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Insecure digital certificates - Chain of trust

Need​

Implementation of proper certificate validation and verification mechanisms

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of X509Certificates for System Security and Cryptography
  • Usage of System.Net.Security for secure network communication
  • Usage of System.Net for network communication in .NET development

Description​

Non compliant code​

using System.Security.Cryptography.X509Certificates;
using System.Net.Security;
using System.Net;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(IgnoreCertificateErrorsCallback);

private static bool IgnoreCertificateErrorsCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    return true;
}

In the C# code snippet above, the IgnoreCertificateErrorsCallback method is set as the ServerCertificateValidationCallback for the ServicePointManager. This callback is used for validating server certificates.

The IgnoreCertificateErrorsCallback method is defined to always return true, which means it will accept any server certificate without performing any validation. The parameters of this method (sender, certificate, chain, sslPolicyErrors) are not used in any way to validate the server certificate.

This is a critical security issue because it allows a "Man in the Middle" attack. An attacker can present any server certificate, even an invalid one, and the method will accept it. Once the attacker has established this trust, they can intercept and potentially alter the communication between the client and the server.

Steps​

  • Implement proper certificate validation logic in the TrustManager checkServerTrusted method.
  • Ensure that the TrustManager throws a certificate exception when invalid certificates are detected.
  • Do not accept any incoming certificate without proper validation.
  • Implement a secure chain of trust to verify the authenticity and integrity of the digital certificates.
  • Use a trusted certificate authority (CA) to issue and validate the server certificates.
  • Regularly update the list of trusted CA certificates to ensure the latest security standards.
  • Consider implementing certificate pinning to further enhance the security of the certificate validation process.
  • Perform thorough testing and validation to ensure the proper functioning of the certificate validation logic.

Compliant code​

using System.Security.Cryptography.X509Certificates;
using System.Net.Security;
using System.Net;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(ValidateServerCertificate);

private static bool ValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    if (sslPolicyErrors == SslPolicyErrors.None)
    {
        return true;
    }

    // Do not allow this client to communicate with unauthenticated servers.
    return false;
}

The updated code now includes a proper implementation of the ValidateServerCertificate method. This method is used as a callback for the ServicePointManager.ServerCertificateValidationCallback delegate, which is responsible for validating the server's SSL certificate.

In the ValidateServerCertificate method, we first check if there are any SSL policy errors by comparing the sslPolicyErrors parameter to SslPolicyErrors.None. If there are no errors, the method returns true, indicating that the certificate is valid.

If there are any SSL policy errors, the method returns false. This means that the client will not be allowed to communicate with the server if the server's SSL certificate is not valid. This prevents the client from accepting any incoming certificate without proper validation, thus mitigating the risk of Man in The Middle attacks.

This solution ensures that the TrustManager throws a certificate exception when invalid certificates are detected. It also implements a secure chain of trust to verify the authenticity and integrity of the digital certificates.

To further enhance the security of the certificate validation process, consider implementing certificate pinning and regularly updating the list of trusted CA certificates to ensure the latest security standards.

Finally, perform thorough testing and validation to ensure the proper functioning of the certificate validation logic.

References​

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