The lftp command doesn't verify the TLS certificate by default in Red Hat Enterprise Linux 6

Issue

• I used the lftp command to connect the TLS enabled vsftpd server, and I saw a difference of the behavior between RHEL6 and RHEL7.
• The server certificate on the vsftpd server was not valid, and the lftp command verified the certificate and failed to connect it on RHEL7 as expected.
• But, the lftp command on RHEL6 doesn't verify the certificate and could connect the server unexpectedly.
• What caused the difference?