See https://wiki.samba.org/index.php/Samba_Release_Planning for a matrix showing the currently supported versions of Samba.

Please report all security issues or defects to [email protected] and never on IRC, public mailing lists or in Bugzilla!

Once reported and if warrented by the specific issue, the Samba Team will follow the process outlined in https://wiki.samba.org/index.php/Samba_Security_Process to produce a security release.