ORA-28040: No matching authentication protocol after Oracle 12c upgrade.

After Oracle 12c upgrade users started seeing below errors

ORA-28040: No matching authentication protocol

The main issue is older versions of drivers will not able to connect 12c server. In Oracle 12.1, the default value for the SQLNET.ALLOWED_LOGON_VERSION parameter is set to 11. This means that database clients using pre-11g drivers cannot authenticate to 12.1 database servers unless the SQLNET.ALLOWED_LOGON_VERSION parameter is set to the old default of 8 in $ORACLE_HOME/network/admin/sqlnet.ora

SQLNET.ALLOWED_LOGON_VERSION_CLIENT = 8
SQLNET.ALLOWED_LOGON_VERSION_SERVER = 8

Note that SQLNET.ALLOWED_LOGON_VERSION has been deprecated in 12c.

Even after updating above values users might receive below error.
ORA-01017: invalid username/password: logon denied

In this case users need to add following parameter to sqlnet.ora
SQLNET.AUTHENTICATION_SERVICES = (NONE)

Also few users received ORA-28040 error when creating older version of databases (for ex: 10g) using DBCA

Thanks
Satishbabu Gunukula, Oracle ACE
https://oracleracexpert.com