One would suspect that the terrorists, wanting to be able to infiltrate CTU, would investigate the employees looking for a weak link.
Once that link is found, it's easy enough to expolit a couple of druggies looking for a big payday.
That said, how Rudy got into the building is a mystery and sloppy writing.
Also, a secure building should log people in and out. A second attempt to enter a building using a card with someone already logged in should raise a flag.
This show is compelling and all, but the writing is increasingly lazy.
SD
Looking at any work too critically can take the enjoyment out of ~anything~. :~D