • Risk Severity: High
  • Mitigation Strategies:
    • Content Security Policy (CSP): Implement a strong CSP that restricts the sources from which scripts can be loaded.
    • Output Encoding: If you must display the contents of uploaded files, HTML-encode the output to prevent script execution.
    • Serve as application/octet-stream: Serve all uploaded files (especially those with unknown or untrusted content) with the Content-Type: application/octet-stream header and the X-Content-Type-Options: nosniff header.
    • File Type Validation: Restrict uploads to known safe file types. Avoid allowing HTML uploads unless absolutely necessary and thoroughly validated on the server-side.

Attack Surface: 3. Cross-Site Scripting (XSS) via Filenames

  • Description: Attackers upload files with filenames containing malicious JavaScript. If the application displays these filenames without proper escaping, the script executes.
  • How jquery-file-upload Contributes: The library directly handles and processes these filenames as part of the upload. It's the component that receives and makes the filename available to the application.
  • Example: An attacker uploads a file named <script>alert('XSS');</script>.jpg. If the application displays this filename unsanitized in a list of uploaded files, the script executes. jquery-file-upload provided the filename to the application.
  • Impact: Session hijacking, cookie theft, website defacement, phishing attacks.
  • Risk Severity: High
  • Mitigation Strategies:
    • Output Encoding: Always HTML-encode filenames (and any other user-supplied data) before displaying them in the UI.
    • Filename Sanitization: Sanitize filenames on the server-side, removing or replacing potentially dangerous characters.

Attack Surface: 4. Path Traversal

  • Description: Attackers attempt to use ../ or similar sequences in the filename to save the uploaded file outside the intended directory.
  • How jquery-file-upload Contributes: The library directly handles the filename provided by the user, which is then (potentially unsafely) used by the server-side code to determine the file's save location. The library is the conduit for the malicious filename.
  • Example: An attacker uploads a file named ../../../etc/passwd. If the server-side code (which receives the filename from jquery-file-upload) doesn't sanitize it, the file might overwrite a critical system file.
  • Impact: System compromise, data loss, unauthorized access.
  • Risk Severity: High
  • Mitigation Strategies:
    • Never Use User Input Directly: Never use user-supplied input directly in file paths.
    • Generate Unique Filenames: Generate unique filenames on the server (e.g., using UUIDs). This is the best defense.
    • Sanitize Filenames: If you must use part of the original filename, sanitize it thoroughly on the server, removing or replacing dangerous characters.
    • Dedicated Upload Directory: Store uploaded files in a dedicated directory, ideally outside the web root.