User Submitted Data
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 15-17. 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);
?>
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).
