File Upload

File Upload Vulnerabilities arise when a web application allows users to upload files without proper validation or sanitization, potentially leading to unauthorized file execution, data breaches, or server compromise.

How File Upload Vulnerabilities Work

• Attackers exploit inadequate validation of uploaded files to upload and execute malicious scripts or overwrite critical files.

• Commonly targeted areas include web forms allowing image, document, or media uploads.

Typical Vulnerable Scenarios

• Lack of proper file type validation, allowing executable scripts to be uploaded.

• Insecure file storage, where uploaded files are accessible through web browsers.

Exploitation Techniques

• Malicious File Upload: Uploading files with scripts (PHP, JavaScript) embedded in seemingly benign files (images, PDFs).

• Extension Tampering: Changing the file extension to bypass validation checks (e.g., evil.php.jpg).

• Path Traversal in File Names: Using path traversal sequences (e.g., ../../) in file names to overwrite critical files.

Mitigation Strategies

• Strict Validation: Validate file type by checking MIME types and file extensions both on client-side and server-side.

• File Type Verification: Implement server-side checks to verify the actual type of the uploaded file (e.g., magic numbers for images).

• Storage and Access: Store uploaded files in a directory outside of the webroot, with indirect access through a script.

• File Execution Prevention: Configure server settings to prevent execution of files in upload directories.

• Regular Scanning: Periodically scan upload directories for unexpected or potentially malicious files.