This came as a surprise when I came back from vacation to discover that all of a sudden can’t do this on my Linux box sqlplus / as sysdba. I was getting ORA-01017. Something like this:

[oracle@dbtest dbs]$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Wed Aug 3 09:59:29 2016

Copyright (c) 1982, 2014, Oracle. All rights reserved.

ERROR:
 ORA-01017: invalid username/password; logon denied

Enter user-name: ^C
 [oracle@dbtest dbs]$

In my case, one of my colleagues, took the liberty to set TWO_TASK variable. After removing/unsetting the variable, all came to my desired situation.

As you may know, this TWO_TASK variable overwrites the ORACLE_SID & ORACLE_HOME variables and prohibits the local connections and is letting you to use the sqlnet path.