ZEND PHP 5 Certification STUDY GUIDE

Security ” 217
mitigate attacks, but there is an inherent problem in blacklist filtering that is evident
here: blacklists must be modified continually, and expanded as new attack vectors
become apparent.
On the other hand, whitelist filtering is much more restrictive, yet it affords the
programmer the ability to accept only the input he expects to receive. Instead of
identifying data that is unacceptable, a whitelist identifies only the data that is acceptable.
This is information you already have when developing an application; it
may change in the future, but you maintain control over the parameters that change
and are not left to the whims of would-be attackers. Since you control the data that
you accept, attackers are unable to pass any data other than what your whitelist allows.
For this reason, whitelists afford stronger protection against attacks than blacklists.
Filter Input
Since all input is tainted and cannot be trusted, it is necessary to filter your input to
ensure that input received is input expected. To do this, use a whitelist approach, as
described earlier. As an example, consider the following HTML form:
Username:
Password:
Favourite colour:
Red
Blue
Yellow
Green
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This form contains three input elements: username, password, and colour. For this
example, username should contain only alphabetic characters, password should contain
only alphanumeric characters, and colour should contain any of “Red,” “Blue,”
“Yellow,” or “Green.” It is possible to implement some client-side validation code
using JavaScript to enforce these rules, but, as described later in the section on