Could not establish trust relationship for the SSL/TLS secure channel.

Working with some older Cisco ASA devices, I’m trying to access the ASDM interface. The browser isn’t giving me luck, so I turned to PowerShell to help me, but I get the following error when trying an Invoke-WebRequest to grab the asdm.jnlp file I need.

The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

Eh, ok. My first thought was to somehow avoid a certificate check but I did not see a native way of doing this with Invoke-WebRequest (at least from an old Server 2008 box with PowerShell v4.0).

StackOverflow to the rescue. Here’s the solution that worked for me.

if (-not("dummy" -as [type])) {
    add-type -TypeDefinition @"
using System;
using System.Net;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;

public static class Dummy {
    public static bool ReturnTrue(object sender,
        X509Certificate certificate,
        X509Chain chain,
        SslPolicyErrors sslPolicyErrors) { return true; }

    public static RemoteCertificateValidationCallback GetDelegate() {
        return new RemoteCertificateValidationCallback(Dummy.ReturnTrue);
    }
}
"@
}

[System.Net.ServicePointManager]::ServerCertificateValidationCallback = [dummy]::GetDelegate()

Now I can add on my Invoke-WebRequest and everything works.