[Microsoft][ODBC SQL Server Driver][DBNETLIB]SSL Security error

One of my clients encountered the following error, when they tried to make a connection from an application to the SQL server. [Microsoft][ODBC SQL Server Driver][DBNETLIB]SSL Security error.

From my experience, most likely this is a TLS issue where the TLS version being used in the application server doesn't match the TLS version required by the database server. Below steps demonstrate how to fix it.

1. In the database server, check the TLS versions being enabled, by checking in below registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.x\<Client and Server>\DisabledByDefault
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.x\<Client and Server>\Enabled
If you see that both TLS 1.0 and 1.1 are DisabledByDefault=1 and Enabled=0 likes below screenshot, then your database server was set to accept TLS 1.2 only.

2. Check the SQL Server version, by running SELECT @@VERSION query. If you use TLS 1.2, not every SQL Server release supports TLS 1.2, please refer to KB3135244 - TLS 1.2 support for Microsoft SQL Server to determine whether you need to upgrade or update your SQL Server.

3. In the application server, download and install the most recent ODBC driver for SQL Server (Microsoft ODBC Driver 17 for SQL Server already support TLS 1.2). Then change the connection string in the application config to use the new driver.