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Fiddler error–HTTPS handshake failed

This is a follow-up on a post from 2013(!). There I talked about how we used Fiddler to inject a client certificate to authenticate the request.

This application is still in use today, and we still use the same trick to authenticate when debugging locally. Last week, a colleague contacted me that this approach didn't work anymore. When he tried to connect to the service it failed with the following error message:

fiddler.network.https> HTTPS handshake to <site> (for #1) failed. System.Security.Authentication.AuthenticationException A call to SSPI failed, see inner exception.

We discovered that the root cause of the issue was that the server was expecting a TLS 1.2 connection and we were using a TLS 1.0 connection.

We had to take 2 steps to solve the issue:

Step 1 – Update Fiddler

We first had to update Fiddler. Therefore go to https://www.telerik.com/download/fiddler and download the latest version.

Notice that we downloaded Fiddler Classic not the cross-platform Fiddler Everywhere version.

Step 2 – Update the list of supported HTTPS protocols

Open Fiddler and click on Tools  --> Options… .

Go to the HTTPS tab and click on the link next to the protocols label:

Add TLS1.2 to the list of protocols:

Click on OK twice to close both popups.

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