How to Detect Password Protected File Uploads in C# .NET Framework

Cloudmersive
4 min readFeb 21, 2024

When password protection is applied to a malicious document, it can prevent antivirus software from detecting the underlying threat. To mitigate insecure file upload attacks with password protected files, we can look at the problem from another angle — by checking files for password protection policies.

Using the below code, we can take advantage of a free API which simultaneously scans files for viruses, malware, and commonly obfuscated content threats like password protected files. This supports all Office document formats, PDF formats and 100+ image formats. We can set custom threat rules to block files with password protection from proceeding through an upload process, and we can also apply policies against invalid files, scripts, executables, and a variety of other threatening content types.

We can structure our API call in two quick steps with ready-to-run code examples. First, however, we’ll need to obtain a free API key — this will allow a limit of 800 API calls per month (with no commitment upon reaching that limit).

With our API key ready, we can begin by installing the SDK. Let’s run the following command in our Package Manager console to install via NuGet:

Install-Package Cloudmersive.APIClient.NET.VirusScan -Version 3.0.4

Next & last on our task list, let’s copy the below code into our file, enter our API key string in the indicated snippet, and then set the allowPasswordProtectedFiles Boolean to “false”. Setting this parameter to “false” means password protected files will receive a CleanResult: “false” response, but even if we choose not to set this policy, files with password protection will be called out in the API response body.

using System;
using System.Diagnostics;
using Cloudmersive.APIClient.NET.VirusScan.Api;
using Cloudmersive.APIClient.NET.VirusScan.Client;
using Cloudmersive.APIClient.NET.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 review the example response object below to better understand the breadth of malware & non-malware content threats this API can protect against:

{
"CleanResult": true,
"ContainsExecutable": true,
"ContainsInvalidFile": true,
"ContainsScript": true,
"ContainsPasswordProtectedFile": true,
"ContainsRestrictedFileFormat": true,
"ContainsMacros": true,
"ContainsXmlExternalEntities": true,
"ContainsInsecureDeserialization": true,
"ContainsHtml": true,
"ContainsUnsafeArchive": true,
"ContainsOleEmbeddedObject": true,
"VerifiedFileFormat": "string",
"FoundViruses": [
{
"FileName": "string",
"VirusName": "string"
}
],
"ContentInformation": {
"ContainsJSON": true,
"ContainsXML": true,
"ContainsImage": true,
"RelevantSubfileName": "string"
}
}

Now we can easily improve security for our file upload processes using minimal code.

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