Chapter 15. Security
PHP is a powerful language and the interpreter, whether included
in a web server as a module or executed as a separate
CGI binary, is able to access files, execute
commands and open network connections on the server. These
properties make anything run on a web server insecure by default.
PHP is designed specifically to be a more secure language for
writing CGI programs than Perl or C, and with correct selection of
compile-time and runtime configuration options, and proper coding
practices, it can give you exactly the combination of freedom and
security you need.
As there are many different ways of utilizing PHP, there are many
configuration options controlling its behaviour. A large
selection of options guarantees you can use PHP for a lot of
purposes, but it also means there are combinations of these
options and server configurations that result in an insecure
The configuration flexibility of PHP is equally rivalled by the
code flexibility. PHP can be used to build complete server
applications, with all the power of a shell user, or it can be used
for simple server-side includes with little risk in a tightly
controlled environment. How you build that environment, and how
secure it is, is largely up to the PHP developer.
This chapter starts with some general security advice, explains
the different configuration option combinations and the situations
they can be safely used, and describes different considerations in
coding for different levels of security.
A completely secure system is a virtual impossibility, so an
approach often used in the security profession is one of balancing
risk and usability. If every variable submitted by a user required
two forms of biometric validation (such as a retinal scan and a
fingerprint), you would have an extremely high level of
accountability. It would also take half an hour to fill out a fairly
complex form, which would tend to encourage users to find ways of
bypassing the security.
The best security is often inobtrusive enough to suit the
requirements without the user being prevented from accomplishing
their work, or over-burdening the code author with excessive
complexity. Indeed, some security attacks are merely exploits of
this kind of overly built security, which tends to erode over time.
A phrase worth remembering: A system is only as good as the weakest
link in a chain. If all transactions are heavily logged based on
time, location, transaction type, etc. but the user is only
verified based on a single cookie, the validity of tying the users
to the transaction log is severely weakened.
When testing, keep in mind that you will not be able to test all
possibilities for even the simplest of pages. The input you
may expect will be completely unrelated to the input given by
a disgruntled employee, a cracker with months of time on their
hands, or a housecat walking across the keyboard. This is why it's
best to look at the code from a logical perspective, to discern
where unexpected data can be introduced, and then follow how it is
modified, reduced, or amplified.
The Internet is filled with people trying to make a name for
themselves by breaking your code, crashing your site, posting
inappropriate content, and otherwise making your day interesting.
It doesn't matter if you have a small or large site, you are
a target by simply being online, by having a server that can be
connected to. Many cracking programs do not discern by size, they
simply trawl massive IP blocks looking for victims. Try not to