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c#authenticationsslssl-certificatefiddler

C# : Authenticate server certificate before sending HTTP GET / POST

I'm building a .net app, and I'd like to make the web calls secure enough that its not easy to monitor the the traffic thru something like fiddler. I'd like to be able to know that the certificate from the server isn't as expected and then never send out a full request with the request data.

Twitter's iOS app does this. Someone I think would have to somehow make a copy of the twitter's https certificate and make that fiddler's certificate. I havn't done it myself, but i think that's how I understand it. What you see in fiddler is that the tunnel has been created, but no request was actually set out. Its same scenario when you don't have the fiddler's HTTPS certificate installed on the device and you open a browser to google.com / a tunnel is created and then the browser knows 'uhoh untrusted server' and displays a message to the user. I'd like to just make it more secure and only allow 1 certificate / my server's certificate.

Make sense? I think i figured out how to do it with making a separate full dummy request / but thats not ideal.

Solution

  • What you're asking is "How do I implement certificate pinning in my client application?"

    The way to do that would be to attach a validation callback on the Service Point responsible for making your HTTPS requests. Your validation callback would override the default behavior ("Accept any certificate trusted by Windows") and would instead validate that the received certificate is EXACTLY the one you expect.

    Now, before you go that route, keep in mind a few things:

    1. An attacker with Admin or Debug privileges can easily change your code in memory to remove your validation. This is called the "Trusted client" problem.
    2. Your validation will break if the code is ever run in a corporate environment where an security appliance is doing HTTPS inspection (e.g. BlueCoat, ISA TMG, etc)
    3. Your validation will prevent "certificate agility" -- if the server cert needs to change, you will need to update the application. If you ever want to use a load-balanced configuration with multiple certificates, or a public HTTPS CDN, you would need to update your validation logic.