ma che stai facendo un sito per una banca?

comunque, tanto per avere un pò più di sicurezza in più:

- https
- SESSION

stavo per scrivere altro, poi ho trovato questo su wikipedia che dà un bello spunto:

Methods to prevent session hijacking include:

- An open source solution is ArpON "Arp handler inspectiON". It is a portable ARP handler which detects and blocks all man-in-the-middle attacks through ARP poisoning and spoofing attacks with a static ARP inspection (SARPI) and dynamic ARP inspection (DARPI) approach on switched or hubbed LANs with or without DHCP. This requires an agent on every host that is to be protected.

- Use of a long random number or string as the session key. This reduces the risk that an attacker could simply guess a valid session key through trial and error or brute force attacks.

- Regenerating the session id after a successful login. This prevents session fixation because the attacker does not know the session id of the user after he has logged in.

- Encryption of the data passed between the parties; in particular the session key. This technique is widely relied-upon by web-based banks and other e-commerce services, because it completely prevents sniffing-style attacks. However, it could still be possible to perform some other kind of session hijack.

- Some services make secondary checks against the identity of the user. For example, a web server could check with each request made that the IP address of the user matched the one last used during that session. This does not prevent attacks by somebody who shares the same IP address, however, and could be frustrating for users whose IP address is liable to change during a browsing session.

- Alternatively, some services will change the value of the cookie with each and every request. This dramatically reduces the window in which an attacker can operate and makes it easy to identify whether an attack has taken place, but can cause other technical problems (for example, preventing the back button from working properly, on the web).
Users may also wish to log out of websites whenever they are finished using them.[2][3]
i punti più ragionevoli, a mio avviso, sono il 2°, 3°, 4° e 5°, (anche se il 4°, criptare i dati, non so se è superfluo usando l'https). Sostanzialmente confrontare gli ip che usano la session può evitare l'attacco dall'esterno, e la "paura" che gli utenti possano essere buttati fuori a metà sessione per il cambio di ip, beh, secondo me è molto remota o comunque potrebbe essere un rischio accettabile.

fonte: http://en.wikipedia.org/wiki/Session_hijacking