Symptoms:
When tyring to login on the db instance using sqlplus / as sysdba, gets ORA-01017: invalid username/password; logon denied.
Cause:
Operating system authentication may have been deliberately disabled.
Solution:
Look at the sqlnet.ora file located at $ORACLE_HOME/network/admin and check the value of parameter sqlnet.authentication_services.
If you get the operating system authentication disabled, this parameter is set with the value:
#SQLNET.AUTHENTICATION_SERVICES = (NONE)
If you want to allow connection using operating system authentication, remove or comment out the line:
#SQLNET.AUTHENTICATION_SERVICES = (NONE)
