How to Implement a Custom Server Certificate Validation Callback in .NET?

Customized .NET HTTPS request server certificate verification callback

The .NET Framework verifies the authenticity of the server certificate when establishing an HTTPS connection. However, in some cases, it may be necessary to bypass this check. This article explores how to implement a callback method that allows you to ignore certificate verification for specific HTTPS requests.

Learn about ServicePointManager.ServerCertificateValidationCallback

ServicePointManager.ServerCertificateValidationCallback is a delegate that defines a function responsible for verifying the server's certificate. When the .NET Framework establishes an HTTPS connection, it calls this callback method to verify the certificate. The method takes the following parameters:

• obj: Provides context for server certificate verification.
• certificate: X.509 certificate provided by the server.
• chain: X.509 certificate chain used to establish trust.
• errors: An enumeration representing any SSL policy errors encountered.

Implementation method

In the sample code, the callback is defined as follows:

<code class="language-csharp">ServicePointManager.ServerCertificateValidationCallback = delegate(
    Object obj, X509Certificate certificate, X509Chain chain, 
    SslPolicyErrors errors)
    {
        return true;
    };</code>

This callback effectively ignores all certificate checks and returns true unconditionally, allowing the connection to be established regardless of certificate status.

Callback position

Callback code should be placed before ServicePointManager.ServerCertificateValidationCallback is executed, which occurs when the .NET Framework attempts to establish an HTTPS connection. In the example code, this means placing it before the Stream stream = request.GetRequestStream(); line.

Other options

For per-request certificate verification, you can use a lambda expression like this:

<code class="language-csharp">request.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;</code>

This method allows you to customize certificate verification for individual requests. Alternatively, you can apply the same callback to a global filter, enabling it for all HTTPS connections made by your application.

By implementing ServerCertificateValidationCallback, you can bypass certificate verification for HTTPS requests, thereby connecting to a server that may provide an untrusted or invalid certificate.

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