The following C program will create a new file test.txt when run. (If you are not familar with Linux syscalls for file operations, read chap 3 and 4 of UEAP.)

        #include <sys/types.h>
        #include <sys/stat.h>
        
        int main ()
        {
          creat ("test.txt", S_IRUSR|S_IWUSR|S_IXUSR|S_IROTH|S_IWOTH|S_IXOTH);
          return 0;
        }
      

Now compile this program using gcc:

        # gcc -o a.out a.c   // note that we are root!
      

Then make the generated executable a.out root set-uid:

        # chmod o+s a.out
      

Then run this program as a non-root user, who is the owner of the newly created file test.txt? Do you think whether or not this is secure? Why?