We deployed a new app last week and got the error ORA-28040: No matching authentication protocol while establishing a connection to the database.
It turns out that this app was using an embedded Oracle client 11.1 hence this error.
Oracle recommends that you upgrade your client software to match the current server software, which in my case is Oracle 12.2.
To fix this error you can upgrade the client or config the new parameter
SQLNET.ALLOWED_LOGON_VERSION_SERVER according to the documentation:
MOS 1957995.1 says that the default for the SQLNET.ALLOWED_LOGON_VERSION_SERVER setting has changed in 12.2 from 11 to 12. So if your client is not at least 188.8.131.52 or includes the CPUOCT2012 patch you will not be able to use the 12 setting.
It looks accurate as client 11.1 fails to connect to Oracle 12.2 and client 11.2 works: