🛡️ File Upload Vulnerabilities

What Are File Upload Vulnerabilities?

File upload vulnerabilities occur when a web server allows users to upload files without properly validating their name, type, contents, or size. This can lead to serious security risks, including remote code execution.

🧠 How It Works

Many web applications allow users to upload files (e.g., images, documents). If the application fails to enforce strict validation, attackers can upload:

• Executable scripts (e.g., .php, .jsp)
• Oversized files to exhaust disk space
• Files with malicious names to overwrite critical files

🧨 Triggering Execution

In some cases, simply uploading the file causes harm. In others, the attacker must send a follow-up request to execute the uploaded file.

💥 Potential Impacts

Consequences of Poor Validation

• Remote Code Execution: Uploading a web shell to gain full server control
• File Overwrite: Replacing critical files with malicious ones
• Directory Traversal: Uploading files to unintended locations
• Denial of Service (DoS): Filling up disk space with large files

🧪 Common Validation Failures

What Goes Wrong

• No restriction on file type or MIME type
• No validation of file contents (magic bytes)
• No filename sanitization
• No file size limits
• Upload directory is within the web root and executable

🛡️ Prevention Tips

Best Practices

• Validate file type using both MIME type and file signature
• Rename uploaded files and store them outside the web root
• Restrict executable file types (e.g., .php, .exe)
• Set strict file size limits
• Use allowlists for accepted file extensions
• Disable execution permissions on upload directories

🔗 Resources

• 🌐 PortSwigger: File Upload Vulnerabilities