Limiting file upload size and type

Limiting file upload size and type is essential for improving the security and performance of your Power Pages portal. By controlling what users can upload, you can prevent malicious files and ensure that your system does not become overloaded with large files that could degrade performance.

Here are some best practices for limiting file upload size and type:

1. Limit File Upload Size

Limiting the file size helps ensure that your server doesn’t get overwhelmed by large files that can consume bandwidth, storage, and processing resources.

• Set Maximum File Size Limit: Specify the maximum file size allowed for uploads. For example, you may limit users to uploading files that are no larger than 10 MB.
• Use Server-Side Validation: Ensure that the server-side application enforces the maximum file size restriction. This prevents users from bypassing the restriction by modifying the client-side validation.

Example:

• If you’re using a form in Power Pages or a custom HTML form, you can set the maxFileSize property to restrict the file size on the client-side. However, always validate the file size on the server side as well.

<input type="file" name="file" accept=".jpg, .jpeg, .png" max-file-size="10485760"> <!-- 10 MB limit -->

• Client-Side Validation Example (JavaScript):

document.getElementById("fileInput").addEventListener("change", function(event) {
  var file = event.target.files[0];
  var maxSize = 10 * 1024 * 1024; // 10 MB
  
  if (file.size > maxSize) {
    alert("File is too large. Maximum size is 10 MB.");
    event.target.value = ""; // Reset the input
  }
});

2. Limit File Types

Limiting the file types ensures that only files of specific formats are allowed to be uploaded. This helps to prevent users from uploading executable files (e.g., .exe, .bat, .js) that could contain malicious code.

• Allow Specific File Types Only: Use the accept attribute in your HTML file input tag to restrict the types of files that can be uploaded.

<input type="file" name="file" accept=".jpg, .jpeg, .png, .pdf"> <!-- Allows only image and PDF files -->

• Server-Side File Type Validation: Although the accept attribute on the client-side restricts file types during selection, users can bypass this check. Therefore, it’s crucial to validate the file type on the server side as well.

Server-Side File Type Validation Example (in C#):

if (file.ContentType != "image/jpeg" && file.ContentType != "image/png") {
    throw new InvalidOperationException("Only JPEG and PNG images are allowed.");
}

3. Use File Type Whitelisting

Instead of blacklisting dangerous file types, whitelist allowed file types. This is a safer approach as it avoids the risk of forgetting to block a new dangerous file extension.

• Allow Only Safe File Extensions: Create a list of allowed file types (e.g., .jpg, .jpeg, .png, .pdf) and reject anything not on the list.

const allowedExtensions = ['jpg', 'jpeg', 'png', 'pdf'];

const fileInput = document.getElementById('fileInput');
fileInput.addEventListener('change', function(event) {
  const file = event.target.files[0];
  const fileExtension = file.name.split('.').pop().toLowerCase();
  
  if (!allowedExtensions.includes(fileExtension)) {
    alert('Invalid file type. Only JPEG, PNG, and PDF are allowed.');
    event.target.value = ''; // Clear the file input
  }
});

4. Use Content-Type (MIME Type) Validation

In addition to file extensions, verify the MIME type of the uploaded files. This ensures that the file content matches its extension. For example, a .jpg file should have the MIME type image/jpeg.

Server-Side Example (in C#):

if (file.ContentType != "image/jpeg" && file.ContentType != "image/png") {
    throw new InvalidOperationException("Only JPEG and PNG images are allowed.");
}

5. Enable Virus Scanning

To further secure file uploads, it’s essential to use virus scanning tools to ensure that the uploaded files don’t contain any harmful content.

• Use Antivirus APIs: There are third-party services (like VirusTotal, ClamAV, or Microsoft Defender) that can scan files for malware before they are accepted.

6. Set a Secure File Storage Location

Ensure that uploaded files are stored in a secure location. Here are some considerations:

• Separate Storage: Store uploaded files in a dedicated, secure storage location that is isolated from sensitive system files.
• File Renaming: To prevent files from being accessed directly through their original name, you can rename uploaded files to a unique identifier.

7. Implement File Size Limitations in Power Automate (for backend workflows)

If you’re using Power Automate workflows to handle file uploads (such as saving uploaded files to SharePoint or OneDrive), you can set file size and type limits in the flow.

• Example in Power Automate:
  1. Use the “When a file is created” trigger for SharePoint or OneDrive.
  2. Add conditions to check the file size and type (via dynamic content or expressions).

Example of size validation in Power Automate:

if (length(fileContent) > 10485760) {  // File size greater than 10 MB
  terminate("File size too large");
}

8. Consider Using a Cloud-Based File Storage Solution

If file uploads are expected to be large or frequent, consider integrating cloud-based storage solutions like Azure Blob Storage or SharePoint for better scalability and management.

9. Testing and Monitoring

• Test Upload Limits: Regularly test the file upload functionality to ensure the size and type restrictions are being enforced correctly.
• Monitor Logs: Keep track of any failed uploads or security warnings in the server logs to spot any potential issues or attacks.