Set-UID is an important security mechanism in Unix operating systems. When a Set-UID program is run, it assumes the owner's privileges. For example, if the program's owner is root, then when anyone runs this program, the program gains the root's privileges during its execution. Set-UID allows us to do many interesting things, but unfortunately, it is also the culprit of many bad things. Therefore, the objective of this lab is two-fold: (1) Appreciate its good side: understand why Set-UID is needed and how it is implemented. (2) Be aware of its bad side: understand its potential security problems.