How to Restrict File Upload Types in C# .NET Core

For most file upload processes, it’s practical to consider implementing file type upload restrictions.

By restricting the types of files that can be uploaded to our servers, we can greatly improve the quality and usability of documents in our system, and we can also improve our threat profile considerably by rejecting dangerous file types.

Using C# code examples provided below, we can take advantage of a free API in our .NET Core application that combines file verification and threat scanning into one simple, low-code, and catch-all solution. We’ll be calling a quick service that simultaneously scans files for viruses and malware and verifies the format of each file upload.

File verification enables us to identify invalid files and other types of threatening content, including executables (which are often disguised as other files), macros, scripts, and more. It also allows us to directly restrict the types of files that can pass through our server application.

In fact, we can directly whitelist files for our upload process by providing a comma-separated list of acceptable file extensions (e.g., ‘.pdf,.png,.jpg’) in the API’s restrictFileTypes request parameter.

We could, for instance, use this to create a feedback loop for client-side uploaders that explains why a file upload was rejected and reiterates which file types are expected in the upload process.

We can structure our API call easily in two quick steps. First, let’s install the .NET Core SDK by running the below command:

Install-Package Cloudmersive.APIClient.NETCore.VirusScan -Version 2.0.4

After that, let’s copy the below code to call the function, and let’s authorize our requests with a free Cloudmersive API key (this will provide a limit of 800 API calls per month with no commitments):

using System;
using System.Diagnostics;
using Cloudmersive.APIClient.NETCore.VirusScan.Api;
using Cloudmersive.APIClient.NETCore.VirusScan.Client;
using Cloudmersive.APIClient.NETCore.VirusScan.Model;

namespace Example
{
    public class ScanFileAdvancedExample
    {
        public void main()
        {
            // Configure API key authorization: Apikey
            Configuration.Default.AddApiKey("Apikey", "YOUR_API_KEY");

            var apiInstance = new ScanApi();
            var inputFile = new System.IO.FileStream("C:\\temp\\inputfile", System.IO.FileMode.Open); // System.IO.Stream | Input file to perform the operation on.
            var allowExecutables = true;  // bool? | Set to false to block executable files (program code) from being allowed in the input file.  Default is false (recommended). (optional) 
            var allowInvalidFiles = true;  // bool? | Set to false to block invalid files, such as a PDF file that is not really a valid PDF file, or a Word Document that is not a valid Word Document.  Default is false (recommended). (optional) 
            var allowScripts = true;  // bool? | Set to false to block script files, such as a PHP files, Python scripts, and other malicious content or security threats that can be embedded in the file.  Set to true to allow these file types.  Default is false (recommended). (optional) 
            var allowPasswordProtectedFiles = true;  // bool? | Set to false to block password protected and encrypted files, such as encrypted zip and rar files, and other files that seek to circumvent scanning through passwords.  Set to true to allow these file types.  Default is false (recommended). (optional) 
            var allowMacros = true;  // bool? | Set to false to block macros and other threats embedded in document files, such as Word, Excel and PowerPoint embedded Macros, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional) 
            var allowXmlExternalEntities = true;  // bool? | Set to false to block XML External Entities and other threats embedded in XML files, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional) 
            var allowInsecureDeserialization = true;  // bool? | Set to false to block Insecure Deserialization and other threats embedded in JSON and other object serialization files, and other files that contain embedded content threats.  Set to true to allow these file types.  Default is false (recommended). (optional) 
            var allowHtml = true;  // bool? | Set to false to block HTML input in the top level file; HTML can contain XSS, scripts, local file accesses and other threats.  Set to true to allow these file types.  Default is false (recommended) [for API keys created prior to the release of this feature default is true for backward compatability]. (optional) 
            var restrictFileTypes = restrictFileTypes_example;  // string | Specify a restricted set of file formats to allow as clean as a comma-separated list of file formats, such as .pdf,.docx,.png would allow only PDF, PNG and Word document files.  All files must pass content verification against this list of file formats, if they do not, then the result will be returned as CleanResult=false.  Set restrictFileTypes parameter to null or empty string to disable; default is disabled. (optional) 

            try
            {
                // Advanced Scan a file for viruses
                VirusScanAdvancedResult result = apiInstance.ScanFileAdvanced(inputFile, allowExecutables, allowInvalidFiles, allowScripts, allowPasswordProtectedFiles, allowMacros, allowXmlExternalEntities, allowInsecureDeserialization, allowHtml, restrictFileTypes);
                Debug.WriteLine(result);
            }
            catch (Exception e)
            {
                Debug.Print("Exception when calling ScanApi.ScanFileAdvanced: " + e.Message );
            }
        }
    }
}

We can now easily restrict file upload types in our .NET Core application while simultaneously scanning files for a range of critical threats.