Server-side file validation

This section describes how you can implement File Validation

Sometimes it might be necessary to validate the contents of a file before it gets saved to disk when uploading trough the backoffice.

To help with this, Umbraco supplies a FileStreamSecurityValidator that runs all registered IFileStreamSecurityAnalyzer implementations on the file streams it receives from it's different file upload endpoints. When any of the analyzers deem the file to be unsafe, the endpoint disregards the file and shows a relevant validation message where appropriate. This all happens in memory before the stream is written to a temporary file location.

Implementing a FileStreamSecurityValidator

The IFileStreamSecurityAnalyzer needs a single method to be implemented:

• IsConsideredSafe: This method should return false if the analyzer finds a reason not to trust the file

Example FileStreamSecurityAnalyzer

The following class shows how one could potentially guard against Cross-site scripting(XSS) vulnerabilities in an svg file.

public class SvgXssSecurityAnalyzer : IFileStreamSecurityAnalyzer
    {
        public bool ShouldHandle(Stream fileStream)
        {
            // reduce memory footprint by partially reading the file
            var startBuffer = new byte[256];
            var endBuffer = new byte[256];
            fileStream.Read(startBuffer);
            if (endBuffer.Length > fileStream.Length)
                fileStream.Seek(0, SeekOrigin.Begin);
            else
                fileStream.Seek(fileStream.Length - endBuffer.Length, SeekOrigin.Begin);
            fileStream.Read(endBuffer);
            var startString = System.Text.Encoding.UTF8.GetString(startBuffer);
            var endString = System.Text.Encoding.UTF8.GetString(endBuffer);
            return startString.Contains("<svg")
                   && startString.Contains("xmlns=\"http://www.w3.org/2000/svg\"")
                   && endString.Contains("/svg>");
        }

        public bool IsConsideredSafe(Stream fileStream)
        {
            var streamReader = new StreamReader(fileStream); // do not use a using as this will dispose of the underlying stream
            var fileContent = streamReader.ReadToEnd();
            return !(fileContent.Contains("<script") && fileContent.Contains("/script>"));
        }
    }

You can register it during startup or with a composer.

This is an example of registering the class with a composer:

public class ServerSideValidationComposer : IComposer
    {
        public void Compose(IUmbracoBuilder builder)
        {
            builder.Services.AddSingleton<IFileStreamSecurityAnalyzer, SvgXssSecurityAnalyzer>();
        }
    }

Then you can upload a file with the following content to the backoffice and see that it is not persisted.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg">
   <polygon id="triangle" points="0,0 0,50 50,0" fill="#009900" stroke="#004400"/>
   <script type="text/javascript">
      alert('xss');
   </script>
</svg>