According to CNN,
The attack involved cruise missiles -- along with drones -- flying at low altitude, the source said, and their trajectory was from the north of the Abqaiq oil plant, which was struck by more than a dozen projectiles in the early hours of Saturday morning.
The missiles, according to the investigators' current assessment, flew over southern Iraq and through Kuwaiti airspace before reaching their targets. Kuwait on Monday announced it had launched an investigation into reports of sightings of drones or missiles shortly before the Saudi targets were hit.
Alertness of Kuwaiti AD left me without words. This looks like ROC spotters in the 21st century.
Another source who has spoken with Saudi government officials has told CNN that based on images of the wreckage that fell in the desert, at least some of the missiles used are known as the Quds 1.
Small problem, Quds 1 is said to be a Houti copy of the Iranian Soumar, itself a copy of Kh-55, a 80's era CM. Nice gene stability through clonings.
However, the Quds 1 does not appear to have the sort of range that would allow it to hit Abqaiq from Houthi-held parts of Yemen, some 1,300 kilometers away.
Fabian Hinz, research associate at the James Martin Center for Nonproliferation Studies, has researched the different varieties of missiles the Houthis have and says of the Quds 1, "both the missile's smaller size and its more fuel-hungry engine make it seem unlikely its range would be anywhere close to the Soumar's/Hoveyzeh's range of 1350 kilometers." The Soumar and Hoveyzeh are longer-range missiles that the Houthis have.
But, according to Wiki, the original Kh-55 is 2,500 km (1,300 nmi) range, so someone lost in the km/mile change, or the clone lost 50% of range while retained excellent accuracy with a bad quality engine...