章 29. 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.
例子 29-1. 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);
?>
|
|
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).
add a note
User Contributed Notes
User Submitted Data
rcphp at littondale dot dyndns dot org
29-May-2003 03:52
As mentioned somewhere else, magic_quotes can be dealt with by:
# Unmangle input if PHP is running in "Magic Quotes" mode.
# I need the input for both DB and HTML generation, so use
# the
appopriate translations in the appropriate places.
if(get_magic_quotes_gpc())
{
$content =
stripslashes($content);
$title = stripslashes($title);
}
Then in HTML I use the htmlspecialchars() function to output, and
in SQL I use addslashes(). Because I'm using the same variable in multiple
places, I need to be aware of what it is and what to do with
it.
Whether fast or not, I use Perl like regular functions for
validity checking. The below checks for a number, though PHP has functions
to handle this (I am only learning)
if(!preg_match("/^[0-9]*$/", $alterExistingID))
{
# showWarning is one of my functions
showWarning("Page
called with malformed comment ID");
$alterExistingID =
"";
}
See the is_numeric() function for a better
solution to this.
hugo at GO_SPAMASSASSIN dot gewis dot nl
21-Dec-2002 06:14
To prevent the above noted abuse, you can always assure that
register_globals is off (or if it isn't, exit).
For
example:
if(ini_get('register_globals')) {
print
'<b>Fatal:</b> This script requires that register_globals in
php.ini is disabled.';
exit;
}
// unlink files
here
Hugo.
ps: in general: try to avoid removing files or
exec'ing system commands in web-accessible scripts
Using Register Globals
