This claim may or may not be true. It may simply be intended to give the Islamic State the appearance of greater reach and power than it actually has. After all, as Muhammad said, war is deceit.
“IS group in Egypt claims it downed Russian airliner,” AFP, October 31, 2015 (thanks to Steve):
Cairo (AFP) – The Islamic State group’s affiliate in Egypt has claimed it downed the Russian passenger plane that crashed Saturday in the Sinai Peninsula, where the jihadists are waging an insurgency, killing all on board.
“The soldiers of the caliphate succeeded in bringing down a Russian plane in Sinai,” said the statement circulated on social media.