I have a UNIX system. Can I use /etc/passwd as my user database?
Although using /etc/passwd might seem convenient, it is advisable that you do not use the existing /etc/passwd file for authenticating users of your website. Otherwise, an attacker who gains access to a user of your website will also gain access to the system. Keep separate databases and encourage users to choose different passwords for their system accounts and web access. Periodically run password checkers that scan for weak passwords and accounts in which the username is also the password.
Why am I asked for my password twice in some websites?
Your browser keeps track of your password so that you do not have to type it for every request. The stored password is based on the realm (AuthName directive) and the hostname of the website. Sometimes you can access a website via different names, such as yourdomain.com and www.yourdomain.com. If you are authorized to access a certain restricted area of yourdomain.com but are redirected or follow a link to www.yourdomain.com, you will be asked again to provide the username and password because your browser thinks it is a completely different website.
Are there any serious security or privacy issues raised by cookies?
A server can access a cookie set only from its own domain. Although a cookie can be stored on the user's hard drive, there is no other access to the user's file system. It is possible, however, to set a cookie in response to a request for an image. So, if many sites include images served from a third-party ad server or counter script, the third party may be able to track a user across multiple domains.