Password file authentication

Taken from Apache FAQ.

7. Can I use my /etc/passwd file for Web page authentication?

Yes, you can - but it's a very bad idea. Here are some of the reasons:

• The Web technology provides no governors on how often or how rapidly password (authentication failure) retries can be made. That means that someone can hammer away at your system's root password using the Web, using a dictionary or similar mass attack, just as fast as the wire and your server can handle the requests. Most operating systems these days include attack detection (such as n failed passwords for the same account within m seconds) and evasion (breaking the connection, disabling the account under attack, disabling all logins from that source, et cetera), but the Web does not.
• An account under attack isn't notified (unless the server is heavily modified); there's no "You have 19483 login failures" message when the legitimate owner logs in.
• Without an exhaustive and error-prone examination of the server logs, you can't tell whether an account has been compromised. Detecting that an attack has occurred, or is in progress, is fairly obvious, though - if you look at the logs.
• Web authentication passwords (at least for Basic authentication) generally fly across the wire, and through intermediate proxy systems, in what amounts to plain text. "O'er the net we go/Caching all the way;/O what fun it is to surf/Giving my password away!"
• Since HTTP is stateless, information about the authentication is transmitted each and every time a request is made to the server. Essentially, the client caches it after the first successful access, and transmits it without asking for all subsequent requests to the same server.
• It's relatively trivial for someone on your system to put up a page that will steal the cached password from a client's cache without them knowing. Can you say "password grabber"?

If you still want to do this in light of the above disadvantages, the method is left as an exercise for the reader. It'll void your Apache warranty, though, and you'll lose all accumulated UNIX guru points.