|Secure Programming for Linux and Unix HOWTO|
|Prev||Chapter 7. Structure Program Internals and Approach||Next|
The counterpart to needing trustworthy channels (see Section 7.11) is assuring users that they really are working with the program or system they intended to use.
The traditional example is a ``fake login'' program. If a program is written to look like the login screen of a system, then it can be left running. When users try to log in, the fake login program can then capture user passwords for later use.
A solution to this problem is a ``trusted path.'' A trusted path is simply some mechanism that provides confidence that the user is communicating with what the user intended to communicate with, ensuring that attackers can't intercept or modify whatever information is being communicated.
If you're asking for a password, try to set up trusted path. Unfortunately, stock Linux distributions and many other Unixes don't have a trusted path even for their normal login sequence. One approach is to require pressing an unforgeable key before login, e.g., Windows NT/2000 uses ``control-alt-delete'' before logging in; since normal programs in Windows can't intercept this key pattern, this approach creates a trusted path. There's a Linux equivalent, termed the Secure Attention Key (SAK); it's recommended that this be mapped to ``control-alt-pause''. Unfortunately, at the time of this writing SAK is immature and not well-supported by Linux distributions. Another approach for implementing a trusted path locally is to control a separate display that only the login program can perform. For example, if only trusted programs could modify the keyboard lights (the LEDs showing Num Lock, Caps Lock, and Scroll Lock), then a login program could display a running pattern to indicate that it's the real login program. Unfortunately, since in current Linux normal users can change the LEDs, the LEDs can't currently be used to confirm a trusted path.
Sadly, the problem is much worse for network applications. Although setting up a trusted path is desirable for network applications, completely doing so is quite difficult. When sending a password over a network, at the very least encrypt the password between trusted endpoints. This will at least prevent eavesdropping of passwords by those not connected to the system, and at least make attacks harder to perform. If you're concerned about trusted path for the actual communication, make sure that the communication is encrypted and authenticated (or at least authenticated).
Countering these problems is more difficult; at this time I have no good technical solution for fully preventing ``fooled'' web users. I would encourage web browser developers to counter such ``fooling'', making it easier to spot. If it's critical that your users correctly connect to the correct site, have them use simple procedures to counter the threat. Examples include having them halt and restart their browser, and making sure that the web address is very simple and not normally misspelled (so misspelling it is unlikely). You might also want to gain ownership of some ``similar'' sounding DNS names, and search for other such DNS names and material to find attackers.