Pro PHP Security: From Application Security Principles to the Implementation of XSS Defenses, Second Edition

C H A P T E R  9

Using CAPTCHAs

We begin our discussion of the components of a secure application here in Chapter 9 by extending the notion of access control to the users of your online application.

In an Internet environment that is typically public, anonymous, always on, and unmonitored, the kinds of websites that are designed to be open to essentially any user are particularly vulnerable to abuse. The security threats for this kind of website are not those involved with unauthorized users, because essentially any user is qualified. Rather, the dangers are those associated with automated or mechanical pseudo-users, or robots: other computers ...