ORA-12537: TNS:connection closed - Oracle RAC 11g and above

 Client was getting Below error while connecting to the RAC database

sqlplus tester@RACDB_1

SQL*Plus: Release 10.2.0.4.0 - Production on Tue Sep 22 11:34:07 2020

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ERROR:

ORA-12537: TNS:connection closed

Issue : 

The Oracle Binary permissions got changed and was not allowing the connections

Current Permissions under Oracle Home

[oracle@host1 ~]$ ls -lrt $ORACLE_HOME/bin/oracle

-rwxr-sr-x 1 oracle asmadmin 243043788 Jun  2 01:12 oracle

It should be set to 6751 and should look like "-rwsr-s--x"

But doing chmod 6751 on oracle binary was not setting the correct permissions

[oracle@host1 bin]$ chmod 6751 oracle

[oracle@host1 bin]$ ls -lrt $ORACLE_HOME/bin/oracle

-rwsr-x--x 1 oracle asmadmin 243043788 Jun  2 01:12 oracle

Running below as RDBMS database Owner user helped , in this case "oracle" user

1) Stop the database instance where the permissions got changed

 srvctl stop instance -d RACDB -i RACDB1

 2) Run as Oracle Database owner, in this case its Oracle OS user.  

[oracle@host1 ~]$ $GRID_HOME/bin/setasmgidwrap o=$ORACLE_HOME/bin/oracle

3) The permissions got changed and resolved the connection issues

[oracle@host1 ~]$ ls -lrt $ORACLE_HOME/bin/oracle

-rwsr-s--x 1 oracle asmadmin 243043788 Jun  2 01:12 oracle

Hope this resolves your issue..