After 12c Upgrade: ORA-28040: No matching authentication protocol

An application could not connect to the Oracle Database that was upgraded to 12c (12.1.0.2). Whenever it tried connecting, the database would return the error ORA-28040: No matching authentication protocol back to the application.

Bug 14575666 states:
In 12.1, the default value for the SQLNET.ALLOWED_LOGON_VERSION parameter (deprecated now) has been updated to 11. This means that database clients using pre-11g JDBC thin drivers cannot authenticate to 12.1 database servers unless the SQLNET.ALLOWED_LOGON_VERSION parameter is set to the old default of 8.

Solution:
As the SQLNET.ALLOWED_LOGON_VERSION is a deprecated parameter, although it will work but will keep flooding your ALERT LOG file, it's best to use the updated parameter(s):

Add the below line(s) in the SQLNET.ORA file on the database server:

SQLNET.ALLOWED_LOGON_VERSION_SERVER=8
SQLNET.ALLOWED_LOGON_VERSION_CLIENT=8

Hope this helps...