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Inappropriate coding practices - invalid file

Need​

Enforcement of proper file validation and verification

Context​

  • Usage of C# for building robust and efficient software applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core MVC
  • Usage of Microsoft.AspNetCore.Http for handling HTTP requests and responses in ASP.NET Core applications

Description​

Non compliant code​

public IActionResult UploadFile(IFormFile file)
{
    var filePath = Path.GetTempFileName();

    using (var stream = System.IO.File.Create(filePath))
    {
        file.CopyTo(stream);
    }

    return Ok();
}

The above code represents a file upload functionality in an ASP.NET Core application. The method UploadFile accepts a file from a form and then writes it to a temporary file on the server.

The vulnerability lies in the fact that the code doesn't validate the content of the uploaded file against its extension. This means that an attacker could upload a file with a malicious content but with a benign extension. For example, an attacker could upload a file with a .txt extension but its content could be a script or a binary executable.

The server, trusting the file extension, would treat the file as a harmless text file. But when the file is opened, the malicious content would be executed, leading to potential security breaches.

This vulnerability can lead to serious security issues like code execution, data leakage, or even complete system compromise depending on the nature of the uploaded malicious file.

Steps​

  • Validate the file extension before saving it to the repository.
  • Use a whitelist of allowed file extensions to ensure that only valid files are accepted.
  • Consider using a file signature or MIME type validation to further verify the file's content.
  • Implement proper error handling and logging to handle any exceptions or errors during the file upload process.

Compliant code​

public IActionResult UploadFile(IFormFile file)
{
    var allowedExtensions = new List<string> { ".jpg", ".png", ".txt", ".pdf" }; // Add more extensions as per your requirements
    var extension = Path.GetExtension(file.FileName).ToLower();

    if (!allowedExtensions.Contains(extension))
    {
        return BadRequest("Invalid file extension");
    }

    var filePath = Path.GetTempFileName();

    try
    {
        using (var stream = System.IO.File.Create(filePath))
        {
            file.CopyTo(stream);
        }
    }
    catch (Exception ex)
    {
        // Log the exception
        return StatusCode(500, "Internal server error");
    }

    return Ok();
}

The updated code now includes a whitelist of allowed file extensions. Before the file is saved to the repository, the code checks the file's extension against this whitelist. If the file's extension is not in the whitelist, the method returns a "Bad Request" response.

The code also includes a try-catch block to handle any exceptions that might occur during the file upload process. If an exception is thrown, the method logs the exception and returns an "Internal Server Error" response.

This approach helps to ensure that only valid files are accepted and that any errors during the file upload process are properly handled and logged.

References​

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