According to those familiar with the testing, Kurdish Peshmerga forces were indeed attacked with chemical weapons back in August by ISIS. The Organization for the Prohibition of Chemical Weapons (OPCW) will confirm in their report that the samples from the incident tested positive for mustard gas.
This will be the first confirmed use of chemical weapons inside Iraq since the Saddam Hussein era, and while the OPCW won’t formally blame ISIS, because they make it a habit never to assign blame for chemical attacks, there is no real dispute that this was an ISIS attack, and 35 Kurdish fighters were sickened in it.
ISIS is sort of a special case in claims of chemical weapons use at any rate, since they don’t really deny the incidents of chemical weapons use, and spend a lot of time bragging about their capability to launch such attacks.
CIA director John Brennan reported last week that ISIS was also believed to have used chemical weapons numerous other times, and that the US believes the group has both the capability to produce more chemical agents, and the caches of munitions to fill with them.