Yes, that is to be expected, there was a vulnerability locked down in 6.40.8
"What's new in 6.40.8 (2018-Apr-23 11:34):
!) winbox - fixed vulnerability that allowed to gain access to an unsecured router;"
I wonder how that worked, and what "unsecured" means.
As a rule I tend to have the following config
/ip firewall filter
add action=accept chain=input connection-state=established
add action=accept chain=input connection-state=related
add action=accept chain=input protocol=icmp
add action=drop chain=input in-interface=wan-interface
/tool mac-server
set [ find default=yes ] disabled=yes
add interface=lan-bridge-1
add interface=lan-bridge-2
/tool mac-server mac-winbox
set [ find default=yes ] disabled=yes
add interface=lan-bridge-1
add interface=lan-bridge-2
Then add extra input rules if needed (e.g. allow ssh on the wan port from a given management address-list), which I believe secures things enough to not be a major concern if I can't keep a given router uptodate (I've got some in horrendous places that are 3 days travel away at best, really don't like risking bricking them!)
Of course it only takes 1 infected box internally to bypass that. I've locked down access to only specific management address ranges in the past, but have been burnt when I've had routing protocols break, and the only way to get in is to ssh from the next hop, which I neglected to put in the config. I wonder if a "allow TTL=254 on wan" would do the trick as a template.
With those precautions, the risk from zero-day exploits is significantly minimized.