I don't know if this answers your question directly, but I'll try.

One important thing to note is that in general, I would haphazard – both from researchers, university, as well as working in the financial industry – that the majority of banking "cracks", so to speak, aren't from talented hackers or such, but from people just being either bribed or making mistakes. As such, what ends up being the major "weakest link" in the system isn't the system itself, but the people who operate it.

The Knights actually enforced quite the strenuous background check and installed extreme levels of checks against what they considered "sin". If you had money on your person when you died, you were considered (if you were a knights templar) to be "out of a state of grace", and therefore damned (denied a Christian burial). Vows of poverty, chastity, and monastic life were paramount – if you broke them, you could be at best punished and at worst expelled from the order. So they were pretty good at keeping their ranks absolutely guarded against spilling secrets, which is quite important to the well-functioning of any banking system. In order for people to withdraw any sort of money, the actual liability holder him or herself had to be at the location in person to actually withdraw the amount – to be sure, this is an imperfect system, but it was pretty good and standard at the time.

Sources:

http://www.newyorkfed.org/research/conference/2006/mpayments/He-Huang-Wright.pdf

The History of Money by Jack Weatherford