Esri is planning to require TLS 1.2 connections for ArcGIS Online services starting on April 16, 2019. When TLS 1.2-only connections to ArcGIS Online resources are in effect, there are certain things to be considered when creating new ArcObjects SDK for the Microsoft .NET Framework applications (including ArcGIS Engine for .NET), or running existing ones. At 10.7, the desktop software is already configured to use TLS 1.2 and no action is necessary to enable TLS 1.2.
If your client applications created with legacy ArcObjects .NET (including Engine applications) that use inbound connections to ArcGIS do not have TLS 1.2 enabled after the switch to TLS 1.2 only is made, your integrations may experience disruption.
The following changes can be made to ArcObjects .NET and Engine applications that were built with .NET versions prior to 4.6:
Note: .NET applications may directly enable TLS 1.2 by setting System.Net.ServicePointManager.SecurityProtocol to enable SecurityProtocolType.Tls12, as shown in the example below.
Example (C#): System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;
Additional resources for transitioning to TLS 1.2 for the Esri platform, can be found on the on the Important Updates for the ArcGIS Platform and Transport Layer Security (TLS) Protocol Support pages, including references to Microsoft developer documents and the Esri Product Security team.
Concerning the use of HTTP in legacy applications, note that with a future security update to ArcGIS Online, all organizations must use HTTPS-only, as this is becoming a standard practice in the industry.
It may be that ArcObjects .NET applications that previously worked with HTTP experience certificate errors with HTTPS, because client machines do not have the relevant certificates in their certificate stores. Client machines, therefore, must be configured accordingly (for example, the relevant certificates added to the certificate stores). For testing and development, it is possible to create server ‘test’ certificates. This can allow developers to test their client code against a test server that only supports HTTPS without requesting a certificate from a Certificate Authority. This is common for intranet scenarios.
For more information on TLS 1.2, refer to the Esri Technical Support document: Important Updates for the ArcGIS Platform and Transport Layer Security (TLS) Protocol Support