Perhaps someone should simply describe what would be the RESULT if allowing the insecure use of the certificate. As we already know, not furnishing an entry to ALLOW or PERMIT an expired certificate to be accepted, any security software in place will default
to BLOCK the connection to that resource.
Therefore, furnishing a system with an explicit ALLOW or PERMIT for the expired certificate would result in: any connection attempt to that resource ONLY FROM ANY MACHINE THAT HAS EXPLICITLY ACCEPTED/PERMITted connections TO THAT RESOURCE (**AND** FROM THAT
ENDPOINT). That is, if you have two computers on the same network, but only one of them has ALLOW'd the insecure connection, then ONLY that machine that has allowed the use of the expired certificate will be able to reach the data on that resource. What does
that mean? Well, since we don't know WHAT is hosted on that resource, we cannot say for sure. If the data is, indeed, coming from Microsoft, then sure- we can be fairly confident that there is not some malicious script residing there waiting to exploit the
machine with which we are connecting.
I would advise that if one is getting by without whatever is hosted at that resource and the only motivation to take any action is a warning from a security software instance, then the correct action to take would be to NOT ALLOW an explicitly declared connection
to the insecure resource (accepting the expired certificate) as a primary action- and ignore/silence the warning for THAT specific connection- as a secondary, optional action. If, in order to ignore the warning, one would be required to ignore ALL WARNINGS
(or all of that type e.g.: all suspicious connections via tcp/443), then I would definitely NOT set to ignore warnings in that case. The bottom line here is that Microsoft has decided that it does not need to renew an expired certificate- that, in itself,
should tell the user enough information such that there isn't the need to explicitly ALLOW the connection through: if they were replacing whatever content/service was hosted there with something relevant to the user, then they would either secure it with a
certificate (for activity sensitive enough to require it) or they would change the URL such that the user experience would not suffer such an annoyance. Microsoft's inaction, in this case, is all we need to know. Personally, I also receive this warning (via
Bitdefender); however, I will not allow the connection simply to not receive the warning in the logs.
One may apply the same logic for blocked connections from any other application logs, in a likewise fashion.
Edit: Does simply ACCEPT'ing the expired certificate mean that one has suffered an exploit? No. However, any user that DOES accept an expired certificate for that URL has OPENED AN AVENUE OF ATTACK, which would from that point in time forward, make it POSSIBLE
to exploit some other vulnerability from that URL on a machine that has accepted it.