The greatest weakness in many PHP programs is not inherent in the
language itself, but merely an issue of code not being written with
security in mind. For this reason, you should always take the time
to consider the implications of a given piece of code, to ascertain
the possible damage if an unexpected variable is submitted to it.
Example 30-1. Dangerous Variable Usage
<?php
// remove a file from the user's home directory... or maybe
// somebody else's?
unlink ($evil_var);
// Write logging of their access... or maybe an /etc/passwd entry?
fwrite ($fp, $evil_var);
// Execute something trivial.. or rm -rf *?
system ($evil_var);
exec ($evil_var);
?>
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You should always carefully examine your code to make sure that any
variables being submitted from a web browser are being properly
checked, and ask yourself the following questions:
Will this script only affect the intended files?
Can unusual or undesirable data be acted upon?
Can this script be used in unintended ways?
Can this be used in conjunction with other scripts in a negative
manner?
Will any transactions be adequately logged?
By adequately asking these questions while writing the script,
rather than later, you prevent an unfortunate re-write when you
need to increase your security. By starting out with this mindset,
you won't guarantee the security of your system, but you can help
improve it.
You may also want to consider turning off register_globals,
magic_quotes, or other convenience settings which may confuse
you as to the validity, source, or value of a given variable.
Working with PHP in error_reporting(E_ALL) mode can also help warn
you about variables being used before they are checked or
initialized (so you can prevent unusual data from being
operated upon).