Symantec Messaging Gateway SSH with backdoor user account + privilege escalation to root due to very old Kernel

Symantec Messaging Gateway 9.5.3-3 includes a high-risk vulnerability: an undocumented "support" SSH account with a default password ('symantec') that allows OS logins, and the appliance runs a very old Linux kernel (circa 2007) with known privilege-escalation flaws; together these could enable an attacker to obtain root access. Symantec published an update to address the issue.