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🧾 Classic ASP – Accessing Certificate Information (2025 Guide)

Learn how to retrieve and use SSL/TLS certificate information in Classic ASP using Request.ServerVariables. Understand how certificates enhance security and how to programmatically handle them in Classic ASP applications.

🚀 Introduction – Why Use Certificate Info in Classic ASP?

SSL/TLS certificates play a critical role in securing web communications. In Classic ASP, you can access certificate-related information using built-in server variables. This is useful for:

Client certificate authentication
SSL security validation
Logging certificate details
Auditing and compliance

🔐 Accessing Server Certificate Information

Classic ASP provides access to certificate-related data through Request.ServerVariables.

📌 Common Certificate-Related Server Variables

Variable Name	Description
`CERT_SUBJECT`	The subject field of the client certificate
`CERT_ISSUER`	The issuer field of the client certificate
`CERT_SERIALNUMBER`	Serial number of the certificate
`CERT_FLAGS`	Certificate validation status
`HTTPS`	Indicates if the connection is secure (`on` or `off`)
`SSL_CLIENT_CERT`	Full client certificate in Base64
`SSL_SERVER_CERT`	Server certificate info

🧪 Example: Retrieving Certificate Info

<%
If Request.ServerVariables("HTTPS") = "on" Then
    Response.Write "<strong>Secure connection detected.</strong><br>"
    Response.Write "Client Certificate Subject: " & Request.ServerVariables("CERT_SUBJECT") & "<br>"
    Response.Write "Client Certificate Issuer: " & Request.ServerVariables("CERT_ISSUER") & "<br>"
    Response.Write "Client Certificate Serial: " & Request.ServerVariables("CERT_SERIALNUMBER") & "<br>"
Else
    Response.Write "This is not a secure (HTTPS) connection."
End If
%>

📌 Use Case: Validating Certificate Flags

<%
Dim certFlags
certFlags = Request.ServerVariables("CERT_FLAGS")

If certFlags = "0" Then
    Response.Write "✅ Certificate is valid."
Else
    Response.Write "❌ Certificate is NOT valid. Flags: " & certFlags
End If
%>

🔧 Prerequisites

SSL must be enabled on the web server (IIS)
Client certificate authentication must be configured on IIS
Ensure Classic ASP is enabled in IIS features
Proper certificate trust chains should be installed

📘 Summary

Classic ASP allows access to certificate information using Request.ServerVariables, enabling you to:

Detect HTTPS usage
Read client certificate subject, issuer, serial number
Validate certificate status via flags

🔐 This is essential for secure applications, especially in financial, healthcare, or government portals where client authentication is critical.

❓ Frequently Asked Questions (FAQ)

🔹 Q1. Does Classic ASP support client certificate authentication?

A: Yes, if configured correctly in IIS. You can access certificate details through server variables in Classic ASP.

🔹 Q2. What does `CERT_FLAGS = 0` mean?

A: It indicates that the certificate passed all validation checks and is considered valid.

🔹 Q3. How can I force HTTPS in Classic ASP?

A: Redirect HTTP to HTTPS at the beginning of your script:

<%
If Request.ServerVariables("HTTPS") <> "on" Then
    Response.Redirect "https://" & Request.ServerVariables("HTTP_HOST") & Request.ServerVariables("URL")
End If
%>

🔹 Q4. Can I get server-side SSL certificate info?

A: Server variables focus on client-side certificates. To access server-side details, use IIS logs or PowerShell/Win32 APIs.

🔹 Q5. Is `SSL_CLIENT_CERT` always available?

A: No, only if IIS is configured to require client certificates and a client actually presents one.

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