IsSignatureValid doesn't actually verify the signature

[FlorianHockmann]

The check for PeFile.IsSignatureValid is implemented in the private method AuthenticodeInfo.CheckSignature which basically does this:

var hash = ComputeAuthenticodeHash();
return SignedHash.SequenceEqual(hash);

This shows that the check only compares whether the hash from the signature matches the computed hash of the PE file, but the signature itself isn't checked at all. It should be verified with the certificate.

[secana]

Initial idea for a fix by @FlorianHockmann
FlorianHockmann@e73a1de

[secana]

It seems that we are running into a known problem: https://github.com/dotnet/corefx/issues/31073

I've verified that this is the case with the following code:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.Pkcs;

namespace Framework
{
    class Program
    {
        static void Main(string[] args)
        {
            var rawCerts = File.ReadAllBytes("cert.dump");
            var signedCms = new SignedCms();
            signedCms.Decode(rawCerts);

            try
            {
                signedCms.CheckSignature(true);
                Console.WriteLine("Valid siganture.");
            }
            catch(CryptographicException e)
            {
                Console.Error.WriteLine(e.Message);
            }

            Console.ReadKey();
        }
    }
}

On .Net Core: "Invalid Siganture."
On .Net Framework: "Valid Signature."

[secana]

Not sure how to handle this. Technically .Net Core is right, since DigiCert doesn't comply to the CMS standard. On the other hand, the "real world" standard would be to accept the signature, like .Net Framework does it.

I've tried to update System.Security.Cryptography.Pkcs to a preview version (4.6.0-preview.18571.3), but now the signedCms.Decode(rawCerts) fails with an CryptographicException: ASN1 corrupted data. exception.

[secana]

I've requested help in the CoreFX issue: dotnet/corefx#31073

[secana]

Opened an issue in CoreFX: https://github.com/dotnet/corefx/issues/34202

[secana]

Fixed in 0.8.2