I guess airport security in Egypt isn't up to western standards and you could bribe couple of people if you had enough money (which ISIS has)
Bribing may not be a requirement, there's a fair amount of adherents to the cause in the region.
Sharm was listed as a high risk location from ISIS terrorism earlier this year. Russia has just started engaging with ISIS in Syria. Now, this is
circumstantial, of course, in relation to the destruction of the Russian operated A321.
Lets wait and see what a trusted investigation reveals from the FDR/CVR.