How did the hijackers get through security on the September 11th attacks?

They got through security because they were not actually carrying "dangerous" weapons and were not detected carrying explosives.

They held the air crew outside the cabin hostage with box-cutters and innocent-looking toiletry knives, forcing the pilots to open the door.

The terrorists also had C4 explosive vests, which they threatened to detonate in mid-air if the pilots didn't open the door or if they flew the plane down to land.

Either way, for the pilots, it was a lose-lose situation. Whether they opened the door to the cabin or not, everybody on the plane would still have died.

The events led to the USA and UK placing undercover armed Air Marshals on every British and American flight, with guns specially designed to penetrate muscle and bone, but not holes in the plane.

The events also led to Allied governments re-designing doors on national plane cockpits, making them able to withstand being forced open or blown open with small explosives.

Allied airports have also upgraded security scanners which now take an x-ray photograph of people passing through. They also have dog patrols with canines specially trained to detect the smell of explosives. Also, blades, no matter how small or innocent-looking, and passengers liquids (drinks, shampoos containing peroxide, etc) have been banned on planes.