Category: Server

When you connect to ftp server create with Microsoft IIS using Filezilla Client you should have this error

GnuTLS error -48: Key usage violation in certificate has been detected. Could not connect to server

Your configuration settings are something like this :

• Protocol: FTP – File Transfer Protocol
• Encryption: Require explicit FTP over TLS

The problem is with self signed certificate on server side. This is a problem with the certificate generation of Microsoft IIS, as it does not allow the certificates to be used for digital signatures.

How to generate a valid certificate with IIS

This is a server-side issue, and it did not appear previously because earlier versions of FileZilla shipped with a GnuTLS version that didn’t make this check.

Quoting Tim Kosse’s post in the FileZilla forum thread:

In any case, the problem is with your server’s X.509 certificate chain: Either the server certificate itself or another certificate in the chain has a key usage restriction that is violated. For example a certificate with a key usage restriction to signing cannot be used to authenticate TLS connections. See section 4.2.1.3 of RFC 5280.

This is a problem with the certificate generation of Microsoft IIS (but may also happen if you incorrectly generated a certificate with another method), as it does not allow the certificates to be used for digital signatures. OpenSSL is much more relaxed about this and won’t fail because of it, so it may work with other apps.

On the client side, you can either disable TLS, downgrade to an earlier version of FileZilla (neither of these is recommended due to potential security risks), or use a different client which uses another library such as OpenSSL for now.

How to generate a valid certificate with IIS

This needs to be done on the server side, Yobviously.you can generate the certificate with PowerShell instead until the issue is fixed by Microsoft. Open PowerShell in admin mode.

The following powershell command will create our self-signed certificate for our binding and store it in the Personal Store (Note how I also store a reference to the certificate in a variable called $cert this will be needed further on):

$binding = "192.168.1.70"
$cert = New-SelfSignedCertificate -DnsName "$binding" -CertStoreLocation "cert:\LocalMachine\My"

However, this is not enough to make the certificate work for HTTPS in our browser. We need to add our newly created certificate to the Trusted Root Certificate store. To do this we take our $cert variable which references our created certificate and add it to our Trusted Root Certificate store like so:

$DestStore = new-object System.Security.Cryptography.X509Certificates.X509Store([System.Security.Cryptography.X509Certificates.StoreName]::Root,"localmachine")
$DestStore.Open([System.Security.Cryptography.X509Certificates.OpenFlags]::ReadWrite)
$DestStore.Add($cert)
$DestStore.Close()

Now you have to set the new certicate on your ftp site using IIS Admin.