One of our clients receive this error message when deploying his ASP.NET application.

“System.Net.WebException: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.”. Here is how we solved that issue.

There are 3 related and important issues:

1. the remote site uses a Server Name Indication (SNI) certificate, installed on a different domain name
2. the web application was published to a IIS 6.0 (Windows Server 2003) web server
3. a System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure. This error message is caused because the process is not able to validate the certificate supplied by the server during an HTTPS (SSL) request

IIS 6.0 + Server Name Indication (SNI) certificates = System.Net.WebException #

A Server Name Indication (SNI) certificate basically means that you can install oneSSL/TLS certificate on a web server, to use on multiple domain names. The TLS part takes the negotiation, and that enables the server to select the correct virtual domain early and present the browser with the certificate containing the correct name. Therefore with clients and servers that support SNI, a single IP address can be used to serve a group of domain names for which it is impractical to get a common certificate.

Windows Server 2003 (IIS 6.0), Windows Server 2008 (IIS 7.0) and Windows Server 2008 R2 (IIS 7.5) do not support SNI-certificates.

How to Solve this Error Message

You might wonder what the solution to this error message was. Well, simple: Move the website to an IIS 8.0+ (Windows Server 2012) web server. This version supports Server Name Indication certificates. Microsoft calls this SSL Scalability in IIS 8.0. Because of SNI, or SSL-scalability, support in Windows Server 2012, the ASP.NET System.Net.WebException went away.

It’s simple, right? :)