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Top 7 PHP Security Blunders (1)

Unvalidated Input Errors

One of -- if not the -- most common PHP security flaws is the unvalidated input error. User-provided data simply cannot be trusted. You should assume every one of your Web application users is malicious, since it's certain that some of them will be. Unvalidated or improperly validated input is the root cause of many of the exploits we'll discuss later in this article.

As an example, you might write the following code to allow a user to view a calendar that displays a specified month by calling the UNIX cal command.

$month = $_GET['month'];
$year = $_GET['year'];

exec("cal $month $year", $result);
print "<PRE>";
foreach ($result as $r) { print "$r<BR>"; }
print "</PRE>";

This code has a gaping security hole, since the $_GET[month] and $_GET[year] variables are not validated in any way. The application works perfectly, as long as the specified month is a number between 1 and 12, and the year is provided as a proper four-digit year. However, a malicious user might append ";ls -la" to the year value and thereby see a listing of your Website's html directory. An extremely malicious user could append ";rm -rf *" to the year value and delete your entire Website!

The proper way to correct this is to ensure that the input you receive from the user is what you expect it to be. Do not use JavaScript validation for this; such validation methods are easily worked around by an exploiter who creates their own form or disables javascript. You need to add PHP code to ensure that the month and year inputs are digits and only digits, as shown below.

$month = $_GET['month'];
$year = $_GET['year'];

if (!preg_match("/^[0-9]{1,2}$/", $month)) die("Bad month, please re-enter.");
if (!preg_match("/^[0-9]{4}$/", $year)) die("Bad year, please re-enter.");

exec("cal $month $year", $result);
print "<PRE>";
foreach ($result as $r) { print "$r<BR>"; }
print "</PRE>";

This code can safely be used without concern that a user could provide input that would compromise your application, or the server running it. Regular expressions are a great tool for input validation. They can be difficult to grasp, but are extremely useful in this type of situation.

You should always validate your user-provided data by rejecting anything other than the expected data. Never use the approach that you'll accept anything except data you know to be harmful -- this is a common source of security flaws. Sometimes, malicious users can get around this methodology, for example, by including bad input but obscuring it with null characters. Such input would pass your checks, but could still have a harmful effect.

You should be as restrictive as possible when you validate any input. If some characters don't need to be included, you should probably either strip them out, or reject the input completely.

By: Pax Dickinson from


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