Secure Programming Cookbook.

A coworker lent me a copy of O'Reilly's Secure Programming Cookbook. Just by reading the table of content you get an idea of what you should be thinking about, the rest really are (as being a cookbook) implementation details.

Some notes on file use:

• Don't create a file using fopen as the permissions are 0666 (umask modified.) I never liked the f function anyways.
• Avoid TOCTOU (Time of Check Time of Use) race conditions (before the time you check and the time you use, the attacker as done something to your target and you operate on something that's not what you thought of in the first place.) The best way to avoid these when dealing with files is always to operate on the file descriptor, as the underlying file object doesn't change. If you're using function relying on a string to determine the filename you're 1) wasting cycles and 2) exposing yourself to a TOCTOU based attack.
• Unix/Linux doesn't support mandatory file locks very well. Windows gets it right (a file lock is a lock that, once aquired on a file, prevents other processes from accessing the file in the way described by the lock while the lock is held.)
• To make sure a temporary file can't be used by anyone else: after the file has been opened, delete the file. The process owning the file descriptor will still be able to use the file, while no one else will because the file doesn't exist by name.