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>"));
}
}
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.
