How to Block File Uploads Containing HTML Code using C#/.NET Core

Cloudmersive
4 min readMay 31, 2023

The security of our most sensitive web applications depends entirely on the strength and scope of the policies we put in place to protect them. Securing our applications starts with implementing powerful anti-virus and anti-malware policies, but it should also include policies against non-malware content threats, too. Allowing uploads from external sources to contain HTML is asking for trouble because a variety attacks — including XSS (Cross-Site Scripting) and local file access — can be carried out this way. As a result, it’s best to identify and delete uploads containing HTML before they can harm our web applications and exploit our trusted users.

Thankfully, we can provide 360-degree content protection for any web application using the free API provided below. This API will automatically scan files for virus and malware threats, and it will allow us to detect and block HTML-bearing files by setting a simple request parameter — allowHtml — to “false”. We can also choose to block a variety of other non-malware threats by configuring their respective Booleans in the request body, and we can even restrict file upload types by supplying a comma-separated list of acceptable extensions.

We can easily take advantage of this API in two quick steps using ready-to-run C# code examples. To start, let’s run the following command in our Package Manager console to install the .NET Core SDK via NuGet:

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

Next, let’s structure our request by copying and pasting the below code:

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 );
}
}
}
}

After we configure our custom threat policies, we can authenticate our request by supplying a free-tier API key. This can be obtained by registering a free account on the Cloudmersive website.

That’s all there is to it — now we can instantly remove a variety of threats and keep our web applications safe.

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