Many apartment buildings choose to use cheap locks for their buildings. Not only do these locks tend to wear out faster they also are much easier to attack.
Above is how easy it is to attack master key systems. Both my roommate and I had separate rooms with individual locks. We also both had keys for our doors but none of the other rooms in the building. I dissembled the first lock and measured all of the pins in the 5 cylinders, the first row in the graphic above depicts this. If I only had one lock then I would have a lot of possible master keys to cut since each cylinder can shear at 2 locations that is 2*2*2*2*2 possible shear lines or 32 keys to cut. But since I had access to two locks,with it’s measurements in the second row, I narrowed it down to 1 possibility. I should mention that having a second lock will not guarantee you to one master key combination. Depending on how the system is cross keyed you might have to use trial and error to find the grand master key and not just a sub master key.
Above is the key that I hand filed to the master key dimensions, and it worked on all of the doors.
If the key system used patent controlled keys this attack would have been a little bit more of a hassle since I would have to destroy my key to make the master. Ways to protect against this would be to use a patent controlled key and a 1 cut for the master shear line, since its easier to remove material then add it to an existing key. Also a determined person will always be able to exploit a master keyed system so it’s important to take any rooms that need to be secure off the master key system. Room examples would be server rooms, records rooms, main office etc.
My only motivation in all of this was curiosity, no ill intent.Please do not use any of this material for illicit activities. You should only pick locks that belong to you, or where you have explicit permission from the owner.