This was always, in theory a legit concern, but I felt would not and should not override the more pressing humanitarian need. Hopefully that sentiment stays, but damned if this won't help...
The problem here isn't if the attackers were or were not Syrian refugees but if society is willing to start a heavy postponed dialogue about the nature of the open borders to immigrants and refugees without shouting of Islamophobia or racism. Because regardless if the passport is false or not, there has to be a discussion of what societies at large should do with this issue.
Some folks will want to close all borders, others will want to keep the open arms and don't search anyone or see if they are potential terrorist and let them enter their countries without problems. The reality should be somewhere in the middle. Since you can't identify an extremist by asking "are you an extremist Sunni Muslims?" Perhaps a psychological test when entering to see if they are potential threads, establish a travel history, see their connections with potential or known extremist. Of course this doesn't address the issue of potential of extremist (or with extremist tendencies) in the 1st/2nd/3rd generation immigrant of Muslim.
Are people willing to have that chat or the current state of discussion will continue where someone tries to raise the possibility of terrorist taking advantage of the refugees to infiltrate to do their business.